Cell Division Lecture 30 Chapter 19

M phase of Cell Cycle The M phase encompasses two events Nuclear division - mitosis Cell division - cytokinesis In mammalian cells this phase takes just an hour or so to complete. The rest of the cell cycle excluding this phase (encompassing G1, S phase, G2) is simply known as interphase The cell cycle control system regulates these events

Kinases are involved M-phase Cdk (M-Cdk) is the most important factor regulating the M phase. Many events take place during this period The cell loosens its contact with its neighbours and the extracellular matrix The chromosomes condense The nuclear envelope disappears The ER and Golgi apparatus reorganize The cytoskeleton rearranges M phase ends when M-Cdk is inactivated

Prior to M phase During S phase the entire DNA is duplicated The sister chromatids are kept together by protein complexes called cohesins The chromatids are then condensed by proteins called condensins. These proteins are phosphorylated by M-Cdk

19_03_Cohesins_conde.jpg 19_03_Cohesins_conde.jpg

The involvement of the cytoskeleton Microtubules rearrange to for the mitotic spindle for segregating the chromosomes Actin and myosin filaments constrict the plasma membrane to divide the cell into two daughter cells.

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M phase has 5 stages Prophase - condensing of the chromosomes & formation of the mitotic spindle Prometaphase - Nuclear membrane breaks down & chromosomes attach to spindle fibers Metaphase - Chromosomes aligned in the middle of the cell along the ‘metaphase plate’ Anaphase - the chromosomes are pulled apart to different spindle poles Telophase - Nuclear envelope appears and the cell begins to contrict in the center Cytokinesis is the sixth phase but it overlaps some of the phases above - it begins during late anaphase and concludes after telophase

Microtubules & the centrosome Prior to M phase the cells’ one centrosome duplicates into two The two centrosomes move apart to different poles of the nuclear envelope Between them form many stable microtubules Other microtubules grow to other regions of the cell When the nuclear envelope eventually breaks down the spindle fibers capture and bind the sister chromatids and prevent them from drifting away

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The microtubules are relatively rigid They originate from each centrosome and attach to one or the other sister chromatid of each chromosome and begin a ‘tug-of-war’ The region around the centromere of each sister chromatid gets coated with special proteins that form a polar structure called the kinetochore A number of spindle fibers from one pole attach to one side of the kinetochore of one sister chromatid It then pulls on this by reducing the tubulins along its length The other sister chromatid is pulled by the other spindle pole

The sister chromatids are held together by cohesins The sister chromatids are held together by cohesins. However, when they are dissolved the microtubules are free to pull the chromatids apart and they become known as chromosomes at this time. 19_06_sister_chromatid.jpg 19_06_sister_chromatid.jpg

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19_13_02_mitotic_spindle.jpg Sister chromatids at the metaphase plate stained with fluorescent dyes. 19_13_02_mitotic_spindle.jpg

19_16_APC_triggers.jpg APC - anaphase promoting c*omplex APC triggers the destruction of the cohesins via another protein 19_16_APC_triggers.jpg

ANAPHASE - the separation of chromosomes ANAPHASE - the separation of chromosomes. Microtubules play a prominent role in the process. Pulling and pushing forces are used to segregate the chromosomes 19_17_process.jpg 19_17_process.jpg

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CYTOKINESIS - The plasma membrane begins to invaginate due to the action of the contractile ring of actin and myosin fibers inside the cell. This ring always orientates itself along the metaphase plate 19_19_cleavage_furrow.jpg 19_19_cleavage_furrow.jpg

19_22_phragmoplast.jpg Plant cells with their cell walls have a different mechanism for cytokinesis. The cell wall forms from the special microtubule-based structure called the phragmoplast, along the metaphase plate. 19_22_phragmoplast.jpg