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Mechanics of Cell Division

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Biological Sciences
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Mechanics of Cell Division Mitosis and Cytokinesis • Interphase  DNA is replicated (S phase) • G1 and G2 phases  cell growth • M phase  nuclear division (mitosis) and cell division (cytokinesis) M Phase • Chromosome condensation • Then specialized cytoskeletal machines pull the chromosomes apart (mitotic spindle) • Finally the cell is split in half (contractile ring) Chromosome condensation • Sister chromatids bound tightly together by protein complexes called cohesions after replication in S phase • Cohesions are deposited along the length of each chromatid and are broken only late in mitosis • At the onset of mitosis  chromosomes compacted and become 50x shorter o Condensins mediate this process  use ATP hydrolysis to drive the DNA compaction rxn M Phase depends upon the duplication of centrosomes in the preceding interphase • Centrosomes  microtubule organizing center (MTOC), nucleate the growth of microtubules o Contain a fibrous centrosome matrix containing a pair of centrioles (which contain about 50 copies of γ-tubulin ring complex) o The process of centrosome duplication and separation is called the centrosome cycle o As mitotosis begins, the 2 centrosomes split apart and move to opposite ends of the nucleus o Each centrosome (pair of centrioles) nucleates a radial array of microtubules called an aster, which will become the two poles of the mitotic spindle 5 stages of mitosis • Prophase  sister chromatids condense, and mitotic spindle begins to assemble outside the nucleus • Prometaphase  nuclear envelope breaks down (in mammals) • Metaphase  chromosomes are aligned at the equator of the spindle, medway between the spindle poles. Kinetochore microtubules attach sister chromatids to opposiute spindle poles • Anaphase  sister chromatids synchronously separate and the daughter chromosomes move towards the spindle poles. Kinetochore microtubules get shorter and spindle poles move outward • Telophase  the daughter chromosomes arrive at the two poles, and a new nuclear envelope assembles around each set. Mitotic spindle • Segregates replicated chromosomes • Constructed of microtubules and associated proteins • 3 classes of microtubules play a role: o astral microtubules o kinetochore microtubules o overlap microtubules • kinesins and dyneins also mediate the assembly and fxn of the spindle Processes that mediate the formation and function of the spindle • Microtubule instability increases at M phase o The relatively few, long microtubules radiating out from centrosome convert to larger number of shorter, more dynamic microtubules. o Increasing the amount and/or activity of catastrophins (and decreasing MAPs) destabilizes microtubules by increasing dynamic instability • Actions of opposing motor proteins halp set up the spindle by arranging and moving microtubules • Kinetochores attach chromosomes to microtubules  search and capture mechanism o Microtubu
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