Bio Cell Biology PDF

A cell is the smallest unit that can live on its own and that makes up all living organisms and the tissues of the body. most cells are between 1 and 100 micrometres in diameter, and their components are even smaller, as are viruses. Light microscopes use visible light and magnifying lenses to all...

A cell is the smallest unit that can live on its own and that makes up all living organisms and the tissues of the body. most cells are between 1 and 100 micrometres in diameter, and their components are even smaller, as are viruses. Light microscopes use visible light and magnifying lenses to allow us to see cells Fluorescent light microscopes capture fluorescence (a form of glow) Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation The locations of specific molecules in the cell can be revealed by labelling the molecules with fluorescent molecules Electron microscopes use beans of electrons A transmission electron microscope (TEM) is used to study the internal structure of thin sections of cells A scanning electron microscope (SEM) is used to study the fine details of cell surfaces Most of a cell is water (70%). The remaining 30% contains varying proportions of structural and functional molecules. Large molecules are either lipids or polymers 1.Polysaccharides = polymers 2\. Lipids = not true polymers but can be large molecules 3\. Nucleic acids (DNA, RNA) = polymers 4\. Proteins = polymers On the molecular scale, members of three of these classes -- carbohydrates, proteins, and nucleic acids -- are huge and are therefore called macromolecules Macromolecules are polymers, built from monomers A polymer is a long molecule consisting of many similar or identical building blocks linked by covalent bonds In addition to forming polymers, some monomers also have other functions of their own Polymers grow by dehydration reaction's reaction. Polymers are disassembled to monomers by hydrolysis Hydrolysis is essentially the reverse of the dehydration reaction Hydrolysis means water breakage Polysaccharides serve as fuel and as building material Monosaccharides generally have molecular formulas that are some multiple of the unit CH2O. Glucose (C6H12O6) In aqueous solutions, glucose molecules, as well as most other five- and six-carbon sugars, form rings A disaccharide consists of two monosaccharides joined by a glycosidic linkage Polysaccharides are macromolecules, polymers with a few hundred to a few thousand monosaccharides joined by glycosidic linkages Both plant and animals store sugars in the form storage polysaccharides Animals store glycogen Plants store starch, a mix of amylopectin and amylose Vertebrates store glycogen in liver and muscle cells. Hydrolysis of glycogen in these cells releases glucose when the demand for sugar increases Organisms build strong materials from structural polysaccharides Chitin is the carbohydrate used by arthropods to build their exoskeletons The differing glycosidic linkages in starch and cellulose give the two molecules distinct three-dimensional shapes A dehydration reaction joins two glucose molecules to form maltose. The formula for glucose is C6H12O6. What is the formula for maltose? Lipids are a diverse group of hydrophobic molecules Lipids are grouped with each other because they share one important trait: They mix poorly, if at all, with water Lipids are the components of cell membranes Lipids are fats, phospholipids, and steroids A fatty acid has a long carbon skeleton, usually 16 or 18 carbon atoms in length The carbon at one end of the skeleton is part of a carboxyl group, the functional group that gives these molecules the name fatty acid The rest of the skeleton consists of a hydrocarbon chain A fat is constructed from two kinds of smaller molecules: glycerol and fatty acids Triacylglycerols are three fatty acid molecules are each joined to glycerol Phospholipids are only two fatty acids attached to glycerol. The third hydroxyl group of glycerol is joined to a phosphate group The terms saturated fats and unsaturated fats refer to the structure of the hydrocarbon chains of the fatty acids Unsaturated fat has less hydrogens, thus double bonding A phospholipid has a hydrophilic (polar) head and two hydrophobic (nonpolar) tails. Phospholipids are essential for cells because they are major constituents of cell membranes Steroids are lipids characterized by a carbon skeleton consisting of four fused rings Cholesterol is the molecule from which other steroids, including the sex hormones, are synthesized Cells make their own proteins. Proteins are polymer of amino acids Proteins include a diversity of structures, resulting in a wide range of function Proteins account for more than 50% of the dry mass of most cells, and they are instrumental in almost everything organisms do. Enzymatic proteins accelerate specific chemical reactions Antibodies are proteins Storage proteins serve as a source of amino acids Proteins mediate the selective transport of substances Proteins account for more than 50% of the dry mass of most cells, and they are instrumental in almost everything organisms do Cells are 70% water, 15% proteins, 4% small molecules, 6% RNA,2% phospholipids, 1% DNA, 2% polysaccharides An amino acid is an organic molecule with both an amino group and a carboxyl group (side chain is variable) Proteins are made up of 20 amino acids, which have different physical and chemical properties Polypeptides are Amino Acid Polymers The covalent bond between amino acids is called a peptide bond Proteins have four levels of protein structure The primary structure is the sequence of amino acids The secondary structure refers to the regions stabilized by hydrogen bonds between atoms of the polypeptide backbone. Alpha helices and beta-strands are the main secondary structures Tertiary structure is the overall shape of a polypeptide resulting from interactions between the various amino acids Some proteins consist of two or more polypeptide chains aggregated into one functional macromolecule. Quaternary structure is the overall protein structure that results from the aggregation of these polypeptide subunits. Example of quaternary structure: collagen is a fibrous protein that has three identical helical polypeptides intertwined into a larger triple helix Example of quaternary structure: hemoglobin, the oxygen-binding protein of red blood cells Nucleic acids store, transmit, and help express hereditary information The two types of nucleic acids, deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) DNA -- Sugar = deoxyribose, Nitrogenous bases = C, G, A, T, usually double-stranded, stores hereditary information RNA -- sugar=ribose, nitrogenous bases=C,G,A,U, usually singe-stranded, various functions in gene expression, including carrying instructions from DNA to ribosomes. DNA-\>RNA-\>Protein In a eukaryotic cell, DNA in the nucleus programs protein production in the cytoplasm by dictating synthesis of messenger RNA (mRNA) Nucleic acids are macromolecules that exist as polymers called polynucleotides. The monomers are called nucleotides. A nucleotide, in general, is composed of three parts: a nitrogenous base, a five-carbon sugar, and one phosphate group There are two families of nitrogenous bases: pyrimidines and purines Deoxyribose lacks an oxygen atom on the second carbon DNA molecules have two polynucleotides or "strands", that wind around an imaginary axis, forming a double helix (antiparallel) RNA molecules exist as single stands. Complementary base pairing occurs, between two stretches of nucleotides in the same RNA molecule Base paring within an RNA molecule allows it to take on the three-dimensional shape necessary for its function. In base pairing only certain bases are compatible with each other Adenine (A) pairs with thymine (T) Guanine G) pairs with cytosine (C) The nucleus, the mitochondria, and the chloroplast contain DNA Nuclear DNA is in the nucleus, encodes for majority of the genome in eukaryotes Mitochondrial DNA is the circular chromosome found inside the cellular organelles called mitochondria Chloroplast DNA is the DNA located in chloroplasts, which are photosynthetic organelles located within the cells in some eukaryotic organisms DNA in the nucleus programs protein production in the cytoplasm by dictating synthesis of messenger RNA. Genomics and proteomics have transformed biological inquiry and applications Geonomics is the analysis of large sets of genes or whole genomes of different species, an approach called (DNA Sequencing) Proteomics is the analysis of large sets of proteins, including their sequences. Genome sequencing is the process of determine the entirety, or nearly the entirety, of the DNA sequence of an organism's genome DNA and proteins as tape measures of evolution Because DNA carries heritable information in the form of genes, sequences of genes and their proteins products document the hereditary background of an organism. Eukaryotic cells have internal membranes that compartmentalize their functions A prokaryotic cell lacks a true nucleus and the other membrane-enclosed organelles In a prokaryotic cell, the DNA is concentrated in a region that is not membrane-enclosed, called the nucleoid In a eukaryotic cell, most of the DNA is in an organelle called the nucleus, which is bounded by a double membrane The interior of either type of cell is called the cytoplasm -- the region between the nucleus and the plasma membrane The plasma membrane and the membranes of organelles consist of a double layer\ (bilayer) of phospholipids with various proteins attached to or embedded in it. The hydrophobic parts of phospholipids and membrane proteins are found in the\ interior of the membrane, while the hydrophilic parts are in contact with the\ aqueous solutions on either side Carbohydrate side chains may be attached to proteins or ilipids on the outer surface of the plasma membrane. A phospholipid is an amphipathic molecule, meaning it has both a hydrophilic\ ("water-loving") region and a hydrophobic ("water-fearing") region The eukaryotic cell's genetic instructions are housed in the nucleus and carried out by ribosomes The nucleus houses most of the cell's DNA, and the ribosomes use information from the DNA to make proteins Transcription happens in the nucleus and translation in the cytoplasm The nuclear envelope encloses the nucleus, separating its contents from the cytoplasm The nuclear envelope is composed of two membranes Nuclear pores made by the nuclear pore complex regulates the entry and exit of proteins and RNAs, as well as large complexes of macromolecules The nucleolus (plural, nucleoli) is where a type of RNA called ribosomal RNA (rRNA) is synthetized Ribosomes, which are complexes made of ribosomal RNAs and proteins, are the cellular components that carry out protein synthesis. Ribosomes build proteins in two cytoplasmic locales. Free ribosomes are suspended in the cytosol, while bound ribosomes are attached to the outside of the endoplasmic reticulum or nuclear envelope Most of the proteins made on free ribosomes function within the cytosol. Bound ribosomes make ribosomes make membrane proteins and secreted proteins. In eukaryotic cells, mitochondria and chloroplasts are the organelles that convert energy to forms that cells can use for work Mitochondria are the sites of cellular respiration, the metabolic process that uses oxygen to extract energy from sugars, fats, and other fuels Chloroplasts, found in plants and algae, are the sites of photosynthesis Mitochondria and chloroplasts have two membranes surrounding them Chloroplasts also have a third internal membrane called the thylakoid membrane Like prokaryotes, mitochondria and chloroplasts contain ribosomes, as well as multiple circular DNA molecules associated with their inner membranes. the endosymbiont theory states that an early\ ancestor of eukaryotic cells engulfed an\ oxygen-using non-photosynthetic prokaryotic cell\ and established an endosymbiotic relationship. The ancestors of mitochondria were oxygen-using non-photosynthetic prokaryotes; The ancestors of chloroplasts were photosynthetic prokaryotes The mitochondria outer membranes is smooth, but the inner membrane is convoluted, with infoldings called cristae. Mitochondria fuse, divide, and are very dynamic Chloroplasts contain the green pigment chlorophyll, along with enzymes and other molecules that function in the photosynthetic production of sugar The chloroplast is a specialized member of a family of closely related plant organelles called plastids Peroxisomes have a single membrane and contain enzymes that produce hydrogen peroxide H2O2. Peroxisomes use oxygen to break fatty acids into smaller molecules The different membrane-bound organelles of the eukaryotic cell are part of the endomembrane system, - the nuclear envelope - the endoplasmic reticulum - the Golgi apparatus - lysosomes - vesicles and vacuoles - and the plasma membrane George Palade used electron microscopy to give snapshots of the secretory pathway Made proteins radioactive, and followed them through the pathway RER \> Golgi \> Vesicles \> Membrane The endomembrane are related either through direct physical continuity or by the transfer of membrane segments as tiny vesicles (sacs made of membrane) The endoplasmic reticulum consists of a network of membranous tubules and sacs called cisternae (from the Latin cisterna, a reservoir for a liquid) Smooth ER is so named because its outer surface lacks ribosomes. Rough ER is studded with ribosomes on the outer surface of the membrane and thus appears rough through the electron microscope Ribosomes, which are complexes made of ribosomal RNAs and Proteins, are the cellular components that carry out protein synthesis\ Ribosomes build proteins in two cytoplasmic locales. Free ribosomes are suspended in the cytosol, while bound ribosomes are attached to the outside of the endoplasmic reticulum or nuclear envelope. Ribosomes make proteins based on where they are located, free ribosomes make proteins in the cytoplasm The smooth ER functions in diverse metabolic processes which vary with cell type. These processes include synthesis of lipids, metabolism of carbohydrates, detoxification of drugs and poisons, and storage of calcium ions. The rough ER makes membrane proteins and proteins to be secreted The proteins made by the RER ribosomes are fed into the lumen of the ER Secretory proteins depart from the ER wrapped in the membranes of vesicles called transport vesicles Most secretory proteins are glycoproteins, protein with carbohydrates covalently bonded to them. The carbohydrates are attached to the proteins in the ER lumen by enzymes In addition to making secretory proteins, rough ER is a membrane factory for the cell The Golgi Apparatus: shipping and receiving centre Consists of flattened membranous sacs -- cisternae -- looks like a stack of pita bread The Golgi apparatus consists of stacks of flattened sacs, or cisternae, which\ (unlike ER cisternae) are not physically connected A Golgi stack ha distinct structural directionality, with the membranes of cisternae on opposite sides of the stack differing in thickness and molecular composition. Secretory proteins depart from the ER wrapped in the membranes of vesicles called transport vesicles Cis face is the receiving end and trans face is the shipping side The Golgi manufactures and refines its products in stages, with different cisternae containing unique teams of enzymes The cisternae of the Golgi progress forward from the cis to the trans face, carrying and modifying their cargo as they move Golgi stack dispatches its product by budding vesicles from the trans face, it sorts these products and targets them for various parts of the cell Molecular identification tags, such as phosphate groups or sugars added to the Golgi products, aid in sorting by the acting like a postal code on mailing label transport vesicles budded from the Golgi may have external molecules on their\ membranes that recognize "docking sites" on the plasma membrane, thus\ targeting the vesicles Lysosomes: Digestive Compartments A lysosome is a membranous sac of hydrolytic enzymes that many eukaryotic cells use to digest (hydrolyze) macromolecules Lysosomal enzymes work best in the acidic environment found in lysosomes If a lysosome breaks open or leak, the released enzymes are not very active because the cytosol has a new-neutral pH The cell secretes certain molecules by the fusion of vesicles with the plasma\ membrane; this process is called exocytosis. In endocytosis, the cell takes in\ molecules and particulate matter by forming new vesicles from the plasma\ membrane. Unicellular eukaryotes eat by engulfing smaller organisms or food particles, a process called phagocytosis In phagocytosis, a cell engulfs a particle by extending pseudopodia (singular, pseudopodium) around it and packaging it within a membranous sac called a food vacuole. In pinocytosis, a cell continually "gulps" droplets of extracellular fluid into tiny vesicles, formed by infoldings of the plasma membrane The food vacuole in this way then fuses with a lysosome, whose enzymes digest the food Lysosomes also use their hydrolytic enzymes to recycle the cell's own organic material, a process called autophagy. (Using its own organelles as food) During autophagy, a damaged organelle becomes surrounded by a double membrane, and a lysosome fuses with the outer membrane of this vesicle Vacuoles are large vesicles derived from the ER and GA Contractile vacuoles pump excess water out of the cell, thereby maintaining a suitable concentration of ions and molecules inside the cell. Mature plant cells generally contain a large central vacuole is the plant cell's main repository of inorganic ions The cytoskeleton is a network of fibres that organizes structures and activities in the cell. The cytoskeleton gives mechanical support to the cell and maintains its shape. This is especially important for animal cells, which lack walls The cytoskeleton is dynamic. It can be quickly dismantled in one part of the cell and reassembled in a new location, changing the shape of the cell. Bacterial cells also have fibres that form a type of cytoskeleton although with different proteins. The term cell motility includes both changes in cell location and movements of cell parts, and requires interaction of the cytoskeleton with motor proteins Cytoskeletal elements and motor proteins work together with plasma membrane molecules to allow whole cells to move. Microtubules are the thickest of the three types; microfilaments (also called actin filaments) are the thinnest' and intermediate filaments are fibres with diameters in a middle range. Microtubules are hollow rods constructed from a globular protein called tubulin One end can accumulate or release tubulin dimers at a much higher rate than the\ other, thus growing and shrinking significantly during cellular activities. (The "plus\ end,") Microtubules are polar structures with two distinct ends: a fast growing-plus end and a slow-growing minus end Microtubules shape and support the cell and serve as tracks along which organelles equipped with motor proteins can move In animal cells, microtubules grow out from a centrosome, a region that is often located near the nucleus. These function as compression-resisting girders of the cytoskeleton. Anchored by the minus end All face the same way (plus end is all together and minus end is by centrosome) Kinesins are motor proteins that move vesicles and organelles along microtubule. A flagellum had an undulating motion like a tail of a fish. In contrast, cilia work more like oars, with alternating power and recovery strokes, much like the oars of a racing crew boat. Both are made of microtubules. Bending of flagella and motile cilia involves large motor proteins called dyneins that are attached along each outer microtubule. Two will move on one side then switch to other side, and continue to alternate to move Microfilaments are thin solid rods. They are also called actin filaments because they are built from molecules A three-dimensional network formed by microfilaments just inside the plasma membrane (cortical microfilaments) help support the cell's shape. In some kinds of animal cells, such as nutrient-absorbing intestinal cells, bundles of microfilaments make up the core of microvilli, delicate projections that increase the cell's surface area. Myosin is a microfilament motor protein The sliding of myosin over actin drives muscle contraction Intermediate filaments are a diverse class of cytoskeletal elements. Keratin is the most common Unlike microtubules and microfilaments, which are found in all eukaryotic cells, intermediate filaments are only found in some animals, including vertebrates but not insects. Extracellular components and connections between cells help coordinate cellular activates Although animal cells lack cell walls, they do have an elaborate extracellular matrix (ECM). The main ingredients of the ECM are glycoproteins and other carbohydrate-containing molecules secreted by the cells. Fibronectin and other ECM proteins bind to cell-surface receptor proteins called integrins that are built into the plasma membrane. Integrins span the membrane and bind on their cytoplasmic side to associated proteins attached to microfilaments of the cytoskeleton. The collagen fibres are embedded in a network woven out of proteoglycans secreted by cells. A proteoglycan molecule consists of a small core protein with many carbohydrate chains covalently attached, so that it may be up to 95% carbohydrate. Cells in an animal or plant are organized into tissues, organs, and organ systems. Neighbouring cells often adhere, interact, and communicate via sites of direct physical contact. In animals, there are three main types of cell junctions: tight junctions. Desmosomes, and gap junctions. Tight junctions prevent fluid from moving across a layer of cells. Waterproof At tight junctions, the plasma membranes of neighbouring cells are very tightly pressed against each other, bound together by specific proteins. Forming continuous seals around the cells, tight junction establishes a barrier that prevents leakage of extracellular fluid across a layer of epithelial cells. (skin cells) Desmosomes are strong connections between cells. Desmosomes function like rivets, fastening cells together into strong sheets. Intermediate filaments made of sturdy keratin proteins anchor desmosomes in the cytoplasm. (muscle cells) Gap junctions provide cytoplasmic communicating channels. Gap junctions provide cytoplasmic channels from one cell to an adjacent cell and in this way are similar in their functions to the plasmodesmata in plants. Gap junctions consist of membrane proteins that surround a pore through which ions, sugars, amino acids, and other small molecules may pass. (communication between many types of tissues) Cytoplasm of one plant cell is continuous with the cytoplasm of its neighbours via plasmodesmata, cytoplasmic channels through the cell walls (TEM) Cells arrange their internal organization according to their cellular functions ATP powers cellular work by coupling exergonic reactions to endergonic reactions Anabolic -- Synthesis of more complex compounds, endergonic (requires input of energy) Catabolic -- disassembly of complex molecules, exergonic (release energy) ATP (Adenosine triphosphate) contains the sugar ribose, with the nitrogenous base adenine and a chain of three phosphate groups (the triphosphate group) bonded to it The bonds between the phosphate groups of ATP can be broken by hydrolysis. The reaction is exergonic and releases energy. The cell's proteins harness the energy released during ATP hydrolysis in several ways to perform the three types of cellular work -- chemical, transport, and mechanical ATP hydrolysis causes changes in the shapes and binding affinities of proteins ATP is a renewable resource that can be generated by the addition of phosphate to ADP. The free energy required to phosphorylate ADP comes from exergonic breakdown reactions (catabolism) in the cell. Enzymes speed up metabolic reactions by lowering energy barriers A spontaneous chemical reaction occurs without any requirement for outside energy, but it may occur so slowly that it is imperceptible. (take very long) An enzyme is a macromolecule that acts as a catalyst, a chemical agent that speeds up a reaction without being consumed by the reaction. The initial investment of energy for starting a reaction -- the energy required to contort the reactant molecules so the bonds can break -- is known as the free energy of activation. Or activation. The reactant an enzyme acts on is referred to as the enzyme's substrate Only a restricted region of the enzyme molecule binds to the substrate. This region, called the active site, is typically a pocket or groove on the surface of the enzyme. the active site catalyzes the reaction, and the product is released. The enzyme is\ then free to take another substrate molecule Many enzymes require non protein helpers or cofactors for catalytic activity, often for chemical processes like electron transfers that cannot easily be carried out by the amino acids in proteins. Enzyme inhibitors selectively inhibit the action of specific enzymes. They can be competitive or noncompetitive A competitive inhibitor mimics the substrate competing for the active site. A noncompetitive inhibitor binds to the enzyme away from the active site, alternating the shape of the enzyme so that even if the substrate can bind, the active site functions less effectively, if at all. Metabolic pathways that release stored energy by breaking down complex molecules are called catabolic pathways. Anabolic -- synthesis of more complex compounds (endergonic) Catabolic -- disassembly of complex molecules (exergonic) In a redox reaction, the loss of electrons from on substance is called oxidation, and the addition of electrons to another substance is known as reduction. Oxidization and reduction with respect to hydrogen transfer -- oxidation is the loss of hydrogen, reduction is the gain of hydrogen Oxidization and reduction with respect to oxygen transfer -- oxidation is the gain of oxygen, reduction is the loss of oxygen Glycolysis, which occurs in the cytosol, begins the degradation process by breaking glucose into two molecules of a compound called pyruvate In eukaryotes, pyruvate enters the mitochondrion and is oxidized to a compound called acetyl CoA, which enters the citric acid cycle The citric acid cycle is also called the tricarboxylic acid cycle or the Krebs cycle After pyruvate is oxidized the citric acid cycle completes the energy-yielding oxidation of organic molecules. The hydrogen atoms are not transferred directly to oxygen, but instead are usually passed first to an electron carrier, a coenzyme called NAD+ Dehydrogenases are enzymes that remove a pair of hydrogen atoms from the substrate, thereby oxidizing it. The electron transport chain is a series of proteins that pass electrons from NADH to Oxygen, while accumulating protons (H+ ions) across a membrane. The result of the ETC is the accumulation of proton in between the two membranes of the mitochondria ATP synthase uses the energy of an existing ion gradient to power ATP synthesis The power source for the ATP synthase is a difference in the concentration of H+ on opposite sides of the inner mitochondrial membrane ADP + Pi + Squeezing force = ATP When a unicellular cell divides, it is actually reproducing, since the process gives rise to a new organism (another cell) For multicellular eukaryotes, cell division enables each of them to develop from a single cell -- the fertilized egg Cell division continues to function in renewal and repair in fully grown multicellular eukaryotes, replacing cells that die. Your body produces 100 billion blood cells each day. A cell's DNA, its genetic information, is called its genome Each human cell has a total of 6 billion base pairs of DNA Each base pair is around 0.34 nanometres long Each diploid cell contains 2m of DNA Chromatin refers to a mixture of DNA and proteins that form the chromosomes found in cells of humans and other higher organisms A nucleosome is the basic repeating subunit of chromatin packaged inside the cell's nucleus DNA -\> Nucleosomes (added histones) -\> fibre that loops -\> Chromatid DNA molecules are packaged into structures called chromosomes Before the cell divide to form genetically identical daughter cells, all this DNA must be copied, or replicated A cell cycle is a series of events that takes place in a cell as it grows and divides Cell cycle -- Interphase (G1) is a growth period, S phase where chromosomes duplicate, G2 is the last part of interphase, M phase, mitosis occurs, cytokinesis divides the cytoplasm Each duplicated chromosome consists of two sister chromatids, which are joined copies of the original chromosome Each chromatid has a centromere, a region in the chromosomal DNA where the chromatids are attached most closely. When a cell is not dividing, each chromosome is in the form of a long, thin chromatin fibre Duplicated their centrosomes (anchor for microtubules)\ After DNA replication, the chromes condense as a part of cell division Mitosis is the division of the genetic material in the nucleus, is usually followed immediately by cytokinesis, the division of cytoplasm Mitosis is just one part of the cell cycle, the life of a cell from the time it is first formed during division of a parent cell until its own division into two daughter cells. The miotic phase alternates with a much longer stage called interphase, Interphase can be divided into three phases: the G1 phase ("first gap") the S phae (synthesis) and the G2 phase (2^nd^ gap) Mitosis is conventionally broken down into five stages: prophase, prometaphase, metaphase, anaphase, and telophase The mitotic spindle is a structure consists of fibres made of microtubules and associated proteins. Microtubules are hollow rods constructed form a globular protein called tubulin Each tubulin protein is a dimer, a molecule made up of two subunits A tubulin dimer consists of two slightly different polypeptides, alpha-tubulin and beta-tubulin Kinetochore connects the microtubules to the chromosomes The assembly of spindle microtubules starts at the centrosome, a subcellular region that organizes the cell's microtubules. Microtubules grow out from a centrosome is a region that is often located near the nucleus. These microtubules functioning as compression-resisting grinders of the cytoskeleton. Each of the two sister chromatids of a duplicated chromosome has a kinetochore, a structure made up of proteins that have assembled on specific section of DNA at each centromere. Cytokinesis is the physical process of cell division, which divides the cytoplasm of a parental cell into two daughter cells The place where cytokinesis occurs is called the cleavage furrow On the cytoplasmic side of the furrow is a contractile ring of actin microfilaments associated with molecules of the protein myosin Microfilaments are thin solid rods built from molecules of actin subunits. Myosin is a microfilament motor protein The eukaryotic cell cycle is regulated by a molecular control system The frequency of cell division varies with the type of cell. -Skin cells divide frequently \- Liver cells maintain the ability to divide but keep it in reserve until it is required \- Neurons and muscle cells, do not divide at all The eukaryotic cell cycle is regulated by a molecular control system composed of cyclins and cyclin-dependent kinases A kinase is an enzyme that adds phosphate groups to other molecules and modulate protein functions To be active, cyclin-dependent kinases (Cdks) must be attached to a cyclin, a protein that gets its name from its cyclically fluctuating concentration in the cell. Phosphorylation of various proteins of the nuclear lamina promotes fragmentation of the nuclear envelope during prometaphase of mitosis. The steps of the cell cycle are timed by rhythmic fluctuations in the activity of cyclin-dependant kinases (Cdks) The activity of a Cdk rises and falls with changes in the concentration of its cyclin partner Cells have built in stop signals that halt the cell cycle at checkpoints until overridden by go-ahead signals. G1 checkpoint -- must pass before DNA will replicate G2 checkpoint -- must pass before cell starts prophase M checkpoint -- Must pass before cell starts anaphase External signals are converted to responses within the cell Unicellular organisms identify their sexual mates by chemical signalling In yeast, there are two sexes, or mating\ types, called a and ɑ. Each type\ secretes a specific factor that binds to\ receptors only on the other type of cell.\ When exposed to each other's mating\ factors, a pair of cells of opposite type\ change shape, grow toward each other,\ and fuse (mate). Bacterial cells secrete molecules that can be detected by other bacterial cells. Sensing the concentration of such signalling molecules allows bacteria to monitor the local density of cells, a phenomenon called quorum sensing Cells in a multicellular organism usually communicate via signalling molecules targeted for cells that may or may not be immediately adjacent Communication by direct contact between cells is a type of local signalling a\) Cell junctions, both animal and plant cells have cell junctions that allow molecules to pass readily between adjacent cells without crossing plasma membranes Gap junctions between animal cells Plasmodesmata between plant cells b\) Cell-cell recognition, two cells in an animal may communicate by interaction between molecules protruding from their surfaces The orientation of hairs is an example of direct contact signaling Paracrine signalling is another type of local signalling, in which molecules are secreted by the signalling cell and these molecules travel only short distances A more specialized type of local signalling called synaptic signalling occurs in the animal nervous system. Synaptic signalling -- a nerve cell releases neurotransmitter molecules into a synapse, stimulating the target cell, such as a muscle or nerve cell Muscle contractions involve synaptic signalling between a neuron and a muscle A ligand-gated ion channel is a type of membrane channel receptor containing a region that act as a 'gate' opening or closing the channel when the receptor changes shape Axon terminals, synaptic boutons are small swellings that are found at the terminal ends of axons. They are typically the sites where synapses with other neurons are found, and neurotransmitters are stored. Both animals and plants use chemicals called hormones for long-distance signalling Cell signalling can be divided into three stages: Signal reception, signal transduction, and cellular response. Reception: A signalling molecule binds to a receptor protein, causing it to change shape Most water-soluble signalling molecules bind to specific sites on transmembrane receptor proteins that transmit information from the extracellular environment to the inside of the cell A G protein-coupled receptor (GPCR) works with the help of a G protein, a protein that binds the energy-rich molecule GTP GTP is like ATP but with guanine instead of adenine. It mostly is used for signaling Receptor tyrosine kinases (RTKs) belong to a major class of plasma membrane receptors characterized by having enzymatic activity. A kinase is any enzyme that catalyzes the transfer of phosphate groups. Phosphorylation is a widespread mechanism for regulating protein activity. Phosphorylation can activate or inactivate proteins Intracellular receptor proteins are found in either the cytoplasm or nucleus of target cells. Steroid hormones have intracellular receptors Transduction: Cascades of molecular interactions relay signals from receptors to target molecules in the cell In a phosphorylation cascade, a series of different proteins in a pathway are phosphorylated in turn, each protein adding a phosphate group to the next one in line Small molecules and ions can also act as second messengers Calcium ions (Ca2+) and inositol triphosphate (IP3) are other examples of second messengers Enzyme cascades amplify the cell's response to a signal Response: Cell signalling leads to regulation of transcription or cytoplasmic activities Many signalling pathways ultimately regulate protein synthesis, usually by turning specific genes on or off in the nucleus. A signalling pathway may regulate the activity of proteins