CB22 Control Of Cell Division

Question 1

List the 6 phases of the m phase & describe the events associated with them

Answer 1

Prophase - each replicated chromosome condenses & mitotic spindle assembles between centrosomes

Prometaphose - nuclear envelope breaks down & sister chromatids attach to spindle

Metaphase - chromosomes are aligned at the equator of the spindle

Anaphase - sister chromatids are synchronously pulled apart & towards the spindle pole

Telophase - chromosomes arrive at poles nuclear envelope forms around both sets of DNA

Cytokinesis - contractile ring splits cytoplasm into 2 daughter cells

Question 2

Explain the role of MPF in the G2 to m transition

Answer 2

MPF triggers condensation of replicated chromosomes into rod-like structures & induces assembly of mitotic spindle & separates condensed chromosomes aided by condensins

Synthesis of cyclin B starts immediately after s phase & helps time the onset of m phase
The increase in Cyclin B leads to accumulation of MPF

Once activated MPF can activate more MPF by activating more cdc-25 phosphates - removes inhibitory phosphate & inhibits Wee 1

These events result in an explosion of m-cdk activity that droves the cell from G2 to m phase

Question 3

Describe the action of anaphase promoting factor

Answer 3

• Anaphase promoting factor targets securing for degradation → proteosome

• separase is then free to break the codhesin links that hold the sister chromatids together

Question 4

Summarise the movement of sister chromatids in anaphase

Answer 4

Anaphase A:
Shortening of the kinetochore midrotubules by depolymerisation which results in loss of tubulin subunits

Anaphase B:
Further separation of spindle poles by sliding of interpolar microtubles past each other
Pulling forces at aster microtubles move poles apart

• all driven by motor proteins

Question 5

Discuss the role of p53 in G1 arrest

Answer 5

P53 is a master regulator of monitoring DNA repair & damage

• p53 activates the cdki-p21 which binds to cyclin/CdK complexes & inhibits cell cycle progression
• this allows time for the repair of damaged DNA
• irreparable damage leads to apoptosis

Question 6

Comment on the process of nucleotide excision repair of thymine dimers

Answer 6

Nucleotide excision repair is the main DNA repair pathway for uv & carcinogen induced DNA damage in mammalian cells

10-20 nucleotides around the damaged region are cut out by a series of nucleuses

The original DNA sequence is restored by a repair DNA polymerase

DNA ligase seals the nick in the ribose-phosphate backbone