Exam 2 Notes
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Date Created: 10/07/15
Chapter 7 Cell Structure and Function Eukaryotic cells have a nucleus Prokaryotic cells do not Eukaryotic Cell Organelles Nucleus genetic information storage and processing contains chromatin DNA amp associated proteins enzymes for mRNA synthesis and processing enclosed by double membrane nuclear envelope Nucleolus ribosome synthesis Ribosomes protein synthesis Rough ER protein export Smooth ER protein export lipid synthesis amp in liver cells detoxi cation Golgi apparatus Membrane and export proteins packagedprocessed Peroxisomes lipid oxidation Mitochondria Krebs cycle production of ATP by oxidative phosphorylation Has own DNA double membrane Chloroplasts photosynthesis Has its own DNA double membrane Lysosomes Cellular digestion formed from budding off of ER Centrioles nucleate microtubules Cytoskeleton Cell movement amp shape The Nucleus Genetic information Site of transcription DNA l mRNA 0 Translation occurs in cytoplasm mRNA l protein with the help of ribosomes and tRNA Many proteins have an amino acid quotzip codequot signal that targets them to particular locations in the cell The nucleus is surrounded by a double membrane 0 Membrane has nuclear pores that regulate entrance and exit tofrom nucleus 0 Proteins must have a nuclear localization sequence to pass through a nuclear pore Endomembrane System ER golgi apparatus and small transport vesicles Rough ER studded with ribosomes Smooth ER tubular no ribosomes o in some organisms smooth ER plays a role in synthesizing and detoxing lipids The endomembrane system functions in the delivery of 1 digestive enzymes proteins to the lysosome 2 integral membrane proteins to the cell surface and 3 secretory proteins to the outside world The ER also 0 https interacts with endocytic vesicles wwwyoutubecomwatchvc87ZnnKtHuE 0 Receptor Mediated Endocytosis a process that forms an endocytic vesicle 1 2 3 8 9 Vesicles coated with a protein called clathrin The clathrincoated vesicle recruits receptor proteins and their matching ligands The plasma membrane of the vesicle folds in and pinches off from the membrane and enters the cytosol it is now uncoated and is called an quotearly endosomequot The early endosome undergoes processing steps one of which is the activation of protein pumps that lower its pH acidi es it causing the ligand to dissociate from the receptor The dissociated receptor molecules are sent to a distant region of the endosome The region where the receptor molecules are now buds off as a separate vesicle The early endosome continues to acidify becoming a late endosome The endosome acquires digestive enzymes from the golgi apparatus The late endosome either matures into a lysosome or fuses with a lysosome Main ow of proteins through the endomembrane system from the rough ER to and through the golgi apparatus 0 Protei the lu ns synthesized on ERbound ribosomes are delivered into men of the ER and into the membranes of the rough ER and then delivered by vesicle trafficking to the golgi apparatus 0 https wwwyoutubecomwatchvXCLfdGbySAA Some proteins have a sequence that targets them to a receptor in the membrane of the rough ER The signal sequence 710 hydrophobic amino acids on the ribosome binds to a signal recognition particle SRP The SRP w ribosome binds to an SRP receptor in the ER membrane The SRP is released protein synthesis continues protein enters ER Protein synthesis is complete Signal sequence is removed After the protein has entered the lumen the interior of the ER short chains of sugars are attached to the protein via glycosylation o quotNlinked glycosylationquot N for amino nitrogen on the asparagine side chain the addition of a group of branched sugar chains that were previously assembled on a long membrane lipid called dolichol pyrophosphate onto speci c asparagine side chains on a newly formed protein Glycosylated protein travels to the smooth ER Membrane vesicles containing the glycoprotein bud off from the smooth ER to deliver the protein to the golgi apparatus The proteins then travel from the cis to the trans compartment of the golgi during which they undergo further modi cations In the trans department proteins are sorted for their nal destination 0 All the ribosomes bound to the rough ER are translating mRNA s encoding proteins with an ER signal sequence 0 However most of the ribosomes in cells are quotfreequot ribosomes meaning they are not bound to the ER membrane The quotfreequot ribosomes are translating mRNAs encoding cytosolic proteins proteins targeted to the nucleus or to the mitochondria peroxisomes or chloroplasts Cytoskeleton a network of bers that helps cells maintain shape by providing structural support its brous proteins move to alter the cell s shape and structure or shift contents 0 The cytoskeleton is composed of three different types of protein laments micro laments actin laments intermediate laments and microtubules Micro lamentsActin Filaments Subunits Actin Gactin globular actin 0 Structure Two intertwined strands o The cytoplasm of the cell is lled with networks of actin laments o Actin laments serve as quottracksquot for myosin motor proteins to walk along ex Muscle contraction 0 Some actin laments regulate cell shape by polymerizing and depolymerizing making itself bigger or smaller and thereby pushing out surface protrusions 0 Certain actin laments are stable in length actin subunits are not leaving or being added on because they are quotcappedquot by proteins 0 Ex Actin in muscle 0 Actin functions in muscle contraction o Sarcomere an array of actin laments attached at both ends on a quotZdiskquot structure with bundles of myosin proteins in the middle 0 When muscle contracts the myosin walks along the actin laments pulling the end quotZlinesquot closer together 0 ATP is hydrolyzed in this process 0 Actinmyosin molecules work similarly to muscle contraction to permit contraction in nonmuscle cells such as the movement of a broblast cell 0 When a cell moves forward lamellipodia thin sheets of membrane are formed by adding on branched actin laments which pushes the cell out in the front The cell then adheres to the surface 0 Other function of actin Cel division in animals Intermediate Filaments Subunits varies among different cell types Keratin neuro lament proteins vimentin desmin lamins etc o Lamins A B C make up the nuclear intermediate laments in all cell types These line the inside of the inner membrane of the nucleus Fibers wound into thick cables Function to maintain cell shape by resisting tension anchor organelles Give toughness and exibility to whole assemblies of cells at the level of tissue organization Intermediate laments run between desmosomes structures that touch both adjacent cells to join the individual cells together as a unit Microtubules Subunits Tubulin heterodimer Alphatubulin betatubulin act as a single unit Hollow tube structure Func on 0 Maintain cell shape by resisting compression 0 Assist with motility via agella or cilia 0 Move chromosomes during cell division 0 Move organelles During mitosis microtubules rearrange to form the spindle apparatus Organized parallel arrays of microtubules form cilia and agella A cilium or agellum consists of an outer ring of 9 pairs of microtubules and a central pair of single microtubules 9 2 o The 9 outer pairs consist of a hemitubule B built on to the side of a whole tubule A Pairs of motor molecules called dyneins line the walls of the A tubule 0 Using the energy of ATP hydrolysis dyneins are able to walk on the surface of adjacent B tubule o This produces the bending wavy motion of the cilia and agella Kinesins another class of motor molecules that walk along microtubules o Intracellular vesicles have kinesin and dynein motors and are thus able to move throughout the cell by walking along microtubule quottracksquot 0 ATP hydrolysis provides the energy Chapter 9 Respiration Respiration process where cells use oxygen to convert food molecules to C02 and H20 httpswwwyoutubecomwatchv00jbGchuQ FOOd 02 I C02 H20 1 Oxidation of glucose food release of energy 0 Similar reaction methane CH4reduced encounters oxygen activation energy it forms C02 and H20 and releases a large amount of energy CH bonds are successively replaced with C0 and then CO bonds In each successive reaction oxidation increases 0 Glucose is a 6 carbon molecule that is at an oxidation state corresponding roughly to that of formaldehyde intermediate Glucose is partially oxidized so the oxidation of glucose will provide less energy than the oxidation of a more reduced starting material like hexane 60 BUT glucose is used by almost all cells because glucose is a product of photosynthesis Photosynthesis is the ultimate source of most of the food animals eat 0 Hexane fully reduced 6 carbon molecule Can be used by only a few bacteria for energy Some similar structures are produced by plants and animals and can be oxidized with the production of a very large amount of energy ex fats rich source of energy The insolubility of fats in water cause them to be stored in droplets and fats are more slowly mobilized for oxidation in cells so watersoluble molecules like sugar and starches are used instead 2 The energy released by the oxidation process is stored in ATP molecules 0 ATP has high potential energy and allows cells to do work 0 ATP adenosine triphosphate consists of three phosphate groups ribose and adenine ADP Adenosine Diphosphate AMP Adenosine Monophosphate o How does ATP energy The bond between the two outermost phosphates phosphoanhydride bonds are high energy bonds due to the electrostatic repulsion between adjacent ionized O atoms 0 Hydrolysis the cleavage of chemical bonds by the addition of water of one of the phosphoanhydride bonds releases energy 0 So it takes energy to make ATP and once it s been made that energy can later be released by hydrolyzing that ATP molecule 3 The ATP created can then be used to drive biosynthetic reactions that might otherwise be unfavorable positive AG 0 Ex The formation of glutamine in the cell is usually energetically unfavorable AGnonspontaneous o The AG for the splitting of ATP to ADP Phosphate group is more negative than the addition of ammonia to glutamic acid is positive so the overall reaction is energetically favorable NADNADH in respiration 0 Also important in respiration is NAD and its reduced form NADH o Oxidation the loss of electrons 0 Reduction the gain of electrons quotreduces the chargequot 0 NADH is a carrier of high energy electrons NAD is reduced by the metabolic breakdown products of glucose Metabolism The process whereby cellular molecules are sequentially synthesized or degraded A collective term that includes glycolysis and the Krebs cycle 0 Catabolism breaking down molecules Catabolism Catastrophe o Anabolism using the energy and chemical products from catabolism to synthesize new molecules Anabolism Adding Each individual chemical reaction in a metabolic pathway is catalyzed by a speci c enzyme 0 Substrates molecules that are acted on by an enzyme 0 Most enzymes are proteins 0 The enzyme s quotactive sitequot is the side chain of amino acids where the substrate binds o Enzymes are pH and temperature sensitive Changes in pH and temperature can alter the tertiary structure of the enzyme that changes its ability to bind o lnhibitors any molecule that inhibits an enzyme They can be reversible or irreversible Competitive inhibitors looks like substrate and competes for the active site Noncompetitive inhibitors binds to the enzyme at a location away from the active site but alters the conformation of the enzyme so that the active site is no longer fully functional 0 Allosteric regulation Often molecules that naturally regulate enzyme activity in a cell behave like reversible noncompetitive inhibitors by binding weakly to an allosteric site Most allosteric enzymes have 2 or more subunits each with its own active site The enzyme oscillates between an active and an inactive conformation Allosteric activators stabilize the active conformation Allosteric inhibitors stabilize the inactive conformation Feedback inhibition 0 Many metabolic pathways are switched off by an end product The end product acts as an allosteric inhibitor of an enzyme earlier in the pathway 0 Ex Synthesis of isoleucine As isoleucine builds up some of it binds to an enzyme with prevents the enzyme from working on the initial substrate threonine Thus the whole pathway shuts down until the isoleucine is no longer inhibiting the enzyme The steps of respiration 1 Glycolysis o Glycolysis is a sequence of 10 reactions all of which occur in the cytosol o The rst 5 reactions lead to the production of 2 molecules of glyceraldehyde 3phosphate ATP is used in this phosphorylation thus this is the quotenergy investmentquot phase 0 The last 5 reactions lead to the production of pyruvate and ATP This is the quotenergygeneration phase 0 1 glucose l 2 pyruvates 3 carbon molecule 0 Pyruvate then enters the mitochondrion and is decarboxylated oxidized by NAD and joined with an enzyme to be converted to Acetyl CoA 2 Krebs cycle quotCitric Acid Cyclequot 0 Occurs in the mitochondrial matrix 0 8 steps total begins with two carbons entering as acetate and two different carbons leaving as C02 0 Begins with the entrance of Acetyl CoA catalyzed by pyruvate dehydrogenase o Acetyl CoA reacts with oxaloacetate by donating its acetate group to form citrate Citrate then decomposes back to oxaloacetate releasing C02 as a byproduct The Krebs cycle is cyclic due to the regeneration of oxaloacetate O 0 Most oxidation steps occur by creating NADH or FADH2 from the oxidized forms NADHFADH2 quotReducing powerquot because they can go on to donate electrons to other molecules Feedback regulation occurs in the Krebs Cycle 3 Electron transportoxidative phosphorylation O O O O Occurs in the inner membrane of the mitochondria Electrons pulled off NADH by FMN Electrons pulled off of FMN by FeS Sequential reactions occur as electrons are passed to successively stronger oxidizing agents Finally oxygen the ultimate oxidizing agent acquires electrons and becomes reduced to water This movement of electrons generates the movement of protons from the mitochondrial matrix to the intermembrane space the space between the inner and outer mitochondrial membranes This results in a 10fold greater concentration of protons in the intermembrane space relative to the matrix compartment The protons transfer into the intermembrane space by the mitochondrial FlFo ATP synthase F1FOATPase which acts as a driving force from a mechanical spinning rotor on the complex Peter Mitchell was the rst to propose in 1961 that a proton gradient is created by electron transport and that the stored potential energy of a proton gradient is used to synthesize ATP F1FOATPase does the work of making ATP spinning like a turbine as it passes protons across the membrane down their concentration gradient Chemiosmosis the movement of protons through the ATP Synthase Approximately 25 molecules of ATP are generated via oxidative phosphorylation per 1 molecule of input glucose Overall if 4 molecules of ATP are generated by substrate level phosphorylation during glycolysis and the Krebs cycle approx 30 molecules of ATP can be generated by the respiration of 1 molecule of glucose Substrate level phosphorylation vs oxidative phosphorylation Substrate level phosphorylation is ATP made in glycolysis or Krebs by direct enzymatic transfer of a phosphate from a substrate to ADP Oxidative phosphorylation is in the last step and makes ATP with the F1FOATPase Allosteric feedback mechanisms regulate respiration PFK Phosphofructokinase PFK catalyzes entry into glycolysis quotopens the throttlequot Activation If a lot of ATP is being used in the cell AMP and ADP will accumulate 0 AMP binds to an allosteric site of PFK activating the enzyme lnhibition An excess of ATP or citrate the products of respiration leads to allosteric inhibition of PFK and slows the production of these products Fermentation If there is no 02 present electron transport can t happen because oxygen is the ultimate oxidizing agent in the process Fermentation glycolysis reactions that regenerate NAD by the transfer of electrons from NADH l pyruvate or derivatives of pyruvate In yeast fermentation produces ethanol When humans exercise strongly enough to locally deplete 02 our muscles ferment pyruvate to form lactic acid Lactic acid fermentation by certain fungi and bacteria is used to make cheese and yogurt Catabolism of other foods proteins fats nonglucose sugars produces intermediates that feed into the same cellular respiration pathway Wa rburg effect Aerobic glycolysis 10x glucose uptake and 10x lactate production of oxidative phosphorylation Can occur in proliferative tissue or tumors Chapter 10 Photosynthesis Photosynthesis The ability to use light energy to reduce C02 to carbohydrate glucose 0 In plants H20 is the source of hydrogen used to reduce C02 In the 19305 CB Van Niel studied photosynthesis in purple sulfur bacteria 0 These bacteria use photosynthesis to make carbohydrate from C02 but unlike plants they do not release any 02 in the process 0 Instead they oxidize H25 to sulfur Van Niel reasoned that the bacteria used the hydrogen made sugar from C02 0 Proposed that all photosynthesizing organisms require some hydrogen source 0 Van Niel was the rst to hypothesize that plants use water as a source of hydrogen Respiration Energy released from sugar 0 High energy hydrogen electrons in glucose are delivered to oxygen forming water Mitochondrion uses the released energy to make ATP Photosynthesis Reverse of respiration Water is split and the hydrogen is transferred to carbon dioxide reducing it to sugar 0 The hydrogen electrons increase in potential energy as they move from water to sugar 0 The required energy boost comes from sunlight Photosynthesis has two components 0 Light dependent reactions 0 Use light energy to make ATP and NADPH 0 Water converted to 02 which is released as a byproduct Light independent reactions quotdark reactionsquot 0 ATP and NADPH used to reduce C02 to glucose Leaves and chloroplasts Gas exchange between the leaf s mesophyll tissue and the atmosphere occurs through pores called stomata Photosynthesis occurs in chloroplasts Chloroplasts In the membranes of the thylakoids clusters of photosynthetic pigments such as chlorophyll are grouped together in a network called a photosystem Leaves are green because the pigment molecules absorb blue and red light and re ecttransmit green light The electromagnetic spectrum Light is a form of energy called electromagnetic energy or electromagnetic radiation Light has wavelike properties Shorter wavelengths have higher energy Humans perceive light of wavelengths approx 400nm to 750nm Spectrophotometer measures the relative amount of light of different wavelengths absorbed by a sample solution Light has a particlelight character as well Light comes in packets of energy called photons o If the energy of a photon light matches the energy of an electron it can boost it to a higher energy level further orbital from the nucleus Chloroplast Pigments Different chloroplast pigments absorb different wavelengths of light 0 Multiple pigments can be extracted from chloroplast thylakoid membranes 0 Carotene and carotenoids play a role in light absorption and act as antioxidants protecting the cell from oxygen free radicals that can form when plant cells are exposed to UV rays 0 Chlorophyll and other plant pigments contain aromatic rings andor many alternating CC double bonds like all molecules that absorb visible light 0 These molecular bonds are characterized by pi orbitals that permit resonance that stabilizes the excited state following light absorption 0 By shining light of a speci c wavelength on photosynthetic cells and measuring how much 02 is produced in response the effectiveness of that particular wavelength can be measured By doing this at every wavelength you can make an absorption spectrum 0 Chlorophyll a plays a large role in photosynthesis but the absorption of light is not only by chlorophyll a quotAccessory pigmentsquot like chlorophyll b and carotenoids play a role in light collection These molecules capture light and then pass the energy of light to special molecules of chlorophyll a located in a protein complex called a quotreaction centerquot The array of pigments that surround the chlorophyll a in the reaction center are called the antenna complex The transfer of energy from molecule to molecule in the antenna complex occurs by resonance energy transfer It is extremely rapid 0 There are 3 ways for an excited electron in a chlorophyll molecule to return to its original unexcited state Simply drop down giving off energy as heat or light Transfer its energy by resonance to a neighboring molecule such as in the antenna complex Jump to a nearby electronacceptor lts old place is taken by the transfer of an electron from a nearby electron donor This happens in the reaction center Photosystems Light Dependent Reactions 0 Create ATP and reducing power from light energy Photosystem antenna complex a reaction center and a primary electron acceptor There are 2 kinds of photosystems in thylakoid membranes which are linked together by an electron transport chain The reaction center chlorophyll in Photosystem 1 is called P700 because it best absorbs light at 700nm The chlorophyll in Photosystem 2 is called P680 Photosystem 2 The primary electron acceptor in photosystem 2 is a molecule called pheophyUn The electron in a P680 chlorophyll molecule gets excited and passes it off to pheophytin P680 is now a strong oxidizing agent and takes electrons from water molecules reducing it back to P680 neutral This is the step that converts H20 to 02 Water becomes oxygen as it loses electrons to P680 Pheophytin passes off the electron to a mobile carrier PQ plastoquinone PQ passes the electron to a cytochrome complex which pushes protons from the stroma to the lumen of the thylakoid The electron then goes from the cytochrome complex to a small protein called plastocyanin Pc Pc delivers the electron to a p700 chlorophyll molecule in the reaction center of photosystem 1 The p700 accepts the electron because it recently lost one to Ferredoxin when it absorbed a photon A proton gradient is created by PQ when it transmits electrons from pheophytin to the cytochrome complex 0 This proton gradient will be used to synthesize ATP using an ATP synthase like that found in mitochondria Photosystem 1 The primary electron acceptor in photosystem 1 is ferredoxin Ferredoxin transfers electrons to NADP reducing it to NADPH Both photosystems together Photosystems 1 and 2 work together to produce the ATP and reducing power NADPH needed in the Calvin Cycle to make sugar quotZschemequot the pathway of action between the two photosystems named for the sideways Zshape it makes Cyclic photophosphorylation Shortcut of the normal pathway in which ferredoxin hands off the electron to PO on the pathway back to P700 instead of to NADP Doesn t generate NADPH but does generate ATP o The purpose of cyclic photophosphorylation is to keep the production of ATP in sync with the production of NADPH o The Zscheme produces ATP and NADPH in roughly equal amounts but the Calvin Cycle consumes more ATP than NADPH so cyclic photophosphorylation makes up for this difference 0 If the chloroplast runs low on ATP NADPH accumulates which stimulates a temporary shift from noncyclic electron ow Z scheme to cyclic until ATP catches up History of the Calvin Cycle 0 In the 19505 Melvin Calvin worked out the pathway that xes C02 into sugar 0 Calvin realized that C02 could react with RuBP a 5carbon molecule to form a 6 carbon molecule that splits into two molecules of 3 phosphoglycerate G3P 3 carbon molecule Calvin Cycle Lightindependent reactions 0 The Calvin Cycle synthesizes G3P in the stroma of the chloroplast and then transports it to the cytoplasm where it is converted to both glucose and fructose which are then combined to form sucrose 1 Carbon xation prepares C02 for use 0 The enzyme Rubisco is used for C02 xation o Rubisco is the most abundant protein on earth 0 Rubisco is slow and inef cient because it cannot distinguish between 02 and C02 0 As cells use up C02 in the Calvin Cycle they bring in more C02 by opening up their stomata which causes them to lose H20 Causes an increase in the concentration of 02 But Rubisco doesn t differentiate well between 02 and C02 so it accidentally binds to 02 instead This process is called photorespiration Unlike true respiration it generates no ATP and it draws carbon away from the Calvin cycle so it is a wasteful process 2 Reduction 3PGA l G3P using ATP and NADPH 3 Regeneration of RuBP Metabolic adaptations to compensate for photorespiration C4 Plants most plants are C3 plants 0 A separate enzyme xes C02 into a 4 carbon organic acid 0 This enzyme is spatially separated from the cells engaged in the Calvin cycle using Rubisco o CAM plants 0 A separate enzyme xes C02 into an organic acid 0 This enzyme is temporallv separated from the events of the Calvin cycle C02 is taken up at night through stomata and released for delivery to Rubisco during the day for incorporation into the Calvin cycle Chapter 11 The Cell Cycle Replication in Bacteria a prokaryote The bacterial chromosome is a single circular molecule of DNA The replicating bacterial chromosome is attached to the cell membrane When the DNA is replicated so is the membrane attachment complex New membrane and cell wall material is inserted in between the two new attachment complexes causing the two replicated DNA molecules to move away from each other Septum formation is then initiated and separates the two new cells from one another 0 In the bacterial DNA molecule there is a DNA sequence called the origin where DNA polymerase molecules separate the two strands and begins copying them 0 2 molecules of DNA polymerase initiate replication at the origin One moves north and the other moves south This is called bidirectional DNA replication 0 The replication fork is the point at any given moment where the DNA molecule is as it is replicating the DNA Proka ryotes Prokaryotes have a single circular DNA molecule as their chromosome 0 Prokaryotes replicate their DNA continuously when they are growing 0 Because of this rapidly growing bacteria often have multiple copies of replicated DNA per cell Euka ryotes A eukaryotic cell typically has multiple chromosomes each of which is a long linear DNA molecule 0 Each linear molecule of DNA in a eukaryote has multiple origin sites at regular intervals Eukaryotic cells replicate their DNA only once each cell division 0 This replication occurs during the SPhase during interphase 0 Because it only replicates once each cell division a eukaryotic cell under normal conditions never has more than two replicated copies of each particular chromosome DNA Facts 0 Before DNA replication chromosomes still come in pairs 0 These sister pairs are called homologs and they are similar but not identical because one came from Mom and one came from Dad 0 After DNA replication but before cell division the two identical daughter version of each single chromosome are called chromatids Chromosomes contain the genetic material which is replicated during Sphase of interphase The Eukaryotic Cell Cycle Go 0 Gl O O 62 O O Daughter cells born out of mitosis automatically enter Go even if only brie y Growth hormones stimulate cells to leave Go and enter 61 Most cells in an adult organism spend their entire lives in the Go state Different from stationary phase that bacteria enter when they exhaust their food supply Animal cells are metabolically active with high rates of protein synthesis Gap period between mitosis and synthesis 2N DNA DNA Synthesis 2N 4N DNA Second gap period What was once a chromosome is now a pair of chromatids these chromosomes will separate in mitosis and will once again simply be chromosomes 4N DNA To determine experimentally 0 Calculate percentage of cells in each phase this then is probably the percentage of the total duration of cell cycle spent in this phase The overall length of cell cycle 100 is equal to the doubling time Td Mitosis is easily distinguishable but the other phases aren t To determine the duration of Sphase they basically tracked the cells in Sphase 19505 by using T which is only incorporated during the Sphase and pulselabeling then using autoradiography to see them To determine 62 pulse label cells in S phase with HTdR rinse and incubate without HTdR pulsechase experiment By autoradiography the labeled cells show in mitosis The time between when they re labeled in Sphase and the when they show up in mitosis is the duration of Gphase 0 Can calculate Gl duration by subtracting known lengths of S 62 and mitosis from the doubling time Mitosis The purpose of mitosis is to divide the cell into two equal sized daughter cells and segregate one copy of each chromatid to the two daughter cells lnterphase A cell doubles each of its chromosomes to form 2 joined chromatids sister chromatids Prophase Chromosomes condense and mitotic spindles begin to form 0 Chromatin condensation ln eukaryotic cells DNA associates with certain proteins The combination of DNA w these associated proteins is called chromatin A major group of the proteins in chromatin are the histones 0 DNA wraps around a core of histone proteins to form a bead called a nucleosome Much of the DNA in the interphase stage is in the form of a string of nucleosome beads o In prophase the nucleosomes pack together to form a chromatin ber which then forms a scaffold structure 0 This scaffold structure then turns further supercoils to form the condensed chromatin form of the metaphase chromosome quotheterochromatinquot no gene transcription quoteuchromatinquot active gene transcription Prometaphase Nuclear envelope breaks down spindle bers contact chromosomes at kinetochore o Kinetochores are protein structures that connect to the chromosome at the centromere kinetochores capture the ends of microtubules radiating out from the duplicated centrosomes Metaphase Chromosomes line up in the middle of the cell 0 Centromeres connect the two chromatids together and serves as center of attachment of the chromatids to the microtubules of the spindle apparatus 0 Centromeres consist of speci c nucleotide sequences that associate with a speci c set of proteins 0 These same speci c set of proteins that bind the centromeric DNA form a structure called a kinetochore o The kinetochore structure binds the end of the microtubules in the spindle apparatus The ends of the microtubules bind to the kinetochore while the H ends are imbedded in the centrosome O lnitiat The bound microtubules polymerize at the end and depolymerize at the H ends leading to a quottreadmillingquot of tubulin subunits this aids in the positioning of chromatids in the midpoint of the metaphase cell Anaphase sister chromatids separate chromosomes are pulled to opposite ends of the cell 0 The kinetochore complex acts like a little pacman eating its way down the length of the microtubule towards the centrosome This movement is due in part to MT depolymerization in the kinetochore and the action of MT motor proteins that walk towards the H end of the MT ahead of the depolymerization process Telophase the nuclear envelope reforms and the spindle apparatus disintegrates Cytokinesis The cytoplasm is divided 2 daughter cells formed ion of Mitosis quotMitosis promoting factorquot MPF was discovered in the late 1970s as it was discovered that Mphase takes precedence over the other phases leading to the conclusion that the cytoplasm of a cell in Mphase must contain some factor named MPF that causes cells in other parts of the cell cycle to immediately enter mitosis after fusion with a mitotic cell 0 This idea was further solidi ed when cytoplasm from a mature egg was injected into an immature egg and the immature egg immediately began to undergo maturation initiated mitosis this was name quotMaturationpromoting factorquot but it turned out to be the same thing as mitosis promoting factor 0 The injection experiment permitted the cloning of the two genes encoding the two proteins that compose MPF Cyclindependent Kinase cdk one of the two proteins that make up MPF is cyclin an enzyme that functions as a protein kinase phosphorylates its substrate Cyclin a regulatory protein that regulates the kinase and chooses the substrate for the kinase to phosphorylate Kinases attach a phosphate molecule to a substrate protein while brotein phosohatases remove the phosphate o So the level of phosphorylated substrate protein in a cell at any given moment is determined by the relative activity of the kinase versus the phosphatase When cyclin and cdk come together in active form at the GZM boundary the cyclin partner chooses substrates for the cdk 0 Phosphorylation of the proteins it chooses initiates the events of mitosis Cdk and cyclin can also phosphorylate an enzyme called APC that leads to the degradation of cyclin which permits the cell to end mitosis
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