1. Lecture #16 Mitosis Yasir Waheed

2. The main function of the mitosis is to accurately separate and distribute (segregate) its chromosomes, which were replicated in the preceding S phase, so that each new daughter cell receives an identical copy of the genome. With minor variations, all eucaryotes solve this problem in a similar way: they assemble specialized cytoskeletal machines first to pull the duplicated chromosome sets apart and then to split the cytoplasm into two halves.

3. The division of a cell into two daughter occurs in the M phase of the cell cycle. M phase consists of nuclear division (mitosis) and cytoplasmic division (cytokinesis),

5. At prophase, each replicated chromosome consists of two sister chromatids, join together by cohesin. The chromosomes become condense (by condensin). Outside the nucleus the mitotic spindle assembles between the two centrosomes.

6. Figure 18-3. The related structure and function of cohesins and condensins. (A) Both proteins have two identical DNA- and ATP-binding domains at one end and a hinge region at the other, joined by two, long, coiled- coil regions. This flexible structure is well suited for their role as DNA cross- linkers. (B) Cohesins cross-link two adjacent sister chromatids, gluing them together. (C) Condensins mediate intramolecular cross-linking to coil DNA in the process of chromosome condensation.

9. Figure 18-16. Kinetochore microtubules. (A) A fluorescence micrograph of a metaphase chromosome stained with a DNA-binding fluorescent dye and with human autoantibodies that react with specific kinetochore proteins. The two kinetochores, one associated with each chromatid, are stained red. (B) A drawing of a metaphase chromosome showing its two sister chromatids attached to kinetochore microtubules, which bind by their plus ends. The number of microtubules bound to a metaphase kinetochore varies from 1 in budding yeast to over 40 in some mammalian cells.

10. Three types of microtubules are present, Chromosomes attach to the Kinetochore microtubules.

12. Figure 18-26. The major forces that separate daughter chromosomes at anaphase in mammalian cells. Anaphase A depends on motor proteins operating at the kinetochores that, together with the depolymerization of the kinetochore microtubules, pull the daughter chromosomes toward the nearest pole. In anaphase B, the two spindle poles move apart. Two separate forces are thought to be responsible for anaphase B. The elongation and sliding of the overlap microtubules past one another in the central spindle push the two poles apart, and outward forces exerted by the astral microtubules at each spindle pole act to pull the poles away from each other, toward the cell surface.

13. Figure 18-29. A model for how motor proteins may act in anaphase B. Plusend- directed motor proteins cross-link the overlapping, antiparallel, overlap microtubules and slide the microtubules past each other, thereby pushing the spindle poles apart. The red arrows indicate the direction of microtubule sliding. Minus-end-directed motor proteins bind to the cell cortex and to those astral microtubules that point away from the spindle and pull the poles apart. These astral microtubules shorten as the spindle poles are pulled toward the cortex.

15. The contractile ring contains both actin and myosin filaments and forms around the equator of the cell, just under the plasma membrane; as the ring contracts, it pulls the membrane inward, thereby dividing the cell in two.

17. THANKS