1. Cell Division
Lecture 30
Chapter 19

2. 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

3. 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

4. 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

5. 19_03_Cohesins_conde.jpg
19_03_Cohesins_conde.jpg

6. 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.

7. 19_04_mediate_M.jpg
19_04_mediate_M.jpg

8. 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

9. 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

10. 19_05_centrosome.jpg
19_05_centrosome.jpg

11. 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

12. 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

13. 19_07_mitotic_spindle.jpg
19_07_mitotic_spindle.jpg

14. 19_09_Kinetochores.jpg
19_09_Kinetochores.jpg

15. 19_13_01_mitotic_spindle.jpg
19_13_01_mitotic_spindle.jpg

16. 19_13_02_mitotic_spindle.jpg
Sister chromatids at the metaphase plate stained with fluorescent dyes.
19_13_02_mitotic_spindle.jpg

17. 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

18. 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

19. 19_18_envelope breaks.jpg
19_18_envelope breaks.jpg

20. 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

21. 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