Lecture 1- The cell cycle

Question 1

What is the cell cycle clock?

Answer 1

A network of interacting proteins that receives signals from outside and inside the cell, integrates them and decides the cell’s fate.

Question 2

What 2 fates can the cell cycle clock decide?

Answer 2

1. Proliferation- cell cycle of growth a division

2. Quiescence- non-proliferation state imposed on the cell

Question 3

What is the structure of the cell cycle?

Answer 3

INTERPHASE:

1. G1 phase- cell increase in size
   - ribosome, RNA produced
   - preparation for DNA synthesis
2. S phase- DNA synthesised (chromosomes duplicated)
3. G2 phase- cell checks fidelity of DNA
   - preparation for nuclear division

MITOSIS (cell division): prophase, prometaphase, metaphase, anaphase, telophase and cytokinesis

Question 4

What are the 2 major cell cycle control factors?

Answer 4

1. The normal control by different cyclins. This drives the cycle forward.
2. Mechanisms to stop the cycle (and correct) if there are problems i.e checkpoints

Question 5

Give the pairings of cyclins with CDKs and the activity they modulate.

Answer 5

G1: CDK4 and CDK6 pairs with cyclin D

After the R point: Cyclin E associate with CDK2 —> lead to phosphorylation of substrates required for entry in S phase

Early S phase: Cyclin A replace Cyclin E in complex with CDK2 —> leads to S phase progression.

Later S phase: Cyclin A associate with CDK1

G2: Cyclin B replace Cyclin A in the CDK1 complex

M phase: Cyclin B/CDK1 —> mitosis triggering

G0 to G1: mediated by Cyclin C/CDK3 complex

Question 6

How do Cyclin E, A and B levels fluctuate during the cell cycle?

Answer 6

Cyclin E: low levels throughout most of G1, rapid increase after the R point.

Cyclin A: levels increase in concert with the entrance in S phase.

Cyclin B: levels increase in anticipation of mitosis

Question 7

Which Cyclin is an exception and why?

Answer 7

Cyclin D

• controlled by extracellular signals
• removal of growth factors leads to rapid collapse of Cyclin D1 levels
• cyclin D convey messages from the extracellular environment to the cell Cyclin clock in the nucleus
• cyclin D are synthesised in the cytoplasm and transported in the nucleus

Question 8

What controls cyclins levels during the cell cycle?

Answer 8

Controlled by intracellular signals and coordinated with cell cycle advance

Cyclin/CDKs activate complexes of the subsequent phase and inhibit those active in the previous phase

EXCEPTION: Cyclin D is controlled by extracellular signals

Question 9

What controls the cell cycle before and after the R point?

Answer 9

Before R point, programme is influenced by extracellular signals.

After R point, programme is cell autonomous.

Question 10

What do CKIs regulate?

Answer 10

Cyclin/CDKs

Question 11

What are the cell cycle checkpoints and what do they check for?

Answer 11

G2 checkpoint: is all DNA replicated, is the cell big enough, is the environment favourable?

Metaphase checkpoint: are all chromosomes aligned on spindle?

G1 checkpoint: is the cell big enough, is environment favourable, is there DNA damage?

Restriction points: sense environmental changes

Question 12

Describe G1/S restriction point progression.

Answer 12

• Cyclin D has a high turnover and its levels can only be maintained under continuous mitogen signalling.
• As levels of cyclin D/CDK4 are maintained, Rb is hypo-phosphorylated
• This allows some E2F transcription
• E2F causes Cyclin E/CDK2 to accumulate and this hyper-phosphorylates Rb to fully release E2F transcription and enter S phase

Question 13

What are the 3 places where DNA damage is detected and acted upon to stop the cell cycle?

Answer 13

1. G1
2. Entry to S phase
3. Entry into mitosis

Question 14

What happens to damaged DNA?

Answer 14

1. Break in DNA
2. ATM/ATR get activated and associate with the site of DNA damage and activates other kinases (Chk1/Chk2) to block the cell cycle
3. p53 is phosphorylated which stabilises and turns on p21 (a CKI)
4. p21 renders the G1/S-CDK and S-CDK complexes inactive, preventing cycle progression
5. DNA is repaired or apoptosis occurs

Question 15

What is the G1: growth vs quiescence decision?

Answer 15

A discrete windows to consult the extracellular environment.

From the onset of G1 to a few hours before the G1 to S phase transition.

Question 16

G1: growth vs quiescence decision in cancer.

Answer 16

• late G1 to M progression is similar in normal and cancerous cells
• however, deregulation of the R point decision making machinery accompanies the formation of most types of cancer cells

Question 17

What are the 2 problems with S phase?

Answer 17

1. The DNA has to be replicated accurately to prevent mutations.
2. The DNA must only be copied once

Question 18

Describe DNA replication from G1 —> S —> M

Answer 18

G1: inactive helicases are loaded onto replication origins, forming a PreRC. This is called licensing.

S: Helicases are activated by S-CDKs and the DNA is unwound. DNA is replicated and the forks move away from each other

M: M-CDK trigger chromosome segregation

Question 19

Which phase and how often does licensing occur?

Answer 19

Only occurs in G1 so occurs once per cycle.

Question 20

What happens if the replication forks stall?

Answer 20

• still get a DNA damage response e.g when nucleotides are depleted in a cell
• this response prevents cells segregating partially replicated chromosomes
• Can also get DNA repair in S-phase where a mistake is detected and the DNA around it is resected and filled back in