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Unformatted text preview: -1-LECTURES 34 & 3516 & 18 November 2009 (P. J. Hollenbeck)BIOL 231Mitosis and CytokinesisRead: Chap. 18: 624-638; DVD 18.3-6, 18.8, 9Probs: 88-92; exam IV07, #6-8; ECB, Q18-9, 17, 20I. The Events of Cell DivisionA. Tasks accomplished by cell division<Although some of the changes that occur during the cell cycle are gradual and continuous, the mostimportant ones are temporally discrete and tightly controlled. Mitosis is an example where theobservations that cytologists made more than 100 years ago, and their scheme of dividing the cell cycleup into stages, have held up well in the light of modern mechanistic studies of cells and molecules.> (1) General tasks: Duplicate the genetic material exactly; double the cell mass (usually),meaning organelles, proteins, etc.; then separate everything accurately into daughtercells - in the case of the genome, EXACTLY one copy into each daughter cell.(2) How is this accomplished: The mitotic spindle, a micromachine made of MTs andmotor proteins, binds to the chromosomes and sorts them exactly so that each daughtercell gets one copy of each. Then the cell divides itself in half along a planeperpendicular to the long axis of the spindle, thus partitioning the two sets ofchromosomes and all of the other cellular components into 2 daughter cells. Thespindle has three different kinds of MTs - astral, kinetochore, and polar [see fig below]. (B) Phases and Mechanics[panel 18-1] - order is important!(1) Prophase: Note that before this even begins, the centrioles duplicate to give a centrosome (MTOC)with two centriole pairs. The DNA has also been replicated so that each chromosomehas two chromatids.Organelle dispersion. The nuclear envelope, ER, and Golgi all fragment intosmall vesicles that can disperse throughout the cell.Chromosome condensation. A massive compaction of DNA occurs, mediatedby histones and other DNA-associated proteins, that folds the chromosomes upto a length that can be moved around within the cell. The sister chromatids ofeach chromosome are tightly connected at a discrete location by a centromere-2-and more loosely along their length. Another structure, the kinetochore,assembles onto the centromere region.Cytoskeletal disassembly and mitotic spindle assembly. The interphasecytoskeleton is largely disassembled and MTs are re-assembled to form themitotic spindle. As the spindle MTs assemble, the centrosomes separate,turning the MT array from a single aster to a bipolar structure. (2) Prometaphase: Microtubule attachment. MTs from each spindle pole (centrosome) attach toeach chromosome at the kinetochores. Chromosome congression. The chromosomes undergo oscillations along theMTs between the two spindle poles and end up at the spindle equator....
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