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L14 - Chapter 17 The Cell Cycle A The mechanical events B...

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Chapter 17: The Cell Cycle A) The mechanical events: B) The regulatory events: Pages: 1055-1058; 10-60-1071 exclude Fig 17-20B; 1071-1074; 1105-1108 C) Chapter 18: Apoptosis
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The mechanical events of M phase leading to nuclear division ( mitosis ) and cytoplasmic division ( cytokinesis ) M-phase: is when cells accurately separate and distribute its chromosomes which have been replicated in the preceding S phase. Remember each daughter has to receive an identical copy. All eukaryotes solve this problem in a similar way--they assemble cytoskeletal machines that pull apart the duplicated chromosomes and then split the cytoplasm in half.
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Human mitotic chromo-somes stained to reveal the condensin complex A) Mitotic chromosome B) Metaphase chromosome The sister chromatids are separated by breaking cohesins although enough cohesin is left to keep the chomatids together at the centromere until anaphase While the condensins ensure that each individual sister chromatid is organized and condensed.
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Cohesin : 4 subunit protein complex: Smc1, Smc3: coiled coil proteins Scc1, Scc3: connecting protein Condensin : 5 subunit protein complex: Smc2, Smc4: coiled coil proteins CAP-G, CAP-H, CAP-D2: connecting protein
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Electron micrograph showing the duplicated centrosome. There are now two centrioles but they remain in one complex. Check out how you section one cross-wise and one longitudinally-this is how we know that they are aligned at right angles to each other The centrosome duplicates
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Metaphase: Mitotic Spindle-- 3 classes of microtubules are involved: Kinetochore microtubules (blue): attach each chromosome to the spindle pole Interpolar microtubules (red): hold the two halves of the spindle together Astral microtubules (green): radiate out and interact with the cell plasma membrane
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Metaphase: mitotic spindle assembly: MAPs, Motors and Kinetochores Role of MAPs is to generate microtubule instability Role of Motors in to govern assembly Role of Kinetochores is to attach sister chromatids to the spindle Once assembled, microtubule flux keeps the system dynamic
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Metaphase : MAPs control microtubule instability: Old friends: XMAP215 versus catastrophin 1) There are more catastrophes/ minute in mitotic versus interphase extracts. 2) If the extract is depleted of XMAP215, the # of catastrophes increases. Remember XMAP215 stabilizes microtubules. 3) If catastrophin is then inactivated in the depleted extract, you see a reduction in the # of catastrophes/min to almost the level that occurs in the interphase extract. This strongly suggests that catastrophin is responsible for the catastrophes occurring in the depleted mitotic extract! Interphase or mitotic Xenopus egg extracts were incubated with centrosomes and the behavior of individual microtubules nucleated from the centrosomes was followed by fluorescence video microscopy. .
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