Cell cycle control

Question 2

In what cell cycle phase does replication happen?

Answer 2

S-phase

Question 3

Where does replication start on the DNA?

Answer 3

There are multiple origins of replication across the chromosome. This happens by the recruitment of pre-replication complexes (preRCs). And helicase opens up the two DNA strands.

Question 4

What are the 7 steps of mitosis?

Answer 4

1. Replicated chromosomes are kept together
2. The DNA is condensed
3. Nuclear membrane is removed
4. The mitotic spindle is ‘build’
5. The chromosomes are attached to the spindle
6. The sister chromosomes are separated
7. New nuclei are made in daughter cells, and the DNA is condensed

Question 5

What is APC/C?

Answer 5

Anaphase promoting complex (APC) is a very big E3 ubiquitin ligation. Promotes degradation of proteins.

Question 6

What is the spindle checkpoint?

Answer 6

Mitosis only starts when the last chromosome lines up. CyclinB disappears together with securin. This facilitates that chromosomes get decondensed (cohesin lost) and they can segregate.

Question 7

How does Rb regulate checkpoint blocking during G1/S phase?

Answer 7

Rb = tumor supressor. When active, it suppressess E2F which is a TF. This blocks continuation from G1 into the S-phase. Activation of Rb can also be caused upon growth inhibiting signals such as P53 and P21.

Question 8

Upon mitogenic stimuli, how can G1/S phase checkpoint inhibition be canceled?

Answer 8

CyclinD/CDK4 phosphorylates Rb, making it inactive. E2F is not suppressed anymore and can transcribe genes, such as Cyclin E. Cyclin E also phosphorylates RB and this creates a forward loop. And continuation into S-phase

Question 9

How is G1/S checkpoint inhibition evaded in cancer?

Answer 9

Mutations in growth inhibition factors P53 or P21. Or mutations in RB losing its function as a tumor suppressor.