BIO101 Lecture Note (1) PDF

EDWARD CARES BIO LIVING THINGS, CHARACTERISTICS AND REPRODUCTION Living things exist and are alive and are made of microscopic structures called cells. They grow and exhibit movement or locomotion. They experience metabolism, which includes anabolic and catabolic reactions. Living things are capa...

EDWARD CARES BIO LIVING THINGS, CHARACTERISTICS AND REPRODUCTION Living things exist and are alive and are made of microscopic structures called cells. They grow and exhibit movement or locomotion. They experience metabolism, which includes anabolic and catabolic reactions. Living things are capable of producing a new life which is of their own kind through the process of reproduction. CHARACTERISTICS OF LIVING THINGS MOVEMENT: All living things move. It is very obvious that a leopard moves but what about the thorn tree it sits in? Plants too move in various different ways. The movement may be so slow that it is very difficult to see RESPIRATION: Respiration is the release of energy from food substances in all living cells. Living things break down food within their cells to release energy for carrying out the various processes of life. NUTRITION: living things take in food to stay alive. This is digested, absorbed and then assimilated into the body, used for growth, repair and replacement of worn-out tissues and a source of energy for its various activities. IRRITABILITY: the ability to detect and respond to changes in the internal or external environment. All living things are able to sense and respond to stimuli around them such as light, temperature, water, gravity and chemical substances. Organisms can respond to diverse stimuli. For example, plants can grow toward a source of light, climb on fences and walls, or respond to touch. Even tiny bacteria can move toward or away from chemicals (a process called chemotaxis) or light (phototaxis). Movement toward a stimulus is considered a positive response, while movement away from a stimulus is considered a negative response.  GROWTH: Growth is seen in all living things. It involves using food to produce new cells. The permanent increase in cell number and size is called growth. Growth occurs in plants by cell division in special tissues, meristem. EXCERETION: All living things excrete. Excretion is defined as the removal of toxic materials, the waste products of metabolism and substances in excess from the body of an organism. As a result of the many chemical reactions occurring in cells, they have to get rid of waste products which might poison the cells. This does not involve the removal of undigested food i.e. egestion. REPRODUCTION: All living organisms have the ability to produce offspring. Organisms reproduce sexually by making special cells (gametes) which fuse to produce new offsprings or asexually when cell divides into two to form offsprings. COMPETITION: the utilization of the same resources by one or more organisms of the same or different species living together in a community. When the resources such as food, light, space, water, shelter etc are in short supply to meet the needs of all the organisms. Organisms in the population that are unable to compete successfully for these resources die (survival of the fittest). ADAPTATION; Plants have adaptation that help them to survive, live and grow in certain environments. These adaptations help them to make the most of the surrounding area. Differences between living and non-living things: Living Things Non-Living Things They possess life. They do not possess life. Living things are capable of giving birth to their Non-living things do not reproduce. young ones i.e. reproduce  For survival, living things depend on water, air and Non-living things have no such requirements food. Non-living things are not sensitive and do not Living things are sensitive and responsive to stimuli. respond to stimuli. Metabolic reactions constantly occur in all living There are no metabolic reactions in Non-living things. things. Living organisms undergo growth and development. Non-living things do not grow or develop. They have a lifespan and are not immortal. They have no lifespan and are immortal. Living things move from one place to another. Non-living things cannot move by themselves. They respire and the exchange of gases takes place in Non-living things do not respire. their cells. Example: Humans, animals, plants, insects. Example: Rock, pen, buildings, gadgets. REPRODUCTION IN PLANTS Reproduction is one of the most important characteristics of all living beings. It is the production of ones own kind. It is necessary for the continuation of the species on earth and also to replace the dead members of the species. The process by which living organisms produce their offsprings for the continuity of the species is called reproduction. The modes of reproduction vary according to individual species and available conditions. It may be simply by division of the parent cell as in unicellular organisms, by fragmentation of the parent body, by formation of buds and spores, or it may be very elaborate involving development of male and female reproductive organs (stamens and pistils). Irrespective of the mode of reproduction, all organisms pass on their hereditary material to their offsprings during the process of reproduction. MODES OF REPRODUCTION Most plants have roots, stems and leaves. These are called the vegetative parts of a plant. After a certain period of growth, most plants bear flowers. You may have seen the mango trees flowering; it is these flowers that give rise to juicy mango fruit we enjoy. We eat the fruits and usually discard the seeds. Seeds germinate and form new plants. So, what is the function of flowers in plants? Flowers perform the function of reproduction in plants. Flowers are the reproductive parts. In plants, reproduction is carried out via two modes: Asexual Mode Sexual Mode In asexual reproduction plants can give rise to new plants without seeds, whereas in sexual reproduction, new plants are obtained from seeds Asexual Reproduction In Plants In asexual reproduction in plants, plants are reproduced without the formation of seeds. Following are a few ways in which plants reproduce asexually. Vegetative Propagation As the name suggests, reproduction occurs through the vegetative parts of a plant such as stems, leaves, buds, and roots. Plants produced by vegetative propagation take less time to grow and are exact replicas of their parents as they are reproduced from a single parent. Budding It occurs in unicellular plants. A bud-like outgrowth is formed on one side of the parent cell and soon it separates and grows into a new individual e.g. in yeast. Small bulb-like projections arise from yeast cells, eventually detaching itself from the parent cell. This then matures to grow into a new yeast cell. These, in turn, produce more buds and the chain continues forming a number of new yeast cells within a short period of time. Example can also be seen in a multicellular organism; hydra. Fraegmentation You might have seen slimy green patches in ponds, or in other stagnant water bodies. These are the algae. When water and nutrients are available algae grow and multiply rapidly by fragmentation. An alga breaks up into two or more fragments. These fragments or pieces grow into new individual. They multiply rapidly in a short period of time. Other examples can be Planaria (a type of flatworm) and starfish. Spore Formation Spores are present in the air and are covered by a hard protective coat to bear low humidity and high-temperature conditions. Spores germinate and develop into new organisms under favourable conditions. In lower plants including bryophytes and pteridophytes, special reproductive units develop asexually on the parent body. These are called spores. They are microscopic and covered by a protective wall. When they reach the suitable environment they develop into a new plant body e.g. in bread moulds, moss, fern. Advantages of Asexual Reproduction in Plants A large number of plants can be produced within a short period. The exact copies of the parent plant are produced. Many seedless varieties are obtained through the vegetative method. Less attention is required by the plants grown through asexual means than through seeds. Sexual Reproduction In Plants In sexual reproduction, the gametes from male and female reproductive parts of the flower fuse to produce a zygote which develops into an embryo. This embryo remains inside the seed. Seed upon sowing germinates to produce a new plant. The reproductive parts of plants are flowers, Stamen being male reproductive part and pistil being the female reproductive part. If one of these reproductive parts are present in a flower, it is said to be a unisexual flower. Example: papaya. If both Stamen and Pistil are present in flowers they are called bisexual flowers. Example: rose. Pollen grains form the male gametes. The pistil consists of style, stigma, and the Ovary. The ovary consists of one or more ovules. Ovules are where female gametes or the egg is formed. Female and male gametes fuse to form a zygote. The flower is the structure that makes sexual reproduction in flowering plants possible. A wide variety exists in flower appearance, but the function of the flower parts is the same. Their functions are listed below. The stamen – contains the male part of the flower. It produces pollen, a yellow powdery substance. Pollen is produced in the top of the stamen, in a structure called the anther. The pistil – contains the female part of the flower. The top of the pistil is called the stigma. When a pollen grain reaches the pistil, it sticks to the surface of the stigma. The stigma produces a sugar that is used by the pollen to grow a tube. The pollen tube “digs” its way down through the style, allowing delivery of the sperm down to the ovary. This is the enlarged part of the pistil where the female sex cells (eggs) are produced. The eggs are fertilized by the sperm from the pollen tube. The transfer of the pollen from anther to the stigma is called pollination. If allowed to develop without being picked, the ovary dries and splits open to disperse the seeds(s). The petals – of the flower attract insects that carry the pollen from one plant to another. Some plants have no petals and the pollen is carried by the wind. Pollination When pollen is transferred from the anther to the stigma of a flower through carriers such as insects, wind etc it is called pollination. Pollination takes two forms: self pollination and cross pollination. Self-pollination occurs when the pollen from the anther is deposited on the stigma of the same flower, or another  flower on the same plant. Cross pollination is the transfer of pollen from the anther of one flower to the stigma of another flower on a different individual of the same species. FERTILIZATION A zygote is formed as a result of the fusion of gametes which later develops into the embryo. Fruits and seeds are formed post-fertilization. FRUITS AND SEED FORMATION After fertilization, the ovary grows into a fruit and other parts of the flower fall off. The fruit is the ripened ovary. The seeds develop from the ovules. The seed contains an embryo enclosed in a protective seed coat. Some fruits are fleshy and juicy such as mango and orange. Some fruits are hard like almonds and walnut Cell Biology Chapter 1: Understand the cell of as the basic unit of life Chapter 1: Understand the cell of as the basic unit of life 1.1: Cell as a unit of Life. An organism is a life-form—a living entity made up of one or more cells. Although there is no simple definition of life that is endorsed by all biologists, most agree that organisms share a suite of five fundamental characteristics. Cells Organisms are made up of membrane-bound units called cells. The membrane of a cell regulates the passage of materials between exterior and interior spaces. Replication One of the great biologists of the twentieth century, François Jacob, said that the “dream of a bacterium is to become two bacteria.” Almost everything an organism does contributes to one goal: replicating itself. Evolution Organisms are the products of evolution, and their populations continue to evolve today. Information Organisms process hereditary, or genetic, information encoded in units called genes. Organisms also respond to information from the environment and adjust to maintain stable internal conditions. Right now, cells through- out your body are using information to make the molecules that keep you alive; your eyes and brain are decoding information on this page that will help you learn some biology, and if your room is too hot you might be sweating to cool off. Energy To stay alive and reproduce, organisms have to acquire and use energy. To give just two examples: plants absorb sunlight; animals ingest food. 1.2: Biological Levels of Organization: The biological levels of organization of living things follow a hierarchy, such as the one shown in Figure 1.1 From a single organelle to the entire biosphere, living organisms are part of a highly structured hierarchy. Cell Biology Chapter 1: Understand the cell of as the basic unit of life Figure 1.1: Biological Levels of Organization (Campbell and Urry, 2017) Key Points The atom is the smallest and most fundamental unit of matter. The bonding of at least two atoms or more form molecules. The simplest level of organization for living things is a single organelle, which is composed of aggregates of macromolecules. The highest level of organization for living things is the biosphere; it encompasses all other levels. Cell Biology Chapter 1: Understand the cell of as the basic unit of life The biological levels of organization of living things arranged from the simplest to most complex are: organelle, cells, tissues, organs, organ systems, organisms, populations, communities, ecosystem, and biosphere. Key Terms molecule: The smallest particle of a specific compound that retains the chemical properties of that compound; two or more atoms held together by chemical bonds. macromolecule: a very large molecule, especially used in reference to large biological polymers (e.g. nucleic acids and proteins) polymerization: The chemical process, normally with the aid of a catalyst, to form a polymer by bonding together multiple identical units (monomers). Three of the greatest unifying ideas in all of science, which depend on the five characteristics just listed, laid the ground- work for modern biology: the cell theory, the theory of evolution, and the chromosome theory of inheritance. Formally, scientists define a theory as an explanation for a very general class of phenomena or observations that are supported by a wide body of evidence. Note that this definition contrasts sharply with the everyday usage of the word “theory,” which often carries meanings such as “speculation” or “guess.” The cell theory, the theory of evolution, and the chromosome theory of inheritance address fundamental questions: What are organisms made of? Where do they come from? How is hereditary information transmitted from one generation to the next? When these theories emerged in the mid-1800s, they revolutionized the way biologists think about the world. None of these insights came easily, however. The cell theory, for example, emerged after some 200 years of work. 1.3: Cell Theory The microscopes we use today are far more complex than those used in the 1600s by Antony van Leeuwenhoek, a Dutch shopkeeper who had great skill in crafting lenses. Despite the limitations of his now-ancient lenses, van Leeuwenhoek observed the movements of protista (a type of single-celled organism) and sperm, which he collectively termed “animalcules. ” Cell Biology Chapter 1: Understand the cell of as the basic unit of life In a 1665 publication called Micrographia, experimental scientist Robert Hooke coined the term “cell” for the box-like structures he observed when viewing cork tissue through a lens. In the 1670s, van Leeuwenhoek discovered bacteria and protozoa. Later advances in lenses, microscope construction, and staining techniques enabled other scientists to see some components inside cells. By the late 1830s, botanist Matthias Schleiden and zoologist Theodor Schwann were studying tissues and proposed the unified cell theory. The unified cell theory states that: all living things are composed of one or more cells; the cell is the basic unit of life; and new cells arise from existing cells. Rudolf Virchow later made important contributions to this theory. Schleiden and Schwann proposed spontaneous generation as the method for cell origination, but spontaneous generation (also called abiogenesis) was later disproven. Rudolf Virchow famously stated “Omnis cellula e cellula”… “All cells only arise from pre-existing cells. “The parts of the theory that did not have to do with the origin of cells, however, held up to scientific scrutiny and are widely agreed upon by the scientific community today. The generally accepted portions of the modern Cell Theory are as follows: 1. The cell is the fundamental unit of structure and function in living things. 2. All organisms are made up of one or more cells. 3. Cells arise from other cells through cellular division. The expanded version of the cell theory can also include: Cells carry genetic material passed to daughter cells during cellular division All cells are essentially the same in chemical composition Energy flow (metabolism and biochemistry) occurs within cells The generally accepted parts of modern cell theory include: 1. All known living things are made up of one or more cells 2. All living cells arise from pre-existing cells by division. 3. The cell is the fundamental unit of structure and function in all living organisms. 4. The activity of an organism depends on the total activity of independent cells. Cell Biology Chapter 1: Understand the cell of as the basic unit of life 5. Energy flow (metabolism and biochemistry) occurs within cells. 6. Cells contain DNA which is found specifically in the chromosome and RNA found in the cell nucleus and cytoplasm. 7. All cells are basically the same in chemical composition in organisms of similar species. The modern version of the cell theory includes the ideas that: Energy flow occurs within cells. Heredity information (DNA) is passed on from cell to cell. All cells have the same basic chemical composition. 1.4: Size range of cells. Most cells are between 1 and 100 μm in diameter (yellow region of chart) and their components are even smaller (Figure 1.2), as are viruses. Notice that the scale along the left side is logarithmic, to accommodate the range of sizes shown. Starting at the top of the scale with 10m and going down, each reference measurement marks a tenfold decrease in diameter or length. 1.5: Different types of cell Cells—the basic structural and functional units of every organism—are of two distinct types: prokaryotic and eukaryotic. Organisms of the domains Bacteria and Archaea consist of prokaryotic cells. Protists, fungi, animals, and plants all consist of eukaryotic cells. (“Protist” is an informal term refer- ring to a diverse group of mostly unicellular eukaryotes.) All cells share certain basic features: They are all bounded by a selective barrier, called the plasma membrane (also referred to as the cell membrane). Inside all cells is a semifluid, jellylike sub- stance called cytosol, in which subcellular components are suspended. All cells contain chromosomes, which carry genes in the form of DNA. And all cells have ribosomes, tiny complexes that make proteins according to instructions from the genes. Cell Biology Chapter 1: Understand the cell of as the basic unit of life 1 centimeter (cm) = 10–2 meter (m) = 0.4 inch 1 millimeter (mm) = 10–3 m 1 micrometer (μm) = 10–3 mm = 10–6 m 1 nanometer (nm) = 10–3 μm = 10–9 m Figure 1.2: Size ranges of cells (Campbell and Urry, 2017) Cell Biology Chapter 1: Understand the cell of as the basic unit of life As shown in Table 1.1 below, there are some differences between different types of cells (Prokaryotic and Eukaryotic) Table 1.1: Difference between Prokaryotic and Eukaryotic Cells Prokaryotes Eukaryotes Typical bacteria, archaea protists, fungi, plants, animals organisms Name and prokaryotic means “before nucleus” (from Eukaryotic means “true nucleus” evolution the Greek pro, before), reflecting the (from the Greek eu, true, and earlier evolution of prokaryotic cells. karyon, kernel, referring to the nucleus) Typical size ~ 1–5 μm ~ 10–100 μm in diameter Type of nucleoid region; no true nucleus true nucleus with double nucleus membrane DNA circular (usually) linear molecules (chromosomes) with histone proteins DNA location the DNA is concentrated in a region that is most of the DNA is in an organelle not membrane-enclosed, called the called the nucleus, which is nucleoid. bounded by a double membrane. RNA/protein coupled in the cytoplasm RNA synthesis in the nucleus synthesis protein synthesis in the cytoplasm Ribosomes 50S and 30S 60S and 40S Cytoplasmic very few structures highly structured by structure endomembranes and a cytoskeleton Organelle Membrane-bounded structures are absent Membrane-bounded structures are present Cell movement flagella made of flagellin flagella and cilia containing microtubules; lamellipodia and filopodia containing actin Mitochondria none one to several thousand Chloroplasts none in algae and plants Organization usually single cells single cells, colonies, higher multicellular organisms with specialized cells Cell division binary fission (simple division) mitosis (fission or budding) meiosis Chromosomes single chromosome more than one chromosome Membranes cell membrane Cell membrane and membrane- bound organelles Cell Biology Chapter 1: Understand the cell of as the basic unit of life 1.6: Cell size and function Surface Area to Volume Ration The important point is that the surface area to the volume ratio gets smaller as the cell gets larger. Thus, if the cell grows beyond a certain limit, not enough material will be able to cross the membrane fast enough to accommodate the increased cellular volume. When this happens, the cell must divide into smaller cells with favorable surface area/volume ratios, or cease to function. That is why cells are so small. Eukaryotic cells are generally much larger than prokaryotic cells. Size is a general feature of cell structure that relates to function. The logistics of carrying out cellular metabolism sets limits on cell size. At the lower limit, the smallest cells known are bacteria called mycoplasmas, which have diameters between 0.1 and 1.0 μm. These are perhaps the smallest packages with enough DNA to program metabolism and enough enzymes and other cellular equipment to carry out the activities necessary for a cell to sustain itself and reproduce. Typical bacteria are 1–5 μm in diameter, about ten times the size of mycoplasmas. Eukaryotic cells are typically 10–100 μm in diameter. Metabolic requirements also impose theoretical upper limits on the size that is practical for a single cell. At the boundary of every cell, the plasma membrane functions as a selective barrier that allows passage of enough oxygen, nutrients, and wastes to service the entire cell. For each square micrometer of membrane, only a limited amount of a particular substance can cross per second, so the ratio of surface area to volume is critical. As a cell (or any other object) increases in size, its surface area grows proportionately less than its volume. (Area is proportional to a linear dimension squared, whereas volume is proportional to the linear dimension cubed.) Thus, a smaller object has a greater ratio of surface area to volume (Table 1). The scientific skills exercise gives you a chance to calculate the volumes and surface areas of two actual cells— a mature yeast cell and a cell budding from it. The need for a surface area large enough to accommodate the volume helps explain the microscopic size of most cells and the narrow, elongated shapes of others, such as nerve cells. Larger organisms do not generally have larger cells than smaller organisms—they simply have more cells (see Table 1.2). A sufficiently high ratio of surface area to volume is especially important in cells that exchange a lot of material with their surroundings, such as intestinal  Cell Biology Chapter 1: Understand the cell of as the basic unit of life cells. Such cells may have many long, thin projections from their surface called microvilli, which increase surface area without an appreciable increase in volume. Table 1.2 elucidate geometric relationships between surface area and volume. Cells are represented as boxes. Using arbitrary units of length, we can calculate the cell’s surface area (in square units, or units2), volume (in cubic units, or units3), and ratio of surface area to volume. A high surface-to-volume ratio facilitates the exchange of materials between a cell and its environment. Table 1.2: Geometric relationships between surface area and volume Surface area increase while total volume remains constant Total surface area [sum of the surface areas (height × width) of all 6 150 750 box sides × number of boxes] Total volume 1 125 125 [height × width × length × number of boxes] Surface-to-volume (S-to-V) ratio 6 1.2 6 [surface area ÷ volume] 1.7: Eukaryotic Cell Structures and Their Functions The Eukarya domain includes species that range from microscopic algae to 100-meter-tall redwood trees. Protists, fungi, plants, and animals are all eukaryotic. Although multicellularity has evolved several times among eukaryotes, many species are unicellular. The first thing that strikes biologists about eukaryotic cells is how much larger they are on average than bacteria and archaea. Most prokaryotic cells measure 1 to 10 μm in diameter, Cell Biology Chapter 1: Understand the cell of as the basic unit of life while most eukaryotic cells range from about 5 to 100 μm in diameter. For many species of unicellular eukaryotes, this size difference allows them to make a living by ingesting bacteria and archaea whole. Large size has a downside, however. As a cell increases in diameter, its volume increases more than its surface area. In other words, the relationship between them—the surface-areato-volume ratio—changes. Since the surface is where the cell exchanges substances with its environment, the reduction in this ratio decreases the rate of exchange: Diffusion only allows for rapid movement across very small distances. Prokaryotic cells tend to be small enough so that ions and small molecules arrive where they are needed via diffusion. The random movement of diffusion alone, however, is insufficient for this type of transport as the cell’s diameter increases. 1.8: The Benefits of Organelles How are the problems associated with a low surface-area-to volume ratio overcome in eukaryotic cells? The answer lies in their numerous organelles. In effect, the huge volume inside a eukaryotic cell is compartmentalized into many small bins. Because eukaryotic cells are subdivided, the cytosol—the fluid portion between the plasma membrane and these organelles—is only a fraction of the total cell volume. This relatively small volume of cytosol offsets the effects of a low cell surface-area-to-volume ratio with respect to the exchange of nutrients and waste products. 1.9: Compartmentalization also offers two key advantages: 1. Incompatible chemical reactions can be separated. For example, new fatty acids can be synthesized in one organelle while excess or damaged fatty acids are degraded and recycled in a different organelle. 2. Chemical reactions become more efficient. First, the substrates required for particular reactions can be localized and maintained at high concentrations within organelles. When substrates are used up in a particular part of the organelle, they can be replaced by substrates that have only a short distance to diffuse. Second, groups of enzymes that work together can be clustered within or on the membranes of organelles instead of floating free in the cytosol. When the product of one reaction is the substrate for a second reaction, clustering the two enzymes increases the speed and efficiency of both reactions. If bacterial and archaeal cells can be compared to specialized machine shops, then eukaryotic cells resemble sprawling industrial complexes. The organelles and other structures found in Cell Biology Chapter 1: Understand the cell of as the basic unit of life eukaryotes are like highly specialized buildings that act as administrative centers, factories, transportation corridors, waste and recycling facilities, warehouses, and power stations. When typical prokaryotic and eukaryotic cells are compared, three key differences stand out: 1. Eukaryotic cells are generally much larger than prokaryotic cells. 2. Prokaryotic chromosomes are in a loosely defined nucleoid region while eukaryotic chromosomes are enclosed within a membrane-bound compartment called the nucleus. 3. The cytoplasm of eukaryotic cells is compartmentalized into a larger number of distinct organelles compared to the cytoplasm in prokaryotic cells. Eukaryotic Cell Structures: Figure 1.3 provides a simplified view of a typical animal cell and a plant cell. The artist has removed most of the cytoskeletal elements to make the organelles and other cellular parts easier to see. As you read about each cell component in the pages that follow, focus on identifying how its structure correlates with its function. Figure 1.3a Overview of Eukaryotic Cells. Generalized images of an animal cell that illustrate the cellular structures in the “typical” eukaryote. The structures have been color-coded for clarity (Freeman et al., 2017). Cell Biology Chapter 1: Understand the cell of as the basic unit of life Figure 1.3b Overview of Eukaryotic Cells. Generalized images of a plant cell that illustrate the cellular structures in the “typical” eukaryote. the structures have been color-coded for clarity. Compare with the prokaryotic cell, shown at true relative size at bottom left (Freeman et al., 2017). 1.10: Cell inclusions and organelles. What are Cell Organelles? The cellular components are called cell organelles. These cell organelles include both membrane and non-membrane bound organelles, present within the cells and are distinct in their structures and functions. They coordinate and function efficiently for the normal functioning of the cell. A few of them function by providing shape and support, whereas some are involved in the locomotion and reproduction of a cell. There are various organelles present within the cell and are classified into three categories based on the presence or absence of membrane. Organelles without membrane: The Cell wall, Ribosomes, and Cytoskeleton are non- membrane-bound cell organelles. They are present both in prokaryotic cell and the eukaryotic cell. Single membrane-bound organelles: Vacuole, Lysosome, Golgi Apparatus, Endoplasmic Reticulum are single membrane-bound organelles present only in a eukaryotic cell. Cell Biology Chapter 1: Understand the cell of as the basic unit of life Double membrane-bound organelles: Nucleus, mitochondria and chloroplast are double membrane-bound organelles present only in a eukaryotic cell. 1.11: Functions of cell organelles Plasma Membrane The plasma membrane of a cell is a network of lipids and proteins that forms the boundary between a cell’s contents and the outside of the cell. It is also simply called the cell membrane. The main function of the plasma membrane is to protect the cell from its surrounding environment. It is semi-permeable and regulates the materials that enter and exit the cell. The cells of all living things have plasma membranes. Functions of the Plasma Membrane A Physical Barrier The plasma membrane surrounds all cells and physically separates the cytoplasm, which is the material that makes up the cell, from the extracellular fluid outside the cell. This protects all the components of the cell from the outside environment and allows separate activities to occur inside and outside the cell. The plasma membrane provides structural support to the cell. It tethers the cytoskeleton, which is a network of protein filaments inside the cell that hold all the parts of the cell in place. This gives the cell its shape. Certain organisms such as plants and fungi have a cell wall in addition to the membrane. The cell wall is composed of molecules such as cellulose. It provides additional support to the cell, and it is why plant cells do not burst like animal cells do if too much water diffuses into them. Selective Permeability Plasma membranes are selectively permeable (or semi-permeable), meaning that only certain molecules can pass through them. Water, oxygen, and carbon dioxide can easily travel through the membrane. Generally, ions (e.g. sodium, potassium) and polar molecules cannot pass through the membrane; they must go through specific channels or pores in the membrane Cell Biology Chapter 1: Understand the cell of as the basic unit of life instead of freely diffusing through. This way, the membrane can control the rate at which certain molecules can enter and exit the cell. Endocytosis and Exocytosis Endocytosis is when a cell ingests relatively larger contents than the single ions or molecules that pass through channels. Through endocytosis, a cell can take in large quantities of molecules or even whole bacteria from the extracellular fluid. Exocytosis is when the cell releases these materials. The cell membrane plays an important role in both of these processes. The shape of the membrane itself changes to allow molecules to enter or exit the cell. It also forms vacuoles, small bubbles of membrane that can transport many molecules at once, in order to transport materials to different places in the cell. Cell Signalling Another important function of the membrane is to facilitate communication and signalling between cells. It does so through the use of various proteins and carbohydrates in the membrane. Proteins on the cell “mark” that cell so that other cells can identify it. The membrane also has receptors that allow it to carry out certain tasks when molecules such as hormones bind to those receptors. Plasma Membrane Structure Figure 1.4: Shows the fluid mosaic model of the plasma membrane where integral membrane proteins are inserted into the lipid bilayer, whereas peripheral proteins are bound to the membrane indirectly by protein–protein interactions. Most integral membrane proteins are transmembrane proteins with portions exposed on both sides of the lipid bilayer. The extracellular portions of these proteins are usually glycosylated, as are the peripheral membrane proteins bound to the external face of the membrane. Phospholipids The membrane is partially made up of molecules called phospholipids, which spontaneously arrange themselves into a double layer with hydrophilic (“water loving”) heads on the outside and hydrophobic (“water hating”) tails on the inside. These interactions with water are what allow plasma membranes to form. Cell Biology Chapter 1: Understand the cell of as the basic unit of life Proteins Proteins are wedged between the lipids that make up the membrane, and these transmembrane proteins allow molecules that couldn’t enter the cell otherwise to pass through by forming channels, pores or gates. In this way, the cell controls the flow of these molecules as they enter and exit. Proteins in the cell membrane play a role in many other functions, such as cell signaling, cell recognition, and enzyme activity. Carbohydrates Carbohydrates are also found in the plasma membrane; specifically, most carbohydrates in the membrane are part of glycoproteins, which are formed when a carbohydrate attaches to a protein. Glycoproteins play a role in the interactions between cells, including cell adhesion, the process by which cells attach to each other. Fluid Mosaic Model Technically, the cell membrane is a liquid. At room temperature, it has about the same consistency as vegetable oil. Lipids, proteins, and carbohydrates in the plasma membrane can diffuse freely throughout the cell membrane; they are essentially floating across its surface. This is known as the fluid mosaic model, which was coined by S.J. Singer and G.L. Nicolson in 1972. According to the fluid mosaic model, the plasma membranes are subcellular structures, made of a lipid bilayer in which the protein molecules are embedded (Figure 1.4 ). Figure 1.4: Shows the fluid mosaic model of the plasma membrane Cell Biology Chapter 1: Understand the cell of as the basic unit of life Cytoplasm The cytoplasm is present both in plant and animal cells. They are jelly-like substances, found between the cell membrane and nucleus. They are mainly composed of water, organic and inorganic compounds. The cytoplasm is one of the essential components of the cell, where all the cell organelles are embedded. These cell organelles contain enzymes, mainly responsible for controlling all metabolic activity taking place within the cell and are the site for most of the chemical reactions within a cell. Nucleus The nucleus is a double-membraned organelle found in all eukaryotic cells (Figure 1.5). It is the largest organelle, which functions as the control centre of the cellular activities and is the storehouse of the cell’s DNA. By structure, the nucleus is dark, round, surrounded by a nuclear membrane. It is a porous membrane (like cell membrane) and forms a wall between cytoplasm and nucleus. Within the nucleus, there are tiny spherical bodies called nucleolus. It also carries another essential structure called chromosomes. Chromosomes are thin and thread-like structures which carry another important structure called a gene. Genes are a hereditary unit in organisms i.e., it helps in the inheritance of traits from one generation (parents) to another (offspring). Hence, the nucleus controls the characters and functions of cells in our body. The primary function of the nucleus is to monitor cellular activities including metabolism and growth by making use of DNA’s genetic information. Nucleoli in the nucleus are responsible for the synthesis of protein and RNA. Figure 1.5: Diagram of nucleus Cell Biology Chapter 1: Understand the cell of as the basic unit of life Endoplasmic Reticulum The Endoplasmic Reticulum is a network of membranous canals filled with fluid. They are the transport system of the cell, involved in transporting materials throughout the cell. There are two different types of Endoplasmic Reticulum: 1. Rough Endoplasmic Reticulum – They are composed of cisternae, tubules, and vesicles, which are found throughout the cell and are involved with protein manufacture. 2. Smooth Endoplasmic Reticulum – They are the storage organelle, associated with the production of lipids, steroids, and also responsible for detoxifying the cell. Mitochondria Mitochondria are called the powerhouses of the cell as they produce energy-rich molecules for the cell. The mitochondrial genome is inherited maternally in several organisms. It is a double membrane-bound, sausage-shaped organelle, found in almost all eukaryotic cells. The double membranes divide its lumen into two distinct aqueous compartments. The inner compartment is called ‘matrix’ which is folded into cristae whereas the outer membrane forms a continuous boundary with the cytoplasm. They usually vary in their size and are found either round or oval in shape. Mitochondria are the sites of aerobic respiration in the cell, produces energy in the form of ATP and helps in the transformation of the molecules. For instance, glucose is converted into adenosine triphosphate – ATP. Mitochondria have their own circular DNA, RNA molecules, ribosomes (the 70s), and a few other molecules that help in protein synthesis. Figure 1.6: Diagram of Mitochondria Cell Biology Chapter 1: Understand the cell of as the basic unit of life Plastids Plastids are large, membrane-bound organelles which contain pigments (Figure 1.7). Based on the type of pigments, plastids are of three types: Figure 1.7: Diagram of Plasmid Chloroplasts – Chloroplasts are double membrane-bound organelles, which usually vary in their shape – from a disc shape to spherical, discoid, oval and ribbon. They are present in mesophyll cells of leaves, which store chloroplasts and other carotenoid pigments. These pigments are responsible for trapping light energy for photosynthesis. The inner membrane encloses a space called the stroma. Flattened disc-like chlorophyll-containing structures known as thylakoids are arranged in a stacked manner like a pile of coins. Each pile is called as granum (plural: grana) and the thylakoids of different grana are connected by flat membranous tubules known as stromal lamella. Just like the mitochondrial matrix, the stroma of chloroplast also contains a double-stranded circular DNA, 70S ribosomes, and enzymes which required for the synthesis of carbohydrates and proteins. Chromoplasts – The chromoplasts include fat-soluble, carotenoid pigments like xanthophylls, carotene, etc. which provide the plants with their characteristic color – yellow, orange, red, etc. Leucoplasts – Leucoplasts are colorless plastids which store nutrients. Amyloplasts store carbohydrates (like starch in potatoes), aleuroplasts store proteins, and elaioplasts store oils and fats. Cell Biology Chapter 1: Understand the cell of as the basic unit of life Ribosomes Ribosomes are nonmembrane-bound and important cytoplasmic organelles found in close association with the endoplasmic reticulum. Ribosomes are found in the form of tiny particles in a large number of cells and are mainly composed of 2/3rd of RNA and 1/3rd of protein. They are named as the 70s (found in prokaryotes) or 80s (found in eukaryotes) The letter S refers to the density and the size, known as Svedberg’s Unit. Both 70S and 80S ribosomes are composed of two sub-units. Ribosomes are either encompassed within the endoplasmic reticulum or are freely traced in the cell’s cytoplasm. Ribosomal RNA and Ribosomal proteins are the two components that together constitute ribosomes. The primary function of the ribosomes includes protein synthesis in all living cells that ensure the survival of the cell. Golgi Apparatus Golgi Apparatus also termed as Golgi Complex. It is a membrane-bound organelle, which is mainly composed of a series of flattened, stacked pouches called cisternae. This cell organelle is primarily responsible for transporting, modifying, and packaging proteins and lipid to targeted destinations. Golgi Apparatus is found within the cytoplasm of a cell and are present in both plant and animal cells. Microbodies Microbodies are membrane-bound, minute, vesicular organelles, found in both plant and animal cell. They contain various enzymes and proteins and can be visualized only under the electron microscope. Cytoskeleton It is a continuous network of filamentous proteinaceous structures that run throughout the cytoplasm, from the nucleus to the plasma membrane. It is found in all living cells, notably in the eukaryotes. The cytoskeleton matrix is composed of different types of proteins that can divide rapidly or disassemble depending on the requirement of the cells. The primary functions include providing the shape and mechanical resistance to the cell against deformation, the contractile nature of the filaments helps in motility and during cytokinesis. Cell Biology Chapter 1: Understand the cell of as the basic unit of life Cilia and Flagella Cilia are hair-like projections, small structures, present outside the cell wall and work like oars to either move the cell or the extracellular fluid. Flagella are slightly bigger and are responsible for the cell movements. The eukaryotic flagellum structurally differs from its prokaryotic counterpart. The core of the cilium and flagellum is called a axoneme, which contains nine pairs of gradually arranged peripheral microtubules and a set of central microtubules running parallel to the axis. The central tubules are interconnected by a bridge and are embedded by a central sheath. One of the peripheral microtubular pairs is also interconnected to the central sheath by a radial spoke. Hence there is a total of 9 radial spokes. The cilia and flagella emerge from centriole-like structures called basal bodies. Centrosome and Centrioles The centrosome organelle is made up of two mutually perpendicular structures known as centrioles. Each centriole is composed of 9 equally spaced peripheral fibrils of tubulin protein, and the fibril is a set of interlinked triplets (Figure 1.8). The core part of the centriole is known as a hub and is proteinaceous. The hub connects the peripheral fibrils via radial spoke, which is made up of proteins. The centrioles from the basal bodies of the cilia and flagella give rise to spindle fibres during cell division. Figure 1.8: Diagram of centriole Vacuoles Vacuoles are mostly defined as storage bubbles of irregular shapes which are found in cells. They are fluid-filled organelles enclosed by a membrane. The vacuole stores the food or a Cell Biology Chapter 1: Understand the cell of as the basic unit of life variety of nutrients that a cell might need to survive. In addition to this, it also stores waste products. The waste products are eventually thrown out by vacuoles. Thus, the rest of the cell is protected from contamination. The animal and plant cell have different size and number of vacuoles. Compared to the animals, plant cell have larger vacuoles. Table 1.3 below described each cell organelle and their functions Table 1.3: A Brief Summary on Cell Organelles Cell Structure Functions Organelles Cell membrane A double membrane composed of Provides shape, protects the inner lipids and proteins. Present both in organelle of the cell and acts as a plant and animal cell. selectively permeable membrane. Centrosomes Composed of Centrioles and It plays a major role in organizing the found only in the animal cells. microtubule and Cell division. Chloroplasts Present only in plant cells and Sites of photosynthesis. contains a green-coloured pigment known as chlorophyll. Cytoplasm A jelly-like substance, which Responsible for the cell’s metabolic consists of water, dissolved activities. nutrients and waste products of the cell. Endoplasmic A network of membranous Forms the skeletal framework of the Reticulum tubules, present within the cell, involved in the Detoxification, cytoplasm of a cell. production of Lipids and proteins. Golgi Membrane-bound, sac-like It is mainly involved in secretion and apparatus organelles, present within the intracellular transport. cytoplasm of the eukaryotic cells. Lysosomes A tiny, circular-shaped, single Helps in the digestion and removes membrane-bound wastes and digests dead and damaged organelles, filled with digestive cells. Therefore, it is also called as the enzymes. “suicidal bags”. Mitochondria An oval-shaped, membrane- The main sites of cellular respiration bound organelle, also called as the and also involved in storing energy in “Power House of The Cell”. the form of ATP molecules. Cell Biology Chapter 1: Understand the cell of as the basic unit of life Nucleus A largest, double membrane- Controls the activity of the cell, helps bound organelles, which contains in cell division and controls the all the cell’s genetic information. hereditary characters. Peroxisome A membrane-bound cellular Involved in the metabolism of lipids organelle present in the and catabolism of long-chain fatty cytoplasm, which contains the acids. reducing enzyme. Plastids Double membrane-bound Helps in the process of photosynthesis organelles. There are 3 types of and pollination, Imparts colour for plastids: leaves, flowers and fruits and stores 1. Leucoplast –Colourless starch, proteins and fats. plastids. 2. Chromoplast–Blue, Red, and Yellow colour plastids. 3. Chloroplast – Green coloured plastids. Ribosomes Non-membrane organelles, found Involved in the Synthesis of Proteins. floating freely in the cell’s cytoplasm or embedded within the endoplasmic reticulum. Vacuoles A membrane-bound, fluid-filled Provide shape and rigidity to the plant organelle found within the cell and helps in digestion, excretion, cytoplasm. and storage of substances. 1.12: Difference Between Plant cell and Animal cell In an ecosystem, plants have the role of producers while animals have taken the role of consumers. Hence, their daily activities and functions vary, so do their cell structure. Cell structure and organelles vary in plants and animals, and they are primarily classified based on their function. The difference in their cell composition is the reason behind the difference between plants and animals, their structure and functions Table 1.4. Each cell organelle has a particular function to perform. Some of the cell organelles are present in both plant cell and the animal cell, while others are unique to just one. Most of the earth’s higher organisms are eukaryotes, including all plant and animals. Hence, these cells share some similarities typically associated with eukaryotes. For example, all eukaryotic cells consist of a nucleus, plasma membrane, cytoplasm, peroxisomes, mitochondria, ribosomes and other cell organelles. Cell Biology Chapter 1: Understand the cell of as the basic unit of life Differences Between Plant Cell and Animal Cell As stated above, both plant and animal cells share a few common cell organelles, as both are eukaryotes. The function of all these organelles is said to be very much similar. However, the major differences between the plant and animal cells, which significantly reflect the difference in the functions of each cell. Table 1.4: The major differences between the plant cell and animal cell are mentioned below: Plant Cell Animal Cell Cell Shape Square or rectangular in shape Irregular or round in shape Cell wall Present Absent Plasma/cell Present Present membrane Endoplasmic Present Present Reticulum Nucleus Present and lies on one side of the Present and lies in the centre cell of the cell Lysosomes Present but are very rare Present Centrosomes Absent Present Golgi Apparatus Present Present Cytoplasm Present Present Ribosomes Present Present Plastids Present Absent Vacuoles Few large or a single, centrally Usually small and numerous positioned vacuole Cilia Absent Present in most of the animal cells Mitochondrial Present but fewer in number Present and are numerous Mode of Nutrition Primarily autotrophic Heterotrophic Conclusion Both plant and animal cells comprise membrane-bound organelles, such as endoplasmic reticulum, mitochondria, the nucleus, Golgi apparatus, peroxisomes, lysosomes. They also have similar membranes, such as cytoskeletal elements and cytosol. The plant cell can also be larger than the animal cell. The normal range of the animal cell varies from about 10 – 30 micrometres and that of plant cell range between 10 – 100 micrometres. Cell Biology Chapter 1: Understand the cell of as the basic unit of life 1.13: Effects of hypertonic, hypotonic and isotonic solutions on the cell plasma The effects of hypotonic, hypertonic and isotonic solution on animal and plant cells. Hypertonic - Concentration with higher solute concentration and less water concentration Hypotonic - lower solute concentration and more water concentration Isotonic - Solution in which water molecule and solute molecule are equal in concentration. Animal and plant cell In an isotonic solution Isotonic solution is a solution in which the concentration of solutes is equal, so: - Water diffuses into and out of the cell at equal rates. - There’s no net movement of water across the plasma membrane - The cells retain their normal shape Animal and plant cells in a hypotonic solution Solution which contain higher concentration of water and lower concentration of solutes is called as hypotonic solution. Since the concentration of water is higher outside the cell, there is a net movement of water from outside into the cell. Cell gains water, swells and the internal pressure increases. Eventually burst (haemolysis). The effects of hypertonic solution in animal and plant cell Contain higher concentration of solutes and less of water than a cell. Since the concentration of water is higher within the cell, there is a net movement of water from inside to outside of the cell. (water leaves the cell by osmosis) Causes the cell to shrink as its internal pressure decreases. Hypertonic solution on plant cell Water diffuses out of the large central vacuole by osmosis. Water lose from both vacuole and cytoplasm cause to shrink. Plasma membrane pulls away from the cell wall. (plasmolysis). Become flaccid and less turgid. Cell wall doesn’t shrink because it is strong and rigid. If plasmolysis continues, death may result. If we placed the plasmolysed plant cell in a hypotonic solution (pure water), water moves into the cell by osmosis and become turgid again. (deplasmolysis) Cell Biology Chapter 1: Understand the cell of as the basic unit of life Food preservation The concept of osmosis and diffusion are applied in the preservation of food, such as fruits, fish and vegetables by using preservatives (salt, sugar/vinegar) Salt solution of hypertonic to tissue of fish. So water leaves the fish tissue and enter the salt solution by osmosis. Fish become dehydrated and cell crenate. Therefore, bacteria can’t grow in fish tissue and bacteria cell will crenate. Preserved fish don’t decay so soon and last longer. Preservation with vinegar Mangoes are soaked in vinegar which has low pH, vinegar diffuses into the tissues of the mangoes and become acidic. Low pH prevents the growth of microorganism in mangoes and preserved mangoes can last longer.  BIO 101 Topic: Heredity and evolution: Introduction to Darwinism and Lamarkism, Mendelian laws, explanation of key genetic terms By Dr. A. T. Anifowoshe Department of Zoology University of Ilorin, Ilorin, Nigeria ---------------------------------------------------------------------------------------------------------------- Hereditary and Evolution Heredity Heredity refers to the process through which genetic information is passed from one generation to the next. It is mediated through genes, which are segments of DNA that code for specific traits. Types of Heritable Characters: 1. Physical Traits: These traits are controlled by genes and influenced by alleles inherited from both parents. Examples: Eye color, height, skin color, hair texture etc 2. Physiological Traits: Such traits are encoded by specific genes that regulate biochemical and physiological processes. Examples: Blood type, metabolic rates, and immune responses. 3. Behavioral Traits: Examples: Certain instinctual behaviors and predispositions to learning or temperament. Often influenced by both genetic and environmental factors. 4. Disease Susceptibility: Such traits are often controlled by one or multiple genes. Examples: Inherited disorders like sickle cell anemia, cystic fibrosis, or predisposition to conditions like diabetes or heart disease. Role of Chromosomes in Heredity and Heritable Characters Introduction: Chromosomes are vital structures within cells that carry genetic material in the form of DNA. They play a fundamental role in heredity, serving as the medium through which genetic information is passed from parents to offspring. Chromosomes house genes, the functional units of heredity, which determine an organism's traits, or heritable characters. Role of Chromosomes in Heredity 1. Carriers of Genetic Information: Chromosomes are made up of DNA and proteins. The DNA contains sequences called genes, which encode the instructions for the development and functioning of living organisms. In the nucleus of each cell, DNA molecule is packaged into thread-like structures called Chromosomes. It was first described by Straubberg (1875) and was coined by Waldeyer in 1888. Each chromosome is made up of DNA tightly coiled by proteins called Histones. 2. Transmission During Cell Division: i. Mitosis: Ensures that genetic information is faithfully copied and distributed to daughter cells, maintaining genetic consistency within an organism. ii. Meiosis: Halves the chromosome number in gametes (sperm and egg cells), ensuring that offspring inherit the correct number of chromosomes when gametes fuse during fertilization. 3. Chromosome Structure and Function: i. Chromosome Number: Each species has a specific chromosome number (e.g., humans have 46 chromosomes arranged in 23 pairs). This number is critical for maintaining the integrity of an organism's genetic code. ii. Homologous Chromosomes: Chromosomes come in pairs, one from each parent, and contain alleles of the same genes, which may differ in their expression. Organisms No. of Chromosomes (2n) Human 46 Dog 78 Chimpanzee 48 Horse 64 Chicken 78 Fruit-fly 8 Mosquito 6 Nematode 11(m), 12(f) Rice, Tomatoes 24 Maize, Carrot 20  4. Role in Genetic Variation: i. Crossing Over: During meiosis, homologous chromosomes exchange genetic material, leading to new combinations of alleles in offspring. ii. Independent Assortment: The random distribution of chromosomes to gametes during meiosis contributes to genetic diversity. 5. Sex Determination: Chromosomes, particularly sex chromosomes (X and Y in humans), determine the sex of the offspring. Females typically have two X chromosomes (XX), while males have one X and one Y chromosome (XY). Evolution Evolution is the process through which species undergo changes in their genetic makeup over successive generations, leading to the emergence of new species, adaptation to environments, and biological diversity. Concepts in Evolution 1. Natural Selection: Proposed by Charles Darwin, natural selection suggests that organisms with favorable traits are more likely to survive, reproduce, and pass on those traits to their offspring. 2. Variation: Genetic variation within a population is crucial for evolution. It arises from mutations, genetic recombination during sexual reproduction, and gene flow between populations. 3. Adaptation: Traits that improve an organism's ability to survive and reproduce in its environment become more common over generations. 4. Speciation: The process by which new species arise. This can occur due to geographic isolation, reproductive barriers, or adaptive radiation.  5. Evidence of Evolution: i. Fossil Records: Show gradual changes in species over time. ii. Comparative Anatomy: Homologous structures suggest common ancestry. iii. Genetics: Similarities in DNA sequences across species support evolutionary relationships. 1. Darwinism (Theory of Evolution by Natural Selection) Charles Darwin's theory, presented in On the Origin of Species (1859), suggests that species evolve over time through natural selection. This process is based on the following principles:  Variation: Within any species, individuals exhibit variations in traits, some of which are heritable.  Competition: Due to limited resources, there is a struggle for survival among individuals.  Adaptation: Traits that improve an individual’s chances of survival (e.g., camouflage, speed) become more common in the population over generations.  Survival of the Fittest: Individuals with favorable adaptations are more likely to survive, reproduce, and pass these traits to the next generation. 2. Lamarckism (Theory of Inheritance of Acquired Characteristics) Lamarckism was proposed by Jean-Baptiste de Monet Lamarck in the year 1744-1829. This theory was based on the principle that all the physical changes occurring in an individual during its lifetime are inherited by its offspring. For eg., the development of an organ when used many times.  Use and Disuse: Body parts that are frequently used become stronger and more developed, while unused parts weaken and may disappear.  Inheritance of Acquired Traits: Traits developed over an organism's life due to environmental adaptation are passed to offspring. Example: Lamarck suggested that giraffes have long necks because their ancestors stretched their necks to reach leaves on tall trees. Critique: Lamarckism lacks evidence as acquired traits do not alter genetic information in a way that can be inherited. For instance, a bodybuilder’s muscles do not pass to their children. 3. Mendelian Laws of Inheritance Gregor Mendel, known as the "Father of Genetics," discovered basic principles of inheritance by studying pea plants. His findings led to the formulation of Mendelian laws: Law of Dominance This is also called Mendel’s first law of inheritance. According to the law of dominance, hybrid offspring will only inherit the dominant trait in the phenotype. The alleles that are suppressed are called the recessive traits while the alleles that determine the trait are known as the dominant traits. Law of Segregation The law of segregation states that during the production of gametes, two copies of each hereditary factor segregate so that offspring acquire one factor from each parent. In other words, allele (alternative form of the gene) pairs segregate during the formation of gamete and re-unite randomly during fertilization. This is also known as Mendel’s third law of inheritance. Law of Independent Assortment Also known as Mendel’s second law of inheritance, the law of independent assortment states that a pair of traits segregates independently of another pair during gamete formation. As the individual heredity factors assort independently, different traits get equal opportunity to occur together. Example: Crossing a plant with yellow seeds (dominant) and green seeds (recessive) results in yellow seeds if the yellow allele is present. 4. Key Genetic Terms Genetic Term Definition Gene A unit of heredity made up of DNA that codes for a specific trait or protein. Allele A variant form of a gene. Organisms typically have two alleles for each gene, one inherited from each parent. Genotype The genetic makeup of an organism, representing the combination of alleles inherited for a specific trait. Phenotype The observable physical or biochemical characteristics of an organism, determined by its genotype and influenced by environmental factors. Dominant Allele An allele that expresses its trait even when only one copy is present in the genotype (e.g., "A" in Aa). Recessive Allele An allele that is only expressed when two copies are present in the genotype (e.g., "a" in aa); otherwise, it is masked by a dominant allele. Homozygous A genotype in which an organism has two identical alleles for a particular gene (e.g., AA or aa). Heterozygous A genotype in which an organism has two different alleles for a particular gene (e.g., Aa). Haploid Is when an organism has one set of chromosomes in the gamete. Represented by n Diploid Is when an organism has two sets of chromosomes in the gamete. Represented by 2n Mutation A change or alteration in the DNA sequence, which can introduce new genetic variation or cause genetic disorders. DNA Chromosome A thread-like structure made of DNA and proteins that carry genes. Humans have 23 pairs of chromosomes. Locus The specific location or position of a gene on a chromosome. Genetic The exchange of genetic material between homologous chromosomes Recombination during meiosis, leading to new combinations of genes in the offspring. Meiosis A type of cell division that produces four haploid cells (gametes) with half the genetic material, crucial for sexual reproduction. Mitosis A type of cell division that produces two identical diploid daughter cells, necessary for growth, development, and tissue repair. Genome The complete set of genes or genetic material present in a cell or organism.  Codominance A situation where both alleles in a heterozygous genotype are fully expressed, resulting in a phenotype that shows both traits (e.g., AB blood type). Incomplete A condition where the heterozygous phenotype is intermediate Dominance between the two homozygous phenotypes (e.g., red and white flowers producing pink flowers). Sex-linked Traits Traits controlled by genes located on sex chromosomes (usually the X chromosome), such as color blindness or hemophilia. Genetic Drift A random change in allele frequencies in a population, especially in small populations, leading to variation in gene frequencies over time. Natural Selection The process where individuals with advantageous traits survive and reproduce more successfully, leading to an increase in those traits within a population. Polygenic A situation where multiple genes influence a single trait, resulting in Inheritance continuous variation (e.g., height, skin color). Pleiotropy A situation where one gene influences multiple, seemingly unrelated traits (e.g., a single gene affecting both hair and eye color). Epistasis A form of gene interaction where one gene masks or suppresses the expression of another gene. Gene Flow The transfer of genetic material between populations due to migration, contributing to genetic diversity. Examples: Q.1. What will be the appearance of (a) F1 and (b) F2 progenies when a pure (homozygous) tall pea plant is crossed with a pure (homozygous) dwarf pea plant? Tallness (T) gene is dominant over dwarfness (t) gene. Solution: Pure (homozygous) tall pea plant = TT Pure (homozygous) dwarf pea plant = tt (a) Parents: Thus, the off-springs of F1 generation will be heterozygous tall. (b) Here the F1 hybrids, i.e., heterozygous tall (Tt) are self-pollinated which may result into following possibilities: Therefore, 3 plants will be tall and one plant will be dwarf in F 2 generation showing a ratio of 3: 1. Results: (a) The result of F1 would be the production of heterozygous tall (Tt). (b) The result of F2 would be the production of tall and dwarf in a ratio of 3: 1. Out of 3 tall plants one would be pure (homozygous) tall (TT) and two would be heterozygous tall (Tt). The dwarf would be a pure homozygous dwarf (tt). Q.2. When a plant homozygous for tall is crossed with a plant homozygous for dwarf, what will be the appearance of the offsprings of a cross of F1 with its tall parent & dwarf parent. When a plant homozygous for tall (TD is crossed with a plant homozygous for dwarf (tt) the off-springs in the F1 will be the production of heterozygous tall (Tt). Now such a heterozygous tall (Tt), when back-crossed with its tall parent (TT), will show the following results: Q3. Find out the phenotypic value of the offspring of the following cross, in which the genotypes of the parents are given: YYRr X YyRr Q.15. Find out the phenotypic appearance of the off-springs of the following cross, in which the genotypes of the parents are given: YyRr X Yyrr Result: A phenotypic ratio of 3 (yellow round): 3 (yellow wrinkled): 1 (green round): 1 (green wrinkled) will be produced.  BIO 101 a. GENES b. Chromosomes GENES Genes are hereditary units or basic units of inheritance. Traits are controlled by genes. The complete sets of genes within an organism’s genome are called its genotype. The complete set of observable traits of the structure and behavior of an organism is called a phenotype. A heritable trait passes from one generation to the next via DNA (Deoxyribonucleic acid). DNA is a sequence of letters that spell out the genetic code. They are in the chromosome and are responsible for transmission of characters from parents to offspring. DNA organised into words and sentences are called genes. These genes are usually in pairs. Humans have approximately 20,000 genes and each one influences a part of development. One copy is inherited from the mother and one from the father. A change in the spelling of a DNA sequence or the gene is called mutation. Every person’s DNA contains mutation which are harmless. Some mutations however cause specific diseases. CHROMOSOMES Chromosomes are rod or thread-like bodies found in the nucleus of a cell. The chromosomes house and contain the genes. They are carriers of genetic materials. Chromosomes are stainable. Organisms inherit genetic materials from their parents in the form of homologous chromosomes. The specific location of a DNA sequence within chromosome is known as a locus. When DNA sequence within a particular locus change or varies between individuals, the different or alternate forms of this sequence (gene) are called alleles. The chromosome number varies from species to species. Table 1: CHROMOSOME NUMBER OF SOME ANIMALS AND PLANTS Man (Homo sapiens) 46 Goat (Copra hircus) 60 Common rat (Rattus rattus) 42 Frog (Rana tigrina) 26 Dog (Canis familiaris) 78 1  Chicken (Gallus domesticus) 78 Name of plants Diploid chromosome number (2n) Banana (Musa paradisiaca) 22, 44, 55, 77, 88 Orange (Citrus sinensis) 18, 27, 36 Bread mold (mucor heimalis) 2 Tomato ( Solanum lycopersicum ) 24 Rice (Oryza sativa) 24 Papaya (Carica papaya) 18 Structure and Behavior of Chromosome A chromosome can be considered a stainable threadlike nuclear component having special organisation, individuality and function. The term is taken from Greek word chromasoma which means “coloured bodies’’ (chroma = colour; soma = body). Chromosomes have marked affinity for basic dyes because of which they are stained. This property is known as chromaticity. Staining the cell with stains such as aceto orcein , aceto carmine shows that chromosome are not visible in the interphase nucleus or metabolically active nucleus due to their high water content, but can be easily seen during cell division. This is seen during both mitosis and meiosis. During cell division, the chromosome undergoes dehydration, spiralisation and condensation. Therefore, they become progressively thicker and smaller. The stainability of chromosome also increases. Hence the chromosome becomes readily observable under microscope. Therefore, staining of chromosome is generally carried out to make them visible under light microscope. 2  Properties of Chromosome  Chromosomes are capable of duplication and maintaining their morphological and physiological properties through successive cell divisions.  Chromosomes contain DNA (Deoxyribonucleic acids). This carries the genes and thus plays a major role in heredity.  During reproduction of organisms, they are passed on to the next generation through the gametes.  They play an important role in variation, mutation and evolution and in their control of morphogenesis, multiplication and equilibrium of vital processes. Types of Chromosomes The term chromosome is mainly used to describe the chromosome of eukaryotic cell. Most chromosomes found in these cells are called autosomes. This controls all somatic characteristic of an organism. The symbol for autosomes is A. Somatic cells are cells found in all the part of an organism i.e that make up the organism except the germ cells i.e the egg cells. Other types of chromosomes such as those for sex determination (x and y), B- chromosomes, L-chromosomes, M-chromosomes, S-chromosomes, E-chromosomes are called ALLOSOMES. This controls some specialized characteristics of an organism. Autosomes are found in all parts of a eukaryotic organism while allosomes may or may not be present in all organism. CHROMOSOME NUMBER Every cell of the body is provided with a packet of genetic material, the nucleus which contains chromosomes. The number of chromosomes is constant for a particular species. This is important in determining the phylogeny and taxonomy of the species. The number or set of chromosomes of the gamete is referred to as reduced or haploid (n) sets of chromosomes. The somatic or body cells of most organisms contain two haploid cells known as Diploid (2n). Structure of Eukaryotic chromosome CHROMATIDS: At mitotic metaphase each chromosome consists of two symmetrical structures called chromatids. Each chromatids contains a single DNA molecule. Both chromatids are attached to each other only by the centromere and become separated at the beginning of anaphase, when each sister chromatids of a chromosome migrate to the opposite poles. CHROMONEMA (TA): This is seen as very thin filaments during mitotic prophase (the term coined by Vejdovsky in 1912). It represents a chromatid in the early stages of condensation. Therefore, chromatid and chromonema are two names for same structure: a single linear DNA molecule with its associated proteins. The chromonemata form the gene- bearing portions of the chromosomes. MATRIX: (old view) a chromosome may have more than one chromonemata which are embedded in the achromatic and amorphous substance, called matrix. The matrix is 3 enclosed in a shell or pellicle. Both the matrix and pellicle are non-genetic materials and appear only at metaphase when the nucleolus disappears. It is believed that nucleolar materials and matrix are interchangeable. i.e when chromosomal matrix disappears, the nucleolus appears and vice versa. Electron microscopic observations have however queried the occurrence of pellicle and matrix in them. CHROMOMERES: These are bead-like accumulations of chromatin material that are sometimes visible along interphase chromosomes. They are regions of tightly folded DNA. SHAPE OF EUKARYOTIC CHROMOSOMES The shape of the chromosomes is changeable from phase to phase in the continuous process of the cell growth and cell division. In the resting phase or interphase stage of the cell, the chromosomes occur in the form of thin, coiled, elastic and contractile, thread-like stainable structures, the chromatin threads. Chromatin is a nucleoprotein complex in which DNA strand is associated with protein. The associated proteins are Basic proteins (HISTONES) and acidic proteins (NON- HISTONES). Both DNA-histone and histone-histone binding are important for chromatin structure and the histones found in all eukaryotic cell are H1 = very rich in lysine (molecular weight 21,500) H2A and H2B = lysin-rich (14,000 and 13,775 respectively) H3 and H4 = Arginine-rich (15,320 and 11,280 respectively) The molar ratios of histones are 1H1:2 H2A: 2H2B: 2H3: 2H4 4  They are found firmly associated with DNA-histone complex. The molecules of non- histone protein are numerous and heterogenous and may include enzyme involved in DNA and RNA synthesis. Non-histones contents vary throughout cell cycle from one cell to another but histones remain constant. There are two distinct types of chromatins Heterochromatin: This is the region of condensed chromosome which are stained darkly Euchromatin: This is the region of the chromatin that is slightly stained. EUCHROMATIN HETEROCHROMATIN Slightly stained Darkly stained 5 Uncoiled state and unclumped Higher order of folding coiled and clumped DNA active DNA inactive Replicates earlier Replicates later CENTROMERE: Each chromosome has a single differentiated region somewhere along its length that seem to act as the point at which force is exerted in the separation of dividing chromosome. These clear regions are termed centromere. In metaphase and anaphase, the chromosomes become thick and filamentous. Each chromosome contains a clear zone known as centromere or kinetocore, along their length. The centromere divides the chromosome into two parts, each part is called chromosome arm. Structure of eukaryotic chromosome 6  The position of centromere varies from chromosome to chromosome. It provides different shapes to the chromosome. It can be used to identify each chromosome i.e Telocentric, metacentric, submetacentric and acrocentric. VIRAL CHROMOSOME The chromosome of viruses consists of a single nucleic acid molecule. Either DNA or RNA, single or double stranded. They cannot replicate in the absence of their host cell as they depend on host biosynthesis mechanism to duplicate their genetic material and synthese in their protein coat. 7  BACTERIAL CHROMOSOME They are simple relatively in form compare with those of the eukaryotes. It consist of double stranded DNA molecules compacted into a structure referred to as nucleoid. E. coli has a large circular chromosome measuring approximately 1200µm in length. DNA found is associated with several types of DNA proteins and the proteins are small and contain high percentage of positively charged amino acids. 8 Bacterial chromosome structure Genetic disorder An inherited medical condition caused by a DNA abnormality. Most common types Down syndrome A genetic chromosome 21 disorder causing developmental and intellectual delays. Cystic fibrosis An inherited life-threatening disorder that damages the lungs and digestive system. Huntington's disease An inherited condition in which nerve cells in the brain break down over time. Duchenne muscular dystrophy 9 An inherited disorder of progressive muscular weakness, typically in boys. Sickle cell anemia A group of disorders that cause red blood cells to become misshapen and break down. Haemophilia A disorder in which blood doesn't clot normally. Thalassemia A blood disorder involving lower-than-normal amounts of an oxygen-carrying protein. Fragile X syndrome A genetic condition causing intellectual disability. At the end of this unit, you should know the following:  Definition of a gene  What is a genetic material?  Importance of the genetic material  Structure of the chromosome  Types of chromosomes  Shapes of chromosomes 10 SIGNIFICANCE OF CHROMOSOME IN CELLULAR REPRODUCTION AND DEVELOPMENT  In mitosis, the daughter cells receive precisely the same number and type of chromosome as the original parent cell i.e. diploid constitution is maintained from one generation of the cell to the other  Mitosis is the type of division during growth of an organism to healing of cuts. It is the basis of asexual reproduction Meiosis  This is also referred to as reduction division. It is a complex process and is of great significance in the life cycle of plants and animals which reproduces sexually.  If mitosis is the only method of cell division, after every act of fertilization, the chromosomes number of individuals will become doubled in subsequent generations. The increase in number of chromosomes is harmful and creates imbalance between cytoplasm and the nucleus which may ultimately cause several variations and malformations in the organisms.  Failure of meiosis leads to the formation of diploid gametes which after fertilization form polypoid forms. Importance of meiosis  With the exchange of chromatids during meiosis genes are interchanged.  The interchanged of genes leads to recombination of characters in the progeny  It is a reduction division and so prevent unnecessary doubling of genetic material from one generation to the other. Molecular Genetics Chromosome – Definition, Structure, Function, Examples Chromosomes are thread-like structures present in the nucleus. They are important because they contain the basic genetic material DNA. These are present inside the nucleus of plants as well as animal cells. Chromosomes were first discovered by Strasburger in 1815 and the term ‘chromosome’ was first used by Waldeyer in 1888. Human beings have 46 chromosomes in their body. These are arranged into 23 pairs. Let us discuss the chromosome structure in detail. 11  Definition of Chromosome “A Chromosome looks like a thread and is coiled material, made of proteins. Chromosomes are present in the nucleus of all the cells and contain the basic genetic material DNA, which passes from one generation to another”. Structure: A chromosome has generally 8 parts; Centromere or primary constriction or kinetochore, chromatids, chromatin, secondary constriction, telomere, chromomere, chromonema, and matrix.  Centromere or Kinetochore: It is the primary constriction at the center to which the chromatids or spindle fibers are attached. Its function is to enable movement of the chromosome during the anaphase stage of cell division.  Chromatid: During cell division, a chromosome is divided into 2 identical half strands joined by a centromere. A chromatid is each half of the chromosome joined. Each chromatid contains DNA and separates at Anaphase to form a separate chromosome. Both chromatids are attached to each other by the centromere.  Chromatin: It is a complex of DNA and proteins that forms chromosomes within the nucleus of eukaryotic cells. Nuclear DNA is highly condensed and wrapped around nuclear proteins in order to fit inside the nucleus. In other words, it is not present as free linear strands. The chromatin consists of DNA, RNA, and protein.  Secondary Constriction: It is generally present for the nucleolar organization.  Telomere: Telomere is the terminal region of each side of the chromosome. Ach chromosome has 2  Chromonema: It is a threadlike coiled filamentous structure along which chromomeres are arranged. Chromonema controls the size of the chromosome and it acts as a site of gene bearing.  Chromomeres: These are the bead-like structures present on threads or chromonema. These are arranged in a row along the length of chromonema. The number of chromosomes is constant and it is responsible for carrying the genes during cell division to the next generation.  Matrix: Pellicle is the membrane surrounding each of the chromosomes. Matrix is the jelly-like substance present inside pellicle. It is formed of non-genetic materials. Functions of Chromosomes For the first time, Sutton and Bover suggested the role of chromosomes in heredity in 1902. 1. The most important function of chromosomes is to carry the basic genetic material – DNA. DNA provides genetic information for various cellular functions. These functions are essential for growth, survival, and reproduction of the organisms. 2. Histones and other proteins cover the Chromosomes. These proteins protect it from chemical (e.g., enzymes) and physical forces. Thus, chromosomes also perform the 12  function of protecting the genetic material (DNA) from damage during the process of cell division. 3. During cell division, spindle fibers attached to the centromeres contract and perform an important function. The contraction of centromeres of chromosomes ensures precise distribution of DNA (genetic material) to the daughter nuclei. 4. Chromosomes contain histone and non-histone proteins. these proteins regulate gene action. Cellular molecules that regulate genes work by activating or deactivating these proteins. This activation and deactivation expand or contract the chromosome. Examples of Types of Chromosomes  Metacentric Chromosomes: Metacentric chromosomes have the centromere present exactly in the center. Both the sections are metacentric chromosomes are therefore of equal length. Example: Human chromosome 1 and 3 are metacentric.  Submetacentric Chromosomes: In Submetacentric chromosomes, the centromere is not present exactly at the center. The centromere is slightly offset from the center. Both the sections are therefore not of equal length or are asymmetrical. Example: Human chromosomes 4 to 12 are submetacentric.  Acrocentric Chromosomes: Acrocentric chromosomes have a centromere which is highly offset from the center. Therefore, one of the strands is very long and one very short. Example: Human chromosomes 13,15, 21, and 22 are acrocentric.  Telocentric Chromosomes: In telocentric chromosomes, the centromere is present at the very end of the chromosome. Telocentric chromosomes are present in species such as mice. Humans do not possess telocentric chromosomes. Question on Chromosome Structure Q: How do genetic disorders occur? Ans: When DNA sequence in chromosomes changes, genetic disorders occur. The mutation refers to a change in the DNA sequence. The monogenic disorder occurs when a mutation occurs in one gene. Multifactorial genetic disorder occurs when mutation occurs in multiple genes. Human beings have 23 pairs of chromosomes. All the diseases have a genetic component and the mutations pass from one generation to the other. Cancer, diabetes, obesity are examples of such diseases. 13