Chapter 11: Cell Cycle
Chapter 11: Cell Cycle Biol 2311
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This 4 page Class Notes was uploaded by Rachael Couch on Thursday October 8, 2015. The Class Notes belongs to Biol 2311 at University of Texas at Dallas taught by John Burr in Fall 2014. Since its upload, it has received 54 views.
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Date Created: 10/08/15
Chapter 11 The Cell Cycle Replication in Bacteria a prokaryote The bacterial chromosome 0 Single circular molecule of DNA 0 Attached to the cell membrane 0 Has a DNA sequence called the origin where the DNA is separated and DNA replication begins This separation is initiated by 2 molecules DNA polymerase Each molecules moves in a different direction caed bi directional DNA replication 0 Replication fork the point at any given moment where the polymerase molecule is as it is replicating the DNA 0 DNA replication 0 When the DNA is replicated so is the membrane attachment complex 0 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 0 Septum formation is then initiated and separates the two new cells from one another 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 The Eukaryotic Cell Cycle Go OO 61 Gz o o Daughter cells born out of mitosis automatically enter Go 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 replication 2N 4N DNA Second gap period 4N DNA Mitosis M phase 0 O Homologous chromosomes separate 4N l 2N To determine experimentally O OO Mitosis 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 doubling time Td Mitosis is easily distinguishable but the other phases aren t To determine the duration of Sphase they tracked the cells in S phase 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 Can calculate Gl duration by subtracting known lengths of S 62 and mitosis from the doubling time 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 ln eukaryotic cells DNA associates with certain proteins The combination of DNA w these associated proteins often histones is called chromatin 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 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 o The kinetochore complex quoteatsquot 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 Initiation of Mitosis o quotMitosis promoting factorquot MPF 0 Factor that causes cells to immediately enter mitosis after fusing with a mitotic cell 0 Discovered in the late 1970s 0 Also 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 named 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 bh05phatases 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 0 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