Biology Introduction PDF

BIOLOGY Introduction to biology What distinguishes a living organism from an inanimate object is the ability to reproduce, generating other organisms with the same fundamental characteristics, controlled by a genetic program. 1. Characteristics of living organisms Organisms, with the exception...

BIOLOGY Introduction to biology What distinguishes a living organism from an inanimate object is the ability to reproduce, generating other organisms with the same fundamental characteristics, controlled by a genetic program. 1. Characteristics of living organisms Organisms, with the exception of viruses, are made up of cells: either unicellular or multicellular. Organisms grow (increase in size) and develop (structural and physiological changes). Organisms reproduce, thereby ensuring the perpetuation of the species to which they belong. Organisms regulate their metabolism: chemical reactions that ensure homeostasis. Organisms respond to stimuli: they perceive changes and implement processes in response. Organisms possess genetic information: the information is contained in genes made of DNA. Populations of living organisms are subject to evolution: characteristics change over time, adapting. 2. Bioelements Of the 92 chemical elements present in nature, only about twenty are involved in the composition of living matter, and among these, the most quantitatively important are: oxygen, carbon, hydrogen, and nitrogen. These four, along with phosphorus and sulphur, make up over 99% of living substances. Other essential elements include calcium, chlorine, potassium, sodium, magnesium, iodine, and iron. The remaining elements are present in very small amounts. 3. Biomolecules Biomolecules are organic compounds of high molecular weight, belonging to four main groups: - Carbohydrates: ternary compounds containing carbon, oxygen, and hydrogen. Based on their structure, they are divided into: ○ Monosaccharides: glucose, fructose, galactose ○ Disaccharides: sucrose, lactose, maltose, cellobiose ○ Polysaccharides: Storage: starch (plants) and glycogen (animals) Structural: cellulose, chitin, glycosaminoglycans (GAG) - Proteins: biological polymers formed by the union of 20 different amino acids, linked together by peptide bonds to form chains. Their synthesis is controlled by DNA. They can have a structural role or a catalytic function. Of the 20 amino acids that make up proteins, 9 cannot be synthesised and are therefore called essential amino acids; they must be obtained from the diet. A protein is referred to instead of a peptide when the chain is made up of at least 100 amino acids. A protein can present four levels of structure: ○ Primary: amino acid sequence ○ Secondary: spatial arrangement of amino acids ○ Tertiary: three-dimensional structure ○ Quaternary: formed by the presence of two or more peptide subunits - Lipids: a class of substances that are chemically diverse and are characterised by being insoluble in water. They can have structural functions, energy storage, and act as chemical messengers. The main lipids are: ○ Triglycerides: energy storage function ○ Phospholipids: structural function; components of cell membranes ○ Steroids: include sex hormones, cortical hormones, vitamin D, bile acids, cholesterol - Nucleic acids: linear polymers of nucleotides. They are responsible for fundamental functions in heredity and protein synthesis. These include DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). 4. Weak interactions in biology Organic molecules can interact with each other through weak non-covalent forces and ionic bonds. Interactions of biological importance include: hydrogen bonds, Van der Waals forces, dipole-dipole interactions, and hydrophobic forces. These bonds are important because they give macromolecules like DNA and proteins their shape and structure. 5. Brief notes on microscopy Resolving power is the minimum distance below which we can no longer distinguish two points as separate. The resolving power of the human eye is about 0.1 millimetres. To identify objects smaller than 0.1 mm, magnifying lenses and then microscopes have been used. The optical microscope is equipped with two lens systems: the objective and the eyepiece. The objective is directed towards the stage where the sample to be examined is placed; a mirror directs a beam of light through the specimen, and the objective projects an enlarged image towards the eyepiece, which further magnifies it. This allows for the magnification of an image up to about 2000 times, enabling the observation of major cellular structures: nucleus, mitochondria, chromosomes during mitosis, and even bacteria. In the 1930s, the electron microscope was developed, based on the use of a beam of electrons instead of visible light. This makes it possible to observe subcellular structures (membranes). In the transmission electron microscope (TEM), a beam of electrons passes through the object to be enlarged and strikes a fluorescent screen, where the image is formed. This instrument has the highest resolving power, about 0.2 nm, and allows the fine structure of cells to be highlighted. In the scanning electron microscope (SEM), a beam of electrons explores the object; the electrons do not pass through the sample but are reflected from its surface, forming a three-dimensional image. It has a resolving power of about 10 nm. Cell biology 1. Cell theory The term "cell" was first introduced in 1665 by Robert Hooke. Since then, the structure of the cell has been studied, and today there is a unified theory regarding the nature of living organisms, known as cell theory, formulated around 1850 by Schleiden, Schwann, and Virchow. A cell is a small element, bounded by a membrane, filled with a concentrated solution of chemicals in water, and capable of producing copies of itself by growing and dividing into two. Key elements of cell theory: - all living organisms are composed of cells. - the cell is the fundamental unit. - every cell arises from another pre-existing cell. - genetic information resides in the DNA of cells and is transmitted during cell division. 2. Cell types Each cell is surrounded by a cell membrane that separates the internal environment from the external one and regulates the entry and exit of materials. Inside is the cytoplasm, an aqueous solution in which cellular components are immersed and cellular functions occur. These functions are carried out by cytoplasmic organelles. Based on the presence or absence of a nucleus, cells are divided into two groups: - Prokaryotic cells (bacteria): These are the simplest and smallest existing cells (0.5 – 5 µm). They lack membrane-bound cytoplasmic organelles and do not have a true nucleus; the genetic material consists of a single circular DNA molecule, located in a region of the cell called the nucleoid. In addition to the main DNA molecule, plasmids may also be present. Ribosomes, which allow protein synthesis, are present in the cytoplasm but are smaller than those found in eukaryotes. The plasma membrane forms invaginations called mesosomes. Prokaryotic cells are surrounded by a cell wall external to the plasma membrane, made of peptidoglycan (not in archaebacteria). A peptidoglycan is composed of long polysaccharide chains in which units of amino sugars (N-acetylglucosamine and N- acetylmuramic acid) alternate, linked by peptide cross-bridges to form a complex structure. Prokaryotic organisms are always unicellular and reproduce asexually by binary fission, but they can exchange genetic material through transformation, conjugation, or transduction. In some prokaryotes, there is an outer membrane (capsule), a layer rich in phospholipids and carbohydrates, external to the cell wall. It does not have a protective function but can contain toxins responsible for pathogenic processes. The flagella of prokaryotes, when present, are simple filamentous structures used for movement in the environment. They are helical tubes made of the protein flagellin, and their movement is powered by a proton gradient. - Eukaryotic cells Eukaryotic cells are more complex and larger than prokaryotic cells (10 – 100 µm). The cytoplasm contains various organelles with specific structures and functions that enable various cellular activities. The genetic material is made up of multiple chromosomes, enclosed within a well-defined nucleus. Each chromosome consists of a linear DNA molecule associated with specific proteins. Eukaryotic organisms can be unicellular (protists) or multicellular (plants, fungi, and animals). Cells: differences and common aspects The cells that make up living beings are diversified both physiologically and structurally. At the same time, they possess some important common aspects. Differences: - Size: varies from a few micrometres (bacteria) to a millimetre (frog egg). - Movement: some can move using flagella, while others are immobile. - Covering: some have an outer covering beyond the membrane. - Atmospheric oxygen: some use it, while others are poisoned by it. - Production of compounds: some can produce hormones, starch, fats, pigments, or rubber. Common aspects: - Chemical composition: based on the same four classes of compounds (proteins, carbohydrates, nucleic acids, lipids). - Chemical reactions: all cells perform glycolysis. - Genetic information: in all cells, it resides in DNA. 3. Viruses Viruses are living entities, but they are not made up of cells. They consist of a molecule of nucleic acid (DNA or RNA) containing genetic information, enclosed in a protein coat called a capsid. They have various shapes and sizes (10 – 300 nm). They are incapable of independently synthesising the proteins they are made of. To reproduce, they must infect host cells, exploiting their enzymes and energy systems; thus, they can be defined as obligate intracellular parasites. They are specific parasites: some infect only animal cells, some only plant cells, and others, called bacteriophages, infect only bacterial cells. 4. Cell membrane The cell membrane is a thin envelope (7-9 nm) that surrounds the cell and regulates the exchange of materials with the outside. It is mainly composed of phospholipids and proteins, but also contains cholesterol and glycolipids. Phospholipids are amphipathic molecules, characterised by a hydrophilic polar head and two hydrophobic tails. When dispersed in an aqueous medium, phospholipids tend to spontaneously form a bilayer in which the heads face outward and the hydrophobic tails face inward. Membrane proteins can partially or fully span the lipid bilayer. The phospholipids and their associated proteins are free to move laterally; for this reason, this model is called the fluid mosaic model. Membrane proteins perform numerous functions: enzymes, transport proteins, or cellular receptors, which can recognize and bind specific molecules (hormones, neurotransmitters). The main significances of the cell membrane are: - Structural and morphological: defines the shape of the cell, separating the inside from the outside. - Functional: Regulates the exchange of ions, nutrients, and waste products. - Communication and integration: membrane proteins act as receptors that bind hormones and other messengers, which can modify cellular metabolism. They play important roles in recognition and adhesion between cells, can be recognized as antigens by the immune system, and are responsible for contact inhibition, the phenomenon whereby cells stop proliferating when they come into contact with one another. 5. Nucleus The nucleus controls the activities of the cell and plays an important role in replication, growth, and cellular differentiation. It is surrounded by a double membrane that separates it from the cytoplasm, known as the nuclear membrane, which is studded with pores that allow selective exchanges with the cytoplasm. The nucleus contains DNA, which is complexed with structural proteins called histones to form chromatin. When the cell is not dividing, the DNA strands that constitute the different chromosomes are uncoiled, forming an indistinct mass. Before cell division, however, chromatin condenses, and the individual chromosomes assume a compact appearance. The nucleus also contains one or more nucleoli, special structures where rRNA (ribosomal RNA) is synthesised and ribosomes are assembled. 6. Cytoplasmic organelles The other cytoplasmic organelles present in the cell include: - Ribosomes: sites of protein synthesis; assembled in the nucleolus. They consist of two subunits, one larger and one smaller, each made up of rRNA and proteins. They can be free in the cytoplasm or bound to the outer membrane of the endoplasmic reticulum. Eukaryotic ribosomes (80S) are larger than prokaryotic ribosomes (70S). S = Svedberg, a unit of measurement for sedimentation velocity.  - Endoplasmic reticulum: a system of membranes consisting of tubules and sacs. It can be smooth (SER) or rough (RER), depending on whether it is devoid of or covered with ribosomes. The SER participates in lipid synthesis and detoxification from drugs and poisons, while the RER synthesises proteins destined for non-cytoplasmic locations (outside the cell, cell membrane). After being released into the RER, these proteins are transferred to the Golgi apparatus via vesicle flow. - Golgi apparatus: composed of a stack of flattened membrane-bound vesicles called cisternae. It serves as a collection, processing, and distribution centre for the products of the ER: after receiving the vesicles, it modifies their contents, transfers them into new vesicles, and directs them to various cellular compartments or the plasma membrane. The region facing the RER is called the cis face, while the region facing the plasma membrane is called the trans face. - Lysosomes: membrane-bound vesicles comparable to the cell's stomach. They contain hydrolytic (digestive) enzymes capable of breaking down substances and are characterised by a very acidic pH. They are abundant in cells responsible for defending the organism, such as white blood cells. These organelles digest both external substances that have been engulfed and cellular materials that are no longer useful. The digested materials are then released into the cytoplasm for reuse. A cell can undergo suicide by breaking the lysosomal membrane and releasing the enzymes into the cytoplasm. This process is called autolysis and plays an important role during development and bone remodelling. - Microsomes: vesicles similar to lysosomes, but smaller and containing specific substances. Notably, peroxisomes are vesicles containing catalase, an enzyme capable of decomposing highly toxic hydrogen peroxide (H2O2), which can damage membranes. Peroxisomes in liver cells participate in detoxification, meaning they break down harmful molecules, such as alcohol. - Mitochondria: organelles enclosed by a double membrane; the outer membrane is smooth, while the inner membrane has numerous folds called cristae. The inner content is referred to as the matrix. They possess their own circular DNA, ribosomes, and replicate by binary fission. Mitochondria in the zygote come almost exclusively from the egg cell. According to the endosymbiotic theory, mitochondria are descendants of primitive prokaryotic cells that were engulfed. Mitochondria can be considered the energy factories of cells because they are the site of cellular respiration, a process in which organic substances are broken down into CO2 and H2O in the presence of oxygen, releasing energy that is used to synthesise ATP, necessary for cellular activities. - Cytoskeleton: it consists of a dense network of protein filaments that strengthen the cell, determine its shape, control the movement of chromosomes and some molecules within, and allow for cellular movements. It is made up of three types of filaments: ○ Microtubules: composed of 13 filaments of a globular protein called tubulin, aggregated to form a hollow cylinder. They are essential components of centrioles, the mitotic spindle, and cellular appendages (flagella and cilia). ○ Intermediate filaments: made from various types of fibrous proteins, including keratin, and are important for providing mechanical strength to the cell. ○ Microfilaments: filaments of actin, a protein also involved in muscle contraction. They allow the movement of organelles within the cell and the formation of pseudopodia, extensions that enable phagocytosis as well as the movement of amoebas and amoeboid cells. - Centrioles: cylindrical organelles composed of 9 groups of three microtubules. Plant cells lack them, while animal cells possess two, arranged at right angles in the central region of the cell. They play a crucial role in the assembly of microtubules. The region of the centrioles, which serves as the organising centre for cellular microtubules, is called the centrosome. - Cilia and flagella: these are motile cellular appendages formed from bundles of microtubules arranged in a characteristic pattern (9 pairs of paired microtubules arranged in a circle and one pair of separate microtubules at the centre) and covered by the plasma membrane. Free cells use them to move through liquids, while fixed cells use them to move extracellular material. Flagella are long and few in number, while cilia are short and numerous. 7. Plant cell Unlike animal cells, plant cells have: a cell wall, plastids, and vacuoles.  - Cell wall: a rigid outer envelope that gives shape to the cell, protects it, and supports it. It is made up of cellulose fibres and has small pores that allow the passage of cytoplasm and substances from one cell to another; these communication structures between cells are called plasmodesmata. - Plastids: these include chromoplasts, which contain coloured substances (pigments), leucoplasts, which are colourless and contain storage substances, and chloroplasts, which contain green pigments (chlorophylls) and are the site of photosynthesis. Chloroplasts are surrounded by a double membrane and contain a complex system of flattened and interconnected vesicles called thylakoids, to which chlorophylls are associated. Thylakoids stack to form structures called grana. They are the sites of chlorophyll- based photosynthesis. Chloroplasts contain a circular DNA molecule and ribosomes and replicate by binary fission. - Vacuoles: vesicles that contain water and substances, which grow larger as the cell ages, eventually occupying almost all of its volume. The cytoplasm is reduced to a thin layer pressed against the cell wall, while a large vacuole occupies much of the internal space, providing support to the cell. Vacuoles serve as storage for reserve and waste substances. 8. Exchange of materials between the cell interior and exterior Substances can enter and exit the cell in various ways. It is important to distinguish between passive transport and active transport. Passive transport occurs along a concentration gradient, meaning a substance moves from an area of higher concentration to an area of lower concentration. This process is spontaneous and does not require energy. In contrast, active transport occurs against the gradient, meaning a substance moves from an area of lower concentration to an area of higher concentration. This requires energy, which is provided by the hydrolysis of ATP. The concentration gradient is the force that drives the movement of a substance. If the substance moving has an electric charge (i.e., it is an ion), it is necessary to consider not only its concentration but also the distribution of charges on both sides of the membrane. The electrochemical gradient is the net driving force that tends to move a charged solute across the membrane and is the result of the sum of the concentration gradient and the electric potential. - Simple diffusion: the net movement of particles from a region of high concentration to one of lower concentration. This process occurs along the concentration gradient and is a passive process. Osmosis is a specific case of diffusion; it involves the passage of water through a semipermeable membrane that separates two solutions of different concentrations. Water spontaneously moves from the more dilute solution (hypotonic) to the more concentrated solution (hypertonic). The pressure required to be applied to the more concentrated solution to prevent the solvent from passing through is known as osmotic pressure. Substances like ions and sugars, which cannot cross the membrane via simple diffusion, can be transported through membrane proteins. These proteins operate through two mechanisms: - Facilitated diffusion: the transport of a substance along its concentration gradient via a transport protein. It is passive and does not require energy. - Active transport: the transport of substances against their concentration gradient through membrane proteins, known as pumps. This is an active process and requires energy. Transport proteins can be categorised into two types: - Carrier proteins: they bind the substance they transport, then change shape to release it on the opposite side of the membrane. - Channel proteins: they form pores through the membrane. Their opening occurs in response to electrical signals or the binding of a specific molecule. Macromolecules and large particles that cannot pass through the membrane via simple diffusion or transport proteins can be introduced or expelled using vesicles through:  - Endocytosis: formation of membrane invaginations that close inward to form small vesicles encapsulating the substance to be transported. This process is called phagocytosis when the cell engulfs solid particles and pinocytosis when it engulfs droplets of liquid. In receptor-mediated endocytosis, the molecule to be transported binds to a membrane receptor, and the molecule-receptor complex is subsequently engulfed in a vesicle - Exocytosis: this process operates in the opposite direction; vesicles migrate to the membrane and fuse with it, releasing their contents outside. 9. Protein pumps and membrane potential In all cells, there is a different concentration of ions on either side of the membrane, resulting in an electrical potential difference of approximately -70mV (the inside is negative relative to the outside). This difference is known as the membrane potential (or resting potential) and results from the activity of various transport proteins. - Sodium-potassium pump: sodium ions (Na+) are about 10 times more concentrated outside the cell, while potassium ions (K+) are about 30 times more concentrated inside. This gradient is produced by an intrinsic protein, the sodium-potassium ATPase pump, which transports both ions against their gradient: 3 Na+ ions are transported outside and 2K+ ions inside, using energy from ATP hydrolysis. The gradients of Na+ and K+ are responsible for the membrane potential, control cell volume, grant nerve and muscle cells their excitability properties, and are involved in the transport of certain nutrients such as sugars and amino acids. 10. Cell communication Communication between distant cells occurs indirectly through chemical messengers transported in the blood. Cells in close contact can communicate directly by exchanging materials through various types of junctions. Plant cells communicate through plasmodesmata, channels that traverse cell walls and connect the cytoplasm of adjacent cells. Animal cells have different types of junctions. Cells forming a tissue are never in direct contact; there is space between them, but they are connected at regions known as intercellular junctions. - Desmosomes: these are anchoring junctions; they consist of a dense plaque connected to keratin fibres and proteins. These proteins extend from the dense plaque of one cell, across the cell membrane, to the dense plaque of the second cell. This structure ensures the continuity of the tissue and provides it with significant resistance. - Tight junctions: their function is to seal the spaces between cells, preventing the passage of materials. They are formed by rows of membrane proteins that enable the adhesion of two adjacent cells, forming a sort of belt. - Gap junctions: characterised by the presence of protein complexes containing channels, inserted into the membranes of adjacent cells. These channel-proteins protrude into the interstitial space and connect, forming channels that allow the transit of ions and small molecules from one cell to another. 11. Cellular metabolism Cellular metabolism is the set of reactions that transform matter and energy occurring within the cell. The set of reactions that break down complex molecules into simpler substances is called catabolism; the set of reactions that synthesise cellular components from simple compounds is called anabolism. Catabolic reactions, which are exergonic, release the energy necessary for the cell to maintain its structures and level of organisation. Anabolic reactions, which are endergonic, require energy, provided by catabolic reactions through an intermediate carrier: ATP.  - The energy carrier: ATP Adenosine triphosphate (ATP) is formed from adenosine, a nucleoside, linked to three phosphate groups. The bond between the first and second phosphate and that between the second and third are high-energy bonds: forming them requires a lot of energy; conversely, their breakdown releases energy. ATP is synthesised through a condensation reaction from adenosine diphosphate (ADP) and an inorganic phosphate group. The synthesis reaction is endergonic. When ATP is hydrolyzed, the same amount of energy needed for its synthesis is released. - Metabolic reactions and redox reactions Cellular reactions involving the transfer of electrons from one substance to another are called redox reactions. The element undergoing oxidation loses electrons, while the element undergoing reduction gains electrons. The two reactions always occur simultaneously because the electrons donated by the oxidising element, known as the reducing agent, are accepted by the element that is reduced, called the oxidising agent. - Enzymes Metabolic reactions occur thanks to the intervention of proteins called enzymes. These are biological catalysts: they increase the rate of reactions without directly participating in them and without being consumed. The substances that react by binding to an enzyme are called substrates. The substrate binds to the enzyme at a specific site called the active site, forming the enzyme-substrate complex. Each enzyme can catalyse only one reaction. Many enzymes require specific temperature and pH conditions to function and the presence of certain cofactors: ions or small organic molecules called coenzymes. The names of enzymes usually end in -ase and refer to their function, for example: hydrolase (catalyses the hydrolysis reaction), polymerase (catalyses the polymerization reaction). 12. Catabolism of glucose The cell derives energy through the oxidation of organic substances. The primary source of energy for cells is the breakdown of glucose (C₆ H₁ ₂ O₆ ), a process that involves several phases and can proceed to complete oxidation, producing CO₂ , or stop at the level of intermediate compounds. The first phase of oxidation is glycolysis, a series of reactions through which glucose is broken down into pyruvate. Depending on the organism's capabilities and environmental conditions, pyruvate can follow two pathways: in the absence of O₂ , it is reduced via fermentation to lactic acid, ethanol, or other compounds, while in the presence of O₂ , it is oxidised to CO₂ during cellular respiration. - Glycolysis This includes 9 biochemical reactions occurring in the cytoplasm, each catalysed by a specific enzyme. During this metabolic pathway, one molecule of glucose is gradually transformed into two molecules of pyruvic acid (C₃ H₄ O₃ ), releasing energy. The energy released is utilised to produce 2 ATP and 2 NADH. Glycolysis equation: C₆ H₁ ₂ O₆ + 2 Pi + 2 ADP + 2 NAD⁺ → 2 C₃ H₄ O₃ + 2 ATP + 2 NADH + 2 H⁺ - Cellular respiration In the presence of oxygen, pyruvate is oxidised and fully degraded, producing CO₂ and H₂ O in cellular respiration, the second phase of glucose degradation. This process occurs in the mitochondria and is divided into three main phases: ○ Oxidative decarboxylation of pyruvate: pyruvic acid (3C) loses a molecule of CO₂ , transforming into an acetyl group (2C) that binds to coenzyme A (CoA), forming acetyl- coenzyme A, which enters the Krebs cycle, along with 1 NADH. ○ Krebs cycle: a cyclic series of reactions where acetyl-CoA binds to oxaloacetic acid (4C) to form citric acid (6C). Citric acid undergoes a series of oxidations leading to the formation of 2 CO₂ , 1 ATP, 3 NADH and 1 FADH₂. ○ Respiratory chain: the energy contained in NADH and FADH₂ is used to produce ATP. The oxidation of NADH produces 3 ATP, while that of FADH₂ produces 2 ATP. From the complete oxidation of one glucose molecule, 38 ATP molecules are obtained. Of these, 2 are produced by glycolysis, and the other 36 by respiration. Thus, the overall balance of glucose breakdown is: C₆ H₁ ₂ O₆ + 6 O₂ → 6 CO₂ + 6 H₂ O + energy (686 kcal/mol). - Alternative energy sources The cell can also obtain energy to produce ATP from molecules other than glucose: carbohydrates, lipids, and proteins. These substances are converted into glucose or other molecules that enter glycolysis or the Krebs cycle at different levels. Polysaccharides and disaccharides are transformed into monosaccharides, which are then converted into glucose. The glycerol from lipids is converted into glyceraldehyde-3-phosphate, an intermediate of glycolysis, while fatty acids from lipids are transformed into acetyl-CoA, which enters the Krebs cycle. Proteins are hydrolyzed into amino acids. These, after losing the amino group, can be converted into pyruvate, acetyl- CoA, or intermediates of the Krebs cycle.  - Fermentation In the absence of oxygen, cells resort to fermentation, a process in which the pyruvate produced from glycolysis is reduced by NADH and converted into different substances depending on the type of fermentation: ○ Alcoholic fermentation: pyruvate is transformed into ethanol and CO₂. ○ Lactic fermentation: it is transformed into lactic acid. This is carried out by certain milk bacteria that convert lactose into lactic acid. This fermentation also occurs in muscles when the oxygen supply is insufficient to produce enough ATP via respiration. Fermentation does not yield any additional ATP compared to glycolysis, but it allows for the reoxidation of NADH, thus restoring the cellular supply of NAD⁺ necessary for glycolysis to occur. 13. Regulation of metabolism The activities that regulate metabolism are also coordinated by the presence of allosteric enzymes, which, in addition to recognizing their substrate, can bind different molecules at specific sites, distinct from the active site, that act as activators or inhibitors. 14. Photosynthesis This is a process performed by plants and some prokaryotes that allows the capture of solar energy and its conversion into chemical energy in the form of glucose. Light energy is utilised for carbon fixation, meaning the transformation of CO₂ into glucose. To perform photosynthesis, plants absorb CO₂ and H₂ O from the environment, producing glucose and releasing O₂ into the atmosphere. Global equation: 6CO₂ + 6H₂ O + energy → C₆ H₁ ₂ O₆ + 6O₂. This is the exact opposite of the equation for the oxidative breakdown of glucose.In eukaryotes, photosynthesis occurs in chloroplasts, where light energy is captured by pigments: chlorophyll and carotenoids. The pigments and other molecules necessary for capturing light energy are embedded in the thylakoid membranes of the chloroplast. The photosynthetic process occurs in two phases: - Light phase: takes place in the thylakoids and requires light, which, captured by chlorophyll, is transformed into chemical energy in the form of ATP and NADPH. - Dark phase: involves a cyclic series of reactions, known as the Calvin cycle, which occurs in the stroma, independently of light. During these reactions, the energy from ATP and the reducing power of NADPH are used to reduce CO₂ and produce glucose. To fix one molecule of CO₂ , 3 ATP and 2 NADPH are required, so to produce one molecule of glucose, formed by fixing 6 CO₂ , 18 ATP and 12 NADPH are needed. 15. Autotrophic and heterotrophic organisms Autotrophic organisms can produce organic substances (sugars) from simple inorganic substances (CO₂ and H₂ O) obtained from the environment. They are divided into: - Photoautotrophs (photosynthetic): use solar light as an energy source for organic production. Plants and algae utilise water as a reducing compound, while other bacteria use H₂ S. - Chemoautotrophs (chemosynthetic): utilise energy released from redox reactions. Heterotrophic organisms are unable to autonomously synthesise organic molecules from simple inorganic molecules and must therefore obtain them from the environment by feeding on autotrophs, other heterotrophs, organic substances from living organisms, or waste products. Cell division and chromosomes Cell division is the process that allows a cell to give rise to two daughter cells. All cells originate from the division of other cells. Each cell contains a large amount of information in the form of genes, segments of DNA. Before each cell division, DNA duplicates, and each of the two cells arising from the division receives an identical set of information to that of the parent cell. The cell division process must ensure the equitable distribution of DNA, cytoplasm, and cellular organelles. Cell division differs in prokaryotes and eukaryotes. In prokaryotes, equipped with a single DNA molecule, it is a very simple process; in eukaryotes, provided with multiple DNA molecules in the form of chromosomes, it is a complex process that ensures the fair distribution of genetic material (mitosis) and cytoplasmic material (cytokinesis). In addition to mitosis, which allows for the growth and renewal of tissues, in all eukaryotes that reproduce sexually, there exists a particular cell line, that of germ cells, which through a different type of cell division, meiosis, forms reproductive cells. 1. Cell division in prokaryotes Prokaryotic cells contain a single circular DNA molecule, free in the cytoplasm. This DNA molecule is associated with the plasma membrane at the mesosoma. Prokaryotes divide by simple binary fission. At the beginning of the process, the circular DNA molecule, attached to the plasma membrane, duplicates while the cell grows. Subsequently, from the mesosoma, a transverse septum forms, dividing the mother cell into two daughter cells, each equipped with a DNA molecule identical to that of the mother cell. This mode of asexual reproduction is simple, quick, and allows bacteria to undergo a succession of generations at an extremely rapid rate. 2. Cell cycle of eukaryotes The life cycle of a eukaryotic cell, known as the cell cycle, is divided into four phases: G1, S, G2, and M. The first three phases constitute interphase, the period before cell division, while in the fourth phase, mitosis, cell division occurs. The four phases are: - G1 phase: this is a period of intense biosynthetic activity and growth. During this time, the cell doubles its size and produces new organelles, as well as the enzymes necessary for DNA replication that will occur in the next phase. - S phase (synthesis): this phase involves DNA replication, which is necessary for each daughter cell to receive a complete copy of the genome during cell division. - G2 phase: the cell continues to grow and form new organelles in preparation for the upcoming division. The cell prepares for mitosis. The duration of the cell cycle varies from cell to cell. Cells that do not divide remain in G1, which in this case is referred to as G0 and can be temporary or permanent. The latter case is represented by a cell that has reached the end of its development and no longer divides, such as neurons. Programmed cell death, or apoptosis, is a genetically programmed process of cellular self-destruction that helps control the number of cells in a tissue. To trigger the changes that lead to the S phase and those that initiate mitosis, the role of cyclin-dependent kinases (Cdks) is crucial. These are enzymes that catalyse the transfer of a phosphate group from an ATP molecule to a protein, which then becomes phosphorylated. Phosphorylation causes a conformational change in the protein, which can thus be activated or inactivated. Activated enzymes catalyse specific reactions, allowing the initiation of the cellular processes characteristic of the S phase and mitosis. Cdks are not always active; they are activated by molecules called cyclins, produced by the cell and destined to degrade once their action is complete. The S phase and mitosis are activated by specific cyclins that differ in various organisms. Mitosis can also be induced by external factors, such as many hormones or growth factors.  3. Eukaryotic chromosome Eukaryotic DNA is always associated with various types of proteins, and the complex formed by DNA and proteins is called chromatin. A chromosome is a long DNA molecule associated with specific proteins. The most abundant proteins are histones, small positively charged proteins that bind to negatively charged DNA. The function of histones is to wrap and compact the long DNA strands so they can be contained within the nuclear space. DNA wraps around groups of 8 histones, forming nucleosomes, the basic units of chromatin. Between two nucleosomes, there is another histone molecule that helps bring the two neighbouring nucleosomes closer together, compacting the structure. In addition to histones, other proteins are associated with DNA that vary from one cell type to another. Human cells contain 46 chromosomes, 23 pairs, and the total length of DNA in each cell is about 2 metres, which condenses to 200 µm when the chromosomes reach their maximum degree of condensation. The number of chromosomes in the cells of an organism is the same for all cells except for reproductive cells, which, in sexually reproducing organisms, contain half the number of chromosomes. 4. Mitosis Mitosis is the process by which the nucleus of a eukaryotic cell divides, resulting in two daughter nuclei, each with a complete set of chromosomes. The division of the nucleus is generally followed, but not necessarily, by the division of the cytoplasm, referred to as cytokinesis. At the beginning of mitosis, the chromosomes, which during interphase are uncoiled and appear as a mass of undifferentiated chromatin, condense. Since the DNA was duplicated during the S phase, each chromosome of a cell entering mitosis consists of two identical DNA strands called sister chromatids. The chromatids are joined at a region called the centromere, while the ends of the chromosomes are referred to as telomeres. Although it is a continuous process, mitosis is divided into four phases: - Prophase: DNA spirals and condenses, beginning to take on the appearance of visible bodies under a light microscope. The nuclear membrane dissolves, and the nucleoli become less visible. At the same time, in the cytoplasm, the two centrosomes, each containing a pair of centrioles, begin to migrate toward opposite poles of the cell, forming the mitotic spindle, a structure made of microtubules that spans the cell and connects the two pairs. From the centrioles, a set of short microtubules radiates outwards, forming the aster. - Metaphase: the chromosomes, fully condensed, align on the equatorial plane of the cell, forming the metaphase plate after adhering to the spindle fibres through kinetochores, protein structures located at the centromere. - Anaphase: the centromeres split into two, and the sister chromatids of each chromosome separate, migrating toward opposite poles of the cell due to the shortening of the mitotic spindle fibres. - Telophase: the spindle gradually disappears. The chromatids, now becoming the new chromosomes of the two daughter cells, gradually uncoil, returning to the stretched form typical of interphase; the nuclear membrane reforms around them, and the nucleolus reappears. Cytokinesis, the division of the cytoplasm, occurs differently in animal and plant cells. In the former, a groove forms at the cell equator that deepens until it divides the mother cell into two equal parts. In plant cells, however, the cytoplasm is divided by a diaphragm, the cell plate, which originates at the centre of the cell from the fusion of vesicles derived from the Golgi apparatus and extends until it reaches the membrane.  5. Asexual reproduction Mitosis is the basis for mechanisms of asexual reproduction, also known as agametic or vegetative reproduction. In these organisms, offspring originate from a single individual, without the involvement and fusion of specialised reproductive cells. With asexual reproduction, barring the occurrence of spontaneous mutations, the offspring are genetically identical to the parent. This type of reproduction applies to both unicellular and multicellular organisms and can occur in various ways, including binary fission, budding, and sporulation. - Binary fission: this occurs when, after mitosis, the cell simply divides into two equal parts. This way, unicellular organisms, such as paramecia, reproduce. - Budding: in unicellular organisms, such as yeast, this involves mitosis followed by an unequal division of the cytoplasm. The new cell, which is smaller, is destined to grow subsequently. Budding also occurs in some multicellular organisms, such as sponges and jellyfish, where protrusions form on the parent organism and later detach, giving rise to new individuals identical to the originals. - Sporulation: the formation of special reproductive cells, spores, following mitosis. These spores have a thick wall that allows them to withstand adverse environmental conditions, and they can generate a new individual when conditions become favourable. 6. Sexual reproduction Sexual reproduction, also known as gametic or amphigenic reproduction, involves the participation of specialised reproductive cells called gametes (germ cells), generally produced by two individuals of different sexes. Each contributes its own chromosomes to form the genetic makeup of the offspring. Reproduction occurs when the male gamete fuses with the female gamete; the fusion of the two gametes, known as fertilisation, results in a cell called the zygote, which represents the first cell of the new organism, from which the adult individual will develop through a series of subsequent mitotic divisions. Since the fusion of gametes involves the union of chromosomal complements from both maternal and paternal origins, sexual reproduction leads to the mixing of different genetic material. This is why offspring are different from their parents and from each other. Unlike asexual reproduction, sexual reproduction allows for an increase in genetic variability. - Haploid and diploid cells Since the zygote originates from the fusion of two cells, these must have a halved genetic material. Gametes differ from somatic cells in terms of chromosome number: ○ Somatic cells are diploid, meaning they possess a double set of chromosomes: each chromosome is present in two copies that form a pair of homologous chromosomes, one from the father and one from the mother. These chromosomes have the same shape, size, and gene sequence. Notation: 2n. ○ Gametes are haploid, meaning they possess a single set of chromosomes: they contain one chromosome from each pair. Notation: n. The mechanism that allows for the reduction of chromosome number is meiosis, which occurs in the gonads. In animals, the female gonads are the ovaries, while the male gonads are the testes; in plants, the female gonads are the ovaries, and the male gonads are the stamens. Sexual reproduction occurs through the alternation of meiosis, which produces gametes, and fertilisation, during which the gametes unite to form a new individual. Parthenogenesis is a type of reproduction that requires the formation of gametes but not their fusion. In this case, the new individual originates from an unfertilized egg cell (female gamete), triggered by mechanical and physical stimuli, rather than from the entry of a sperm cell into the egg. This process is typical of several invertebrates, such as drone bees and some reptiles (lizards). - Meiosis In the gonads, specialised diploid cells called gametocytes undergo meiosis, resulting in haploid gametes. The process consists of two successive cell divisions that produce four haploid cells from a diploid cell. The two divisions are similar to two mitoses, but only the first is preceded by DNA replication. The four phases of Meiosis I are: - Prophase I: chromatin condenses, and chromosomes become distinct bodies; the spindle apparatus forms, and the nuclear membrane and nucleoli disappear. The two homologous chromosomes of each pair come together and pair up; each paired homologous pair consists of four chromatids and is called a tetrad or synapsis. Sometimes, a phenomenon known as crossing-over occurs between the homologous chromosomes. - Metaphase I: the tetrads align on the equatorial plane of the cell, and each pair of homologous chromosomes attaches to a spindle fibre. - Anaphase I: the two chromosomes of each pair separate and move toward opposite poles of the cell due to the shortening of the spindle fibres. At this point, each chromosome is still made up of two sister chromatids joined at the centromere.  - Telophase I: the starting cell divides into two daughter cells, each containing a haploid number of chromosomes. The two products of the first meiotic division are not identical to the original gametocyte because they are haploid and not identical to each other. Between the first and second meiotic divisions, there may be a brief resting period called interkinesis, during which the chromosomes partially uncoil, or Meiosis II may immediately follow Meiosis I. Each of the two daughter cells undergoes Meiosis II: - Prophase II: the centrioles migrate to opposite poles of the cell, and the spindle apparatus reforms. - Metaphase II: the chromosomes align on the equatorial plane of the cell. - Anaphase II: the sister chromatids of each chromosome separate and move toward opposite poles of the cell, becoming the new chromosomes of the daughter cells. - Telophase II: two nuclei form, and cytokinesis occurs, resulting in the formation of two daughter cells. The first meiotic division is reductive, meaning it is accompanied by a halving of the chromosome number; the second is equational, meaning there is no change in the chromosome number. - Crossing-over During Prophase I, the chromatids of two homologous chromosomes can undergo crossing-over, meaning they break at corresponding points and exchange segments. The exchange point is called a chiasma and is random. Crossing-over occurs between non-sister chromatids of a homologous pair. The chromatids involved in the exchange of corresponding segments are called recombinant, while those that do not recombine are referred to as parental. In summary, meiosis leads to: the production of gametes necessary for sexual reproduction; halving the number of chromosomes in gametes; reassortment between paternal and maternal chromosomes to produce new combinations. Crossing-over allows for the reshuffling of chromosome parts (recombination) and is a random event. 7. Chromosomal and genomic mutations Mutations are alterations in the genetic material that can affect a single gene, but they can also involve changes in the structure of a chromosome, chromosomal mutations. or a variation in the number of chromosomes, genomic mutations. Gene mutations are caused by errors in DNA replication, while chromosomal and genomic mutations arise from mistakes during meiosis. Chromosomal mutations result from the breaking of a chromosome. The resulting fragment may be lost (deletion), attach to the homologous chromosome (duplication), attach to a non-homologous chromosome (translocation), or reattach to the original chromosome after rotating 180° (inversion). Genomic mutations involve the loss or gain of one or more chromosomes or a variation in the entire chromosomal set, resulting in each chromosome being represented by more than two homologs (polyploidy). Aneuploidy is the loss or gain of one or a few chromosomes. This mutation is caused by nondisjunction, which is the failure of two homologous chromosomes to separate during Meiosis I or of two sister chromatids during Meiosis II. The result is abnormal gametes: out of the four gametes produced, two contain two copies of a chromosome and two are devoid of it. - Diseases caused by mutations Chromosomal and genomic mutations often lead to severe pathologies. Down syndrome (trisomy 21) is a case of aneuploidy: affected individuals have three copies of chromosome 21 and exhibit delays in both physical and mental development. This syndrome can also be caused by a translocation of a fragment of chromosome 21. Edwards syndrome (trisomy 18) is characterised by malformed ears, heart defects, spasticity, and other damages, usually leading to death within the first year of life.  8. Gametogenesis The process of gamete formation is called gametogenesis and occurs in the gonads. In animals, male gametes are sperm, while female gametes are egg cells. - Spermatogenesis In male gonads, spermatogonia (2n) differentiate into primary spermatocytes (2n), which undergo the first meiotic division, producing haploid cells of equal size, the secondary spermatocytes (n). These undergo the second meiotic division, producing haploid cells called spermatids, which will mature into sperm. - Oogenesis In female gonads, oogonia (2n) differentiate into primary oocytes (2n), which undergo the first meiotic division, producing two haploid cells of different sizes: a secondary oocyte and a small cell known as the polar body. The latter can undergo a second division, forming two new polar bodies, or it may degenerate and die. The secondary oocyte, if fertilisation occurs, undergoes the second meiotic division, producing an egg cell and another small polar body. Heredity Every living being possesses a genetic program, which is a set of instructions that specify its characteristics and direct its metabolic activities. This set of instructions constitutes biological information, which is hereditary and transferred from one generation to the next through reproduction. The characteristics passed on through biological information via reproduction are called hereditary traits. Heredity, the complex of methods for transmitting hereditary traits from the individuals of one generation to their descendants, is the subject of genetics. Biological information is organised into fundamental units called genes, each of which contributes to the determination of a trait and is inherited from the parents. The different forms of the same gene are called alleles. The combination of an individual’s alleles is called the genotype, while the phenotype is the set of characteristics manifested in an individual, determined by its genotype and the environment. The term genome refers to the complete set of information of an organism and the DNA that physically contains it. 1. Mendelian genetics The fundamental rules governing the transmission of hereditary traits were discovered by Mendel between 1854 and 1864. He used the following experimental procedure: he chose pea plants as experimental material, which are easy to cultivate and reproduce quickly; he focused on 7 pairs of unit traits, i.e., traits that appeared only in two alternative forms; he selected pure lines, i.e., plants that, through self-fertilisation, always produced plants with the same trait; he crossed pure lines that differed by only one trait (monohybrid cross) or by two traits (dihybrid cross); and he conducted a numerical analysis of the results.  - Monohybrid cross He crossed pure lines that differed by a single trait, called the parental generation (P). The individuals of the offspring, the first filial generation (F1), all had the same phenotype, identical to that of one of the two parents, while the other parental phenotype seemed to disappear. He defined the trait that manifested in F 1 as dominant and the one that did not manifest as recessive. When he subjected the F1 individuals to self-fertilisation, the offspring, called the second filial generation (F2), consisted of both plants carrying one trait and plants carrying the other parental trait. The two traits manifested according to a precise numerical ratio: - ¾ exhibited the dominant trait (yellow seed) - ¼ exhibited the recessive trait (green seed) Mendel’s first law, known as the law of dominance, states that when crossing two pure lines differing in a hereditary trait, all offspring (F1) are equal to each other and show the trait of one of the two parents, the dominant one. - Mendel’s hypotheses To explain the results obtained, Mendel put forth several hypotheses: each trait is determined by a gene, transmitted from the parents to the descendants through gametes during reproduction; genes exist in alternative forms (alleles); an organism has two alleles of each gene for each hereditary trait, with each allele derived from one of the two parents; the two alleles of a gene separate during meiosis, so that gametes contain only one allele for each trait. Mendel’s second law, the law of segregation, states that every individual has two copies of each factor, and they separate (segregate) during the formation of gametes. Individuals with two identical alleles for a given trait are called homozygous, while those with two different alleles are called heterozygous. The dominant allele is expressed phenotypically in both the homozygous dominant individual and the heterozygous; the recessive allele is only expressed in the homozygous recessive. Two alleles of the same gene are indicated by the same letter: uppercase for the dominant allele (Y), lowercase for the recessive allele (y). Therefore, three different genotypes are possible, corresponding to two phenotypes: - Homozygous dominant genotype (YY) → dominant phenotype (yellow seed) - Heterozygous genotype (Yy) → dominant phenotype (yellow seed) - Homozygous recessive genotype (yy) → recessive phenotype (green seed) The possible genotypic combinations of the individuals of F2, obtained from the self-fertilisation of the individuals of F1, can be predicted by constructing a Punnett square. - Dihybrid cross Mendel also conducted a cross between pure line plants that differed in two traits: smooth/wrinkled seeds and yellow/green seeds. The plants with smooth yellow seeds have a homozygous dominant genotype (RRYY), while those with wrinkled green seeds have a homozygous recessive genotype (rryy). In this case, the plants of the F1 generation displayed both traits controlled by the dominant alleles, namely smooth and yellow seeds. These are plants with a heterozygous genotype (RrYy) and the same phenotype. By crossing the F1 plants, the recessive alleles reappeared, but in addition to the parental phenotypes, recombinant phenotypes also appeared, namely yellow wrinkled seeds and green smooth seeds. This occurs because hereditary factors distribute independently to different gametes; thus, 4 possible allelic combinations can be obtained. Mendel’s third law, known as the law of independent assortment, states that when crossing pure line individuals differing in two traits, in the F2 generation these traits assort independently of each other during gamete formation, combining according to the laws of chance. The law of independent assortment reflects how chromosomes are distributed to daughter cells during meiosis.  - Test cross An individual with a recessive phenotype must have a homozygous recessive genotype. However, if an individual has a dominant phenotype, its genotype could be either homozygous or heterozygous. The genotype of an individual with a dominant phenotype can be determined by performing a test cross, also known as back-cross. This test involves crossing the individual with an unknown genotype with one that has a recessive phenotype, whose genotype is certainly homozygous recessive. If the dominant phenotype individual is homozygous, the F1 offspring will all be heterozygous and thus have a dominant phenotype. If the dominant phenotype individual is heterozygous, the F1 offspring will consist of half dominant phenotype individuals and half recessive phenotype individuals with a homozygous recessive genotype. 2. Incomplete dominance, codominance and other complex hereditary phenomena The transmission of hereditary traits often occurs in a more complex manner than what is predicted by Mendel's rules. Below are some examples that do not follow Mendel’s laws. - Incomplete dominance This phenomenon occurs when, given two alleles of a gene, neither dominates over the other. In this case, Mendel's first law is not respected because the individuals of the F1 generation (heterozygotes) have a different, often intermediate, phenotype compared to either parent. For example, when crossing red-flowered lion's mouth plants with white- flowered plants, the F1 individuals have pink flowers. When the F1 individuals are crossed with each other, the F2 generation will show three phenotypic classes in a ratio of 1:2:1, instead of two in a ratio of 3:1. In the case of lion's mouth plants, ¼ will have red flowers, 2/4 will have pink flowers, and ¼ will have white flowers. - Codominance This occurs when, in a heterozygote, both alleles of a gene are expressed. This is seen, for example, in the case of antigens that distinguish blood groups in the ABO system.  - Multiple alleles The traits chosen by Mendel presented only two allelic forms. However, many genes have more than two allelic forms, which is referred to as multiple allelism. For example, the gene responsible for human blood types in the ABO system exists with three different alleles (I A, IB, I0), the combinations of which determine blood groups A, B, AB, and O. - Pleiotropy This phenomenon occurs when a single gene determines multiple phenotypic effects. Genes that control hormone production often exhibit this behaviour because each hormone generally exerts multiple effects on the organism. - Epistasis This phenomenon involves interaction between different genes, where the action of one gene interferes with the expression of other genes. For example, congenital deafness in humans is a condition controlled by the recessive alleles of two genes, which manifests when one of them is present in the homozygous form. - Linked genes Mendel's third law holds true only if the two genes are located on different chromosomes; genes that are on the same chromosome, known as linked genes, are usually inherited together. This means that a heterozygote for two linked genes A and B (genotype AaBb) does not produce 4 types of gametes in equal quantities but only 2, for example, AB and ab. The alleles of two linked genes can only be separated if a crossing-over occurs between them. The degree of linkage depends on the physical distance between the two genes: the greater the distance, the higher the probability that a crossing-over event will separate them, and consequently, their recombination frequency. - Quantitative traits Many traits manifest with a wide variety of phenotypes that cannot be accurately classified into a few distinct classes. These traits exhibit continuous variability and are referred to as quantitative traits. In humans, for example, height and foot length are quantitative traits. The continuous variability of these traits is due to the fact that they are polygenic traits, meaning they are controlled by many genes that act cumulatively. The phenotype, in this case, is the result of the sum of the effects of the individual genes. - Genotype and phenotype Genetic information (genotype) does not deterministically define an organism's characteristics. It can be said that what is manifested in the organism (phenotype) results from the interaction between genotype and environmental conditions. For example, height and body structure depend only partially on genetic information; they are largely influenced by environmental variables such as nutrition and physical activity. 3. Chromosomal theory of inheritance In the early 20th century, after advances in microscopy allowed observation of chromosome behaviour during mitosis and meiosis, Sutton and Boveri highlighted the analogy between the factors hypothesised by Mendel and the homologous chromosomes observed during meiosis. The hypothesis that the two copies of each factor (the two alleles of a gene) separate during gamete formation was supported by the observation that the two chromosomes of a homologous pair separate during anaphase of meiosis I. Based on this analogy, the two researchers formulated the chromosomal theory of inheritance (1903). According to this theory, genes are material particles located on chromosomes. The position of a gene on a chromosome is referred to as a gene locus. Chromosomes are the sites of genes, but they are not genes themselves. Each chromosome indeed contains many genes. The principles of Mendelian genetics reflect the linear arrangement of genes on chromosomes. - Human chromosomes In diploid cells, chromosomes are arranged in pairs; of these, one differs in males and females. This pair of chromosomes, known as sex chromosomes (heterosomes), consists of two chromosomes that are typically identical in females and different in males. All other chromosomes in an individual are called autosomes. Human somatic cells (diploid) contain 46 chromosomes, 23 pairs: 22 pairs of autosomes and 1 pair of sex chromosomes. Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY). Gametes, being haploid, contain only 23 chromosomes: 22 autosomes and 1 sex chromosome. Egg cells contain one X chromosome, while sperm cells contain either an X or a Y chromosome. The karyotype is the complete set of chromosomes of a species or an individual, arranged according to length and centromere position. In the karyotype of diploid cells, chromosomes are arranged in homologous pairs. To analyse the karyotype, chromosomes must be isolated during metaphase when they are most clearly visible. They are then stained, and their image is analysed by an electronic processor that identifies homologous pairs based on size and banding patterns. These pairs are organised according to a specific numbering system, making it possible to identify anomalies in number and shape. During pregnancy, it is possible to study the karyotype of the foetus by obtaining cells from chorionic villi (chorionic villus sampling) or foetal cells present in the amniotic fluid (amniocentesis). - Sex determination in humans An individual's sex depends on which of the two sex chromosomes is contained in the sperm at the time of fertilisation. The probability is 50% for females and 50% for males. In all cells of female mammals, starting from the 16th day of fertilisation, one of the two X chromosomes is inactivated and is visible during interphase as a structure called the Barr body. ○ Anomalies in the number of sex chromosomes In humans, there are known syndromes caused by the presence of sex chromosomes in abnormal numbers. Turner syndrome: affects individuals with a single X chromosome (karyotype 45, X0). They are female but sterile. These individuals are characterised by short stature and sometimes exhibit intellectual disabilities. Klinefelter syndrome: affects individuals with genotype XXY. These are sterile males who have particularly small testes and may exhibit mild intellectual disabilities. Jacobs syndrome: caused by an extra Y chromosome (karyotype 47, XYY). Some of these individuals are characterised by above-average height, mild intellectual disabilities, and in some cases, vascular problems. These syndromes do not only affect sexual characteristics but also have repercussions on the development of the nervous system. - Experimental verification of the chromosomal theory The traits determined by genes located on sex chromosomes are called sex-linked traits. The study of their transmission allowed Morgan to experimentally demonstrate the validity of the chromosomal theory. He identified a trait in the fruit fly, eye colour, that was transmitted differently depending on the sex. Normally, the eye is dark red (dominant), but due to a mutation, it can also be white (recessive). Morgan crossed white-eyed females (XX) with red-eyed males (XY), observing that the trait was not transmitted according to Mendelian rules. In the F1 generation, all females were red-eyed (XX) and all males were white-eyed (XY). In the F2 generation, however, there was 25% of each phenotypic class (XX, XX, XY, XY). Morgan also performed the cross between white-eyed males (XY) and red-eyed females (XX), obtaining results that were still inconsistent with Mendel’s laws. All individuals in the F1 generation had red eyes (XX, XY), while in the F2 generation, all females had red eyes (XX, XX), 50% of males had red eyes (XY), and the other 50% had white eyes (XY). [X = mutated chromosome, white eye allele; X = normal chromosome, red eye allele]. The only plausible explanation for these results is that the eye colour trait is determined by a gene located on the X chromosome. - Sex-linked inheritance When discussing sex-linked inheritance, we refer to the transmission of all traits controlled by genes located on the sex chromosomes. The inheritance of these traits follows particular rules since it depends on the sex of the offspring being considered. In humans, traits linked to the Y chromosome can only be inherited paternally; they are present only in males and are never transmitted to female offspring; they are always transmitted to all male offspring; and they always manifest phenotypically. Traits linked to the X chromosome, on the other hand, can be inherited both paternally (transmitted to all female offspring and none of the male offspring) and maternally (from a heterozygous mother; transmitted to 50% of offspring regardless of their sex); they are present in both males and females; in males, they always manifest phenotypically since a male possesses only one allele of a gene located on the X chromosome; in females, to evaluate the resulting phenotype, it is necessary to consider whether the allele in question is dominant or recessive compared to the allele carried by the other X chromosome. 4. Genetic diseases - Sex-linked diseases This term refers to diseases caused by genes located on sex chromosomes. - Haemophilia: abnormalities in the blood coagulation mechanism that cause prolonged bleeding from wounds. There are two main forms of haemophilia (A and B) caused by recessive alleles of two genes located on the X chromosome that control two different stages of the blood coagulation mechanism. Since the allele responsible for the disease is recessive, in females, haemophilia manifests in the homozygous recessive condition (very rare), while in males, whose X chromosome carries that allele, it always manifests. - Colour Blindness (Daltonism): caused by a change in the light-sensitive structures of the retina. The main form of the disease involves the inability to distinguish red from green. Colour blindness is also linked to a gene located on the X chromosome, so it mainly manifests in males. Responsible allele: recessive. Most sex-linked diseases are controlled by genes located on the X chromosome; this is also due to the fact that the Y chromosome is much smaller and contains fewer genes. - Autosomal diseases These are diseases caused by genes located on autosomes. Those caused by a dominant gene manifest in both homozygous dominant and heterozygous individuals; those caused by a recessive gene manifest only in homozygous recessive individuals, while heterozygous individuals are healthy carriers. ○ Autosomal dominant diseases - Achondroplasia: a dominant genetic disorder affecting the osteoarticular system, causing early ossification of growth cartilages. The bones remain short, resulting in a typical dwarfism body structure. The mechanism responsible for this dysfunction is unknown, and there are no known cures. - Huntington's disease: a hereditary dominant disorder that manifests between the ages of 35-50. It involves the progressive degeneration of the basal ganglia, brain centres involved in coordinating movements. Characteristic symptoms include jerky movements of the limbs and face; the disease progresses to a loss of the ability to walk. - Brachydactyly: a set of malformations of the fingers of hands and feet transmitted by a dominant allele. ○ Autosomal recessive diseases - Albinism: a recessive genetic disorder characterised by the inability to synthesise melanin, resulting in skin, hair, and iris that lack pigment. - Alkaptonuria: a recessive genetic disorder affecting the metabolism of the amino acid tyrosine; symptoms include arthritis of the spine or large joints and the production of dark urine. - Phenylketonuria (PKU): a hereditary recessive disorder characterised by the absence of the enzyme phenylalanine hydroxylase. In this condition, the amino acid phenylalanine accumulates in the bloodstream because it is not used for protein synthesis, leading to intellectual disabilities over time. To prevent these severe consequences, the disease must be diagnosed very early, and the child should follow a low-phenylalanine diet to avoid accumulation. - Galactosemia: a hereditary recessive disorder characterised by the absence of an enzyme necessary for converting galactose into glucose. Consequently, galactose accumulates in the liver and red blood cells, resulting in severe malnutrition, liver enlargement, and intellectual disabilities. Prevention relies on early diagnosis and a galactose-restricted diet. - Cystic fibrosis: an autosomal recessive genetic disorder caused by a mutation in a gene that encodes a protein involved in the transport of chloride and sodium (and thus water) across cell membranes. This anomaly results in the secretion of thick and viscous mucus in the exocrine glands, causing obstruction of glandular ducts. Typical clinical manifestations include recurrent lung infections, liver insufficiency, and liver cirrhosis. - Pedigree analysis The pedigree (family tree) is a schematic representation of how a disease appears in a family. From this analysis, it is possible to determine whether a specific disease is linked to sex chromosomes or is autosomal, whether it is a dominant or recessive trait; it is also possible to reconstruct the genotypes of the individuals considered to establish which are healthy carriers and the probabilities for a certain individual of having affected offspring. To analyse a pedigree, it is important to keep in mind some rules: - an autosomal trait manifests with equal frequency in both sexes, while a trait carried by sex chromosomes appears with different frequencies in males and females. - an autosomal dominant trait manifests in all generations, and every affected individual has an affected parent; a child produced by a cross between a healthy individual and a heterozygous affected individual has a 50% chance of being affected. - an autosomal recessive trait does not manifest in all generations; affected individuals generally have healthy heterozygous parents (healthy carriers). The pedigree is created using specific symbols for males and females, whether healthy or affected.  5. Human blood groups The distinction between different blood groups is based on the presence of antigens on the cell membrane of red blood cells and the presence of corresponding antibodies in the blood plasma. Antibodies are circulating proteins produced by certain white blood cells (lymphocytes) that can specifically bind foreign molecules (antigens), neutralising them. Antigens are molecules, typically proteins, that can induce the production of antibodies in a foreign immune system. The most important human blood groups are those of the ABO system and the Rh system. - ABO system The presence or absence of antigens A and B on the surface of red blood cells and the corresponding presence or absence of antibodies against these antigens in plasma define the blood groups 0, A, B, and AB.In blood transfusions, incompatibility between blood groups occurs because the anti-A and anti-B antibodies produced by an individual from one group can recognize the antigens of an individual from another group, leading to agglutination and lysis of the donated red blood cells. An AB individual is a universal acceptor because they have both antigen A and antigen B on their red blood cells and do not produce any antibodies against these antigens. A group 0 individual is a universal donor because they lack any antigens on their red blood cells. However, since they have antibodies against both antigens A and B, they can only receive blood from other group 0 individuals. ○ Genetic determination of the ABO system The presence of antigens A and B on human red blood cells is determined by a system of three alleles (IA, IB, I0): - The allele IA encodes for antigen A. - The allele IB encodes for antigen B. - The allele I0 does not encode for any antigen. The alleles IA and IB are dominant over I0 and codominant with each other. Since IA and IB show codominance, in heterozygotes (IAIB), both alleles are expressed, encoding for their respective antigen. Therefore, blood group AB is determined by the genotype IAIB. - Rh system In this system, two groups are distinguished, Rh+ and Rh-, based on the presence or absence of the Rh antigen on red blood cells. The group is determined by the combination of two alleles of the same gene: D (dominant) and d (recessive). Therefore, an individual with genotype DD or Dd possesses the Rh antigen and does not produce anti-Rh antibodies; an individual with genotype dd does not possess the Rh antigen and produces anti- Rh antibodies. Risk cases include Rh- mothers, where the presence of a potential Rh+ foetus causes the production of anti-Rh antibodies that can enter the foetal bloodstream through the placenta, potentially leading to hemolysis (destruction of red blood cells). This phenomenon, called foetal erythroblastosis, is usually not significant in the first pregnancy, as the antibody production is limited; however, it can cause severe damage to the foetus in a subsequent pregnancy because the antibody production is much more extensive. Molecular genetics Molecular genetics investigates the chemical mechanisms that allow for the expression of genetic information in an individual and the transmission of hereditary traits from one individual to their descendants. The basic unit of heredity is represented by the gene, which in this context can be defined as the segment of DNA responsible for determining a specific trait.  1. DNA In the early 1940s, it was known that genes determine hereditary traits and that they are located on chromosomes, but their exact chemical nature was not known. It was understood that chromosomes are made up of DNA and proteins, so the bearer of biological information had to be one of these two molecules. 1944: Avery, MacLeod, and McCarty demonstrated that the bearer of genetic information was DNA. 1949: Chargaff identified the four nitrogenous bases and discovered that the amount of adenine equals the amount of thymine (A=T / C=G). 1952: Hershey and Chase, using bacteriophages, demonstrated that DNA is involved in viral replication. 1952: Franklin produced an X-ray diffraction image of DNA. 1953: Watson and Crick identified the three-dimensional structure of DNA and proposed the double helix model. 1958: Meselson and Stahl demonstrated that DNA replication is semi-conservative. 1962: Watson, Crick, and Wilkins received the Nobel Prize in Medicine for their discoveries regarding nucleic acids. 1969: Hershey received the Nobel Prize for discovering the mechanism of virus replication. - Nucleotides A nucleotide consists of a nitrogenous heterocyclic base attached to a pentose sugar (ribose in RNA / deoxyribose in DNA), which is in turn linked to a phosphate acid. The nitrogenous bases are divided into two groups: pyrimidines (cytosine, thymine, and uracil) and purines (adenine and guanine). - Structure of DNA DNA is a nucleic acid and thus a linear polymer of nucleotides, which can contain four different nitrogenous bases: adenine, guanine, cytosine, and thymine. According to the double helix model proposed by Watson and Crick, the DNA molecule consists of two polynucleotide strands coiled around a central axis. Each strand is made up of a backbone of alternating sugar and phosphate molecules. Each sugar molecule is linked to a nitrogenous base, so the bases protrude laterally from the polynucleotide strands. The two strands are held together by hydrogen bonds between the nitrogenous bases. The base pairing is not random: the distance between the two strands in the double helix is constant, so pairing occurs necessarily between a purine (which consists of two rings) and a pyrimidine (which consists of one ring). The paired bases are referred to as complementary: A- T pairs through 2 hydrogen bonds; G-C pairs through 3 hydrogen bonds. The structure of DNA can thus be compared to a spiral staircase. The distance between a pair of bases and the next is 3.4 Å, and each complete turn of the helix comprises 10 base pairs. Each strand has a 5’ end and a 3’ end. Because in the double helix the 3’ end of one strand faces the 5’ end of the complementary strand, the two strands are said to be antiparallel. - DNA replication For genetic material, namely DNA, to be transmitted to descendants, it must be able to duplicate itself. The process of DNA duplication occurs before a cell divides and is called replication. During replication, the two strands of the double helix separate due to the breaking of hydrogen bonds between the paired bases. Each strand can thus serve as a template for synthesising a new complementary strand, using the free deoxynucleotides available in the cell. Experiments by Meselson and Stahl demonstrated that DNA replication is semiconservative, meaning that each of the two daughter DNA molecules consists of one strand of parental DNA (conserved) and one newly synthesised strand. The entire process requires energy and many enzymes. An enzyme, DNA helicase, and specific proteins are needed to unwind the double helix at the origin of replication, known as the replication fork. The synthesis of the new strand is catalysed by a group of enzymes known as DNA polymerases. In prokaryotes, there is a single origin of replication, and the process occurs in the cytoplasm. In eukaryotes, replication takes place in the nucleus, with multiple origins of replication on each chromosome. DNA polymerase cannot directly synthesise a new DNA strand on the parental strand but requires a primer, which is a short double-stranded segment from which it can start synthesising. This is facilitated by the synthesis of a short RNA strand. Replication always proceeds in the 5’ → 3’ direction, which has an important consequence: one of the two strands, called the leading strand, can be synthesised continuously using a single primer, while the other strand, called the lagging strand, must be synthesised in the opposite direction in small discontinuous fragments, known as Okazaki fragments, which are subsequently joined together. In addition to adding new nucleotides to the growing chain (polymerase function), DNA polymerase can detect the incorporation of an incorrect nucleotide into the growing strand. If an error occurs, the enzyme reverses its direction, removing nucleotides one by one until it reaches the point of the incorrect nucleotide (exonuclease function). Other enzymes, called DNA repair nucleases, also have the task of removing any remaining errors after replication: they scan along the double helix, identifying incorrect nucleotides and replacing them with the correct ones. - The one gene – one enzyme hypothesis Once it was established that DNA contains all the information necessary to define the development and physiology of a cell, it remained to clarify how this molecule performs that function. Studies were conducted on microorganisms that were biochemically altered due to mutations. The term mutation refers to a change in the genetic information of an organism, and organisms that exhibit a mutation are called mutants. By studying the mutants of bread mould, Beadle and Tatum demonstrated that for each mutation there is a corresponding enzyme that functions abnormally. Based on their experiments, they formulated the "one gene – one enzyme" hypothesis in 1941, which states that a specific gene is responsible for the synthesis of a specific enzyme, meaning it codes for that enzyme. This expression was later modified to "one gene – one protein," as not all proteins, whose synthesis is controlled by DNA, are enzymes. As further experiments showed that proteins are often made up of two or more subunits, polypeptide chains, the original formulation was corrected again to "one gene – one polypeptide chain." 2. From genes to proteins: the role of RNA Once the relationship between genes and the corresponding proteins produced by the cell was identified, it was necessary to clarify how a gene, located in the nucleus and formed by a sequence of only four types of nucleotides, could give rise to a polypeptide chain synthesised in the cytoplasm and composed of a sequence of 20 different amino acids. The transition from genes to proteins is facilitated by RNA, which is also made up of a linear sequence of nucleotides. The information contained in a gene is copied (transcribed) into RNA in the nucleus, in a process called transcription. The RNA then moves from the nucleus to the cytoplasm, where the message it carries is used to synthesise a protein (translation). Thus, the flow of biological information always goes from DNA to RNA to proteins. This sequence has been defined as the central dogma of molecular biology. This dogma allows for one exception: the enzyme reverse transcriptase, found in some RNA viruses, allows for the synthesis of a DNA strand from an RNA molecule. - Structure and function of RNA RNA differs from DNA in several characteristics: the pentose sugar is ribose, not deoxyribose; it consists of a single strand rather than a double helix; it always contains four nitrogenous bases, but instead of thymine, it contains uracil. There are three different types of RNA: messenger RNA (mRNA) carries genetic information from DNA to the cytoplasm, where proteins are synthesised; ribosomal RNA (rRNA) is a structural component of ribosomes; and transfer RNA (tRNA) transports free amino acids in the cytoplasm to the ribosomes during protein synthesis and is used to translate the information contained in the nucleotide sequence of mRNA into a sequence of amino acids.  - Transcription Transcription is the process by which the information contained in a gene is copied into a molecule of mRNA. The synthesis of mRNA is catalysed by a group of enzymes, the most important of which is RNA polymerase. At the binding site of the enzyme, the two strands of the DNA segment corresponding to a gene open up, and one of them serves as a template for the synthesis of a complementary mRNA molecule. RNA polymerase moves along the DNA template strand, progressively adding new ribonucleotides to the 3’ end of the growing mRNA strand; thus, transcription occurs in the 5’ → 3’ direction. To initiate synthesis, RNA polymerase binds to a specific sequence on the DNA called the promoter. Another specific sequence, known as the termination signal, indicates the stopping point of transcription. During transcription, only one of the two DNA strands is copied. As in replication, the newly synthesised RNA strand is complementary, not identical, to the DNA template strand from which it was copied. In prokaryotes, transcription occurs in the cytoplasm, and the produced mRNA can be used immediately for protein synthesis. In eukaryotes, however, transcription occurs in the nucleus, and the mRNA must be modified before migrating to the cytoplasm. Almost all eukaryotic genes are discontinuous, formed by alternating coding sequences called exons and non-coding sequences called introns. The DNA of a discontinuous gene is fully transcribed, copying both exons and introns to form an immature mRNA. Before the mRNA exits the nucleus, introns are removed, and exons are spliced together to form mature mRNA. This process is known as splicing. 3. The genetic code The translation of the message contained in mRNA (written in a language based on 4 nucleotides) into a protein (written in a language based on 20 amino acids) is facilitated by a correspondence system known as the genetic code. If the correspondence were 1:1, the 4 nucleotides could specify only 4 amino acids. If one amino acid corresponded to a sequence of 2 nucleotides, there would be 16 combinations, still insufficient to specify all necessary amino acids. Instead, the code is based on triplets of nucleotides, called codons, which allow for 64 combinations. The genetic code has several characteristics: it contains a start signal represented by the codon AUG; it has stop signals represented by three stop codons; it is unambiguous, meaning a given codon always specifies a single amino acid; it is redundant, as almost all amino acids are specified by more than one codon; and it is universal, being valid for all organisms. 4. Translation / protein synthesis Protein synthesis (translation) is the final stage of gene expression. It occurs in the cytoplasm and takes place on ribosomes. All three types of RNA participate in the process: mRNA, which carries the message, rRNA, which is an integral part of the ribosome, and tRNA, which translates the language of nucleic acids into that of proteins. This last molecule can bind amino acids on one side and recognize the codons of mRNA on the other side via a triplet of nucleotides, known as the anticodon, which is complementary to a specific codon on the mRNA. It has a cloverleaf structure, and each cell contains at least one type of tRNA for each amino acid. Ribosomes have 3 binding sites: one for mRNA (small subunit) and two for tRNA: the P site (peptidyl site) and the A site (aminoacyl site). The synthesis of a protein occurs in three phases: initiation, elongation and termination. Initiation: in this phase, the 5' end of mRNA binds to the small ribosomal subunit. The large subunit then associates with this complex, along with the first tRNA bound to its specific amino acid, which pairs with its anticodon to the start codon and occupies the P site. Elongation: this begins with the insertion of an aminoacyl-tRNA into the A site with an anticodon complementary to the second codon of the mRNA. At this point, a peptide bond forms between the first two amino acids, and simultaneously, the tRNA occupying the P site exits the ribosome. The ribosome shifts one codon along the mRNA (5' → 3'), so that the second tRNA with the two attached amino acids now occupies the P site. A third aminoacyl-tRNA enters the now free A site, forming a new peptide bond. This process repeats, linking amino acids one after another according to the specific sequence contained in the mRNA directing the synthesis, until the polypeptide chain is complete. Termination: this occurs when the ribosome reaches one of the three stop codons. Translation halts, the protein detaches from the tRNA, which in turn leaves the P site, and the two ribosomal subunits dissociate. This process requires a significant amount of energy, provided by the hydrolysis of ATP. As the ribosome elongates and moves along the mRNA, the free 5' end can start being read by another ribosome. Thus, the same mRNA strand can be read simultaneously by multiple ribosomes, collectively known as a polysome. Protein synthesis is a very rapid process. 5. Gene mutations Mutations are sudden changes in the hereditary material; gene mutations affect a single gene. They are caused by errors that can occur during replication, which, if not corrected, result in a change in the nucleotide sequence in the DNA. The simplest mutations involve a single nucleotide and are called point mutations. These mutations can involve the substitution of one nucleotide for another or the loss or addition of a nucleotide. - The effects of nucleotide loss and addition The loss and addition of a nucleotide result in a shift of the reading frame, which is why these are called frame- shift mutations, altering the entire polypeptide chain. For this reason, they are almost always responsible for very serious effects, due to the production of a protein completely devoid of biological activity. The substitution of a nucleotide can lead to the production of a synonymous codon, in which case it will be a silent mutation, or it can cause two types of mutations: missense mutation, which gives rise to a codon that encodes a different amino acid than the original; and nonsense mutation, which produces one of the three stop codons that signal the termination of protein synthesis. Nonsense mutations also always have very severe effects. Mutations are random, spontaneous, and rare events. - Mutagenic agents Although mutations are spontaneous events, their frequency can be increased by chemical or physical factors, known as mutagenic agents. Physical mutagens include UV rays, X-rays, and radiation emitted by radioactive materials (α, β, γ rays). Chemical mutagens include many pesticides and herbicides. An example is nitrous acid (HNO2), which causes the transformation of cytosine into uracil. Only mutations affecting reproductive cells (gametes) can be passed on to subsequent generations; however, those occurring in other cells of the organism can also have significant health consequences. 6. Regulation of gene expression All cells possess many genes, but none express all of them simultaneously. Each cell carries out its activities economically and coordinately, translating into proteins only the genes necessary depending on the circumstances. One of the most studied organisms for understanding the mechanisms of gene expression regulation is the bacterium Escherichia coli. - Regulation of gene expression in prokaryotes In prokaryotes, regulation occurs primarily at the transcription level, meaning that only the DNA segments corresponding to gene sequences that need to be translated into proteins are transcribed into mRNA. Transcription is controlled by specific regulatory proteins encoded by regulatory genes. The regulatory protein can act as a repressor when it binds to the DNA, blocking gene transcription, or as an activator when it facilitates the binding of RNA polymerase to the promoter, thus enabling gene transcription. The efficiency and simplicity of this control system also depend on the fact that in prokaryotes, lacking a nucleus, transcription is coupled with translation, allowing the mRNA to start being translated before its synthesis is complete. ○ The operon model The most well-known system of gene regulation in prokaryotes is the operon model, identified in the 1960s by Jacob and Monod. An operon is a segment of the bacterial chromosome consisting of a promoter, an operator and one or more structural genes, which are genes that encode specific proteins. The promoter is the binding site for RNA polymerase; the operator follows the promoter and is a short sequence of bases to which the repressor protein binds; this repressor is encoded by a regulatory gene, which is not necessarily adjacent to the operon.There are two known types of operons: inducible operons and repressible operons. Inducible operons: normally not expressed; their transcription requires the presence of a substance called an inducer that inactivates the repressor. In this operon, the repressor is bound to the operator, preventing RNA polymerase from binding to the promoter to transcribe the structural genes. In the presence of the inducer, a repressor-inducer complex forms, detaching from the operator, allowing RNA polymerase to bind to the promoter and begin transcription. Example: the lac operon, which contains the three genes necessary for the utilisation of lactose in Escherichia coli. In the absence of lactose, the operon is inactive, and the three genes are not expressed. In the presence of lactose, it acts as an inducer, binding to the repressor and inactivating it. RNA polymerase then binds to the promoter and transcribes the three genes together, forming a single mRNA molecule. Repressible operons: Normally expressed, except when a corepressor is present, which activates the repressor. In this operon, the repressor is normally inactive, so the operon is regularly transcribed. The repressor remains inactive until it binds to a corepressor, forming a repressor-corepressor complex that binds to the operator, preventing RNA polymerase from transcribing the structural genes of the operon. A corepressor is often the final product of the biosynthetic pathway that it controls. Example: the trp operon encodes enzymes responsible for the synthesis of the amino acid tryptophan in Escherichia coli. In the absence of tryptophan, the operon is active, and the genes for the enzymes necessary for the amino acid's biosynthesis are transcribed. In the presence of tryptophan, the enzymes that synthesise it are no longer needed, and their production ceases. Tryptophan acts as a corepressor, binding to the repressor and activating it to block transcription. - Regulation of gene expression in eukaryotes This process is different and more complex than in prokaryotes. The genetic material of eukaryotes is much more complex and consists of discontinuous genes, which contain exons interspersed with introns. The freshly transcribed mRNA undergoes processing before being transported to the cytoplasm for translation, and the regulation of gene expression does not end with transcription regulation but involves multiple phases of the process leading to protein synthesis. Eukaryotic genes are not organised into operons. Furthermore, in prokaryotes, the control of gene expression serves to produce the proteins necessary for utilising available nutrients, while in eukaryotes, it is essential for enabling differentiation. Even when differentiated, all cells in an organism contain the entire genetic program. Differentiation depends on the fact that each cell, while possessing complete genetic information, only expresses the genes coding for its characteristic proteins. Eukaryotes can regulate the expression of their genes by controlling the processes that translate genetic messages into proteins. ○ Conformational control: chromatin exists in two forms: euchromatin, which is loosely condensed and transcribed, and heterochromatin, which is more condensed and not transcribed because it is too compact. ○ Control of transcription: The selective transcription of genes is the main regulatory mechanism of gene expression. Transcription control is linked to chemical modifications of DNA, particularly involving the methylation of certain cytosine nucleotides in non-transcribed gene sequences. Even in eukaryotes, RNA polymerase binds to a specific site cal