Microbiology 23: (Fry) Cell cycle checkpoints

Question 1

What are the cell cycle checkpoints?

Answer 1

G1 -> S (restriction point)
- Are conditions suitable for a further round of cell division?

S->G2 (Dna damage checkpoint)
- Is the DNA damaged

G2 -> M
- Has the cell finished replicating its DNA

M -> cytokinesis (mitotic spindle checkpoint)
- Is the spindle correctly formed

Question 2

What factors does the cell check for before passing the restriction point?

Answer 2

Unfavourable growth conditions

Sensor mechanism

• Decrease of stimulatory growth factors
• Increase of inhibitory growth factors
• Other indicators e.g. reduced nutrients

Signalling mechanism

• Down regulation of classical signalling pathways
  e. g. MAPK, PI3K, GPCR (regulate gene expression)

Effector mechanism

• Decreased abundance of cyclin D -> inactivates Cdk4/6
• Increased expression of p16, inhibiting Cdk4/6
• Increased expression of p27, inhibiting Cdk2

Cell cycle paused at restriction point

Question 3

What factors does the cell check for before passing DNA damage checkpoint? Why?

Answer 3

DNA under constant assault from internal and external factors e.g. UV light, free radicals, carcinogens
- lead to mutations or breaks in double-helix

Cells need to arrest cell cycle to repair DNA -> otherwise damaged DNA is replicated

Sensor

• proteins that recognise mismatches in base-pairing, ss or ds breaks (or other covalent modification)
• changes in chromatin structure

Signalling

• Multiple protein kinases are involved in transducing the DNA damage signal
  e. g. ATM, ATR, Chk1, Chk2

Effectors

• Stabilisation of p53 leads to increase in transcription of p21
• inactivation of cdc25 prevents activation of kinases
• expression of genes that promote apoptosis

Question 4

What factors does the cell check for before passing meitotic spindle checkpoint? Why?

Answer 4

Chromosome allignment and full bipolar attachment to spindle
- Cdk1 active at this time (Prometaphase / metaphase)

Mitotic spindle checkpoint at metaphase

• Cdk1 inactivated after

Sensor mechanism
- involves recognising that not all chromosomes have been attached by microtubules to both poles of the spindle

Effector
- prevent destruction of cyclins that would lead to meitotic exit

Question 5

What happens after the loss of each cell cycle check point (in cancer)?

Answer 5

Loss of restriction point - proliferation in absence of positive growth signals - hyperplasia

Loss of DNA damage checkpoint - cancer cells accumulate mutations at an increased rate - genetic instability + dysplasia

Loss of mitotic spindle checkpoint - chromosome segregation errors and aneuploidy in cancer cells - chromosome instability + dysplasia