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Transcript
Chapter 12 The Cell Cycle Roles of cell division • reproduction • growth/maintenance • repair Eukaryotic chromosome Cell Cycle Cell Cycle Interphase – period when the cell is not dividing consists of: G1 – gap (growth period) S – synthesis of DNA G2 – gap (growth, preparation for division) Mitosis (M) Prophase – centrioles migrate to opposite poles, nucleolus breaks down, spindle begins to form, chromatid pairs get shorter, compact, & more visible Prometaphase - nuclear membrane breaks down, spindle fibers extend to centromeres Metaphase – spindle fibers have attached to kinetochore of ea. sister chromatid at the centromere; sister chromatids are aligned at metaphase plate (equator) Mitosis (M) Anaphase – ea. chromatid of sister chromatid pair is separated and pulled toward opposite poles Telophase – nuclear membrane & nucleolus reappear, spindle breaks down, chromosomes get longer, thinner cytokinesis – division of cytoplasm Differences in plant and animal cell cytokinesis In animal cells, a contractile ring forms a cleavage furrow as the membrane pinches in to divide the 2 daughter cells In plant cells, a cell plate forms as vesicles of the Golgi pinch off containing cell wall material, migrate to the metaphase plate, elongate, & fuse to form a new cell wall to separate the 2 daughter cells Cleavage furrow in an animal cell Cell plate in a plant cell Differences in prokaryotic & eukaryotic cell division Prokaryotes – (bacteria) divide by binary fission; circular DNA in a single chromosome 1) DNA replicates 2) Each copy is attached to cell membrane at opposite ends of the cell 3) Cell membrane forms between the 2 daughter cells being produced as growth continues 4) membrane pinches inward, new cell wall material is deposited between 2 daughter cells Binary fission Differences in prokaryotic & eukaryotic cell division Eukaryotes – (everything else) divide most commonly by mitosis Regulation of the cell cycle • controlled by regulatory proteins • checkpoints – critical points where go/no go signals regulate processes G1 checkpoint – “restriction point” -if it passes this point, it is destined to continue through division -if it doesn’t pass this checkpoint, it goes into G0 Regulation of the cell cycle • kinase – enzyme that catalyzes transfer of PO4 from ATP to a target protein to activate or inactivate it Cdk’s – cyclin-dependent kinases (active only when attached to a particular cyclin) ex: MPF (maturation promoting factor) • cyclins – concentrations change cyclically Control systems 1) density dependent inhibition (contact inhibition) cells stop dividing when they come into contact with one another (usu. due to growth factor decrease) 2) anchorage dependence – require attachment to substrate (like ECM of a tissue) Escaping normal controls • cancer cells – unregulated growth of cells; -divide indefinitely as long as nutrients are available; “immortal”; results in tumor (mass) -benign – remains at original site -malignant – ability to spread (cancer) » unusual # of chromosomes » aberrant metabolism » lost attachments (abnormal surface changes) Escaping normal controls • metastasis – spread beyond original site