L1 Cell Cycle

Question 1

A mixture of signals has to be integrated by the cell to make the decision to proliferate, be quiescent of differentiate

Answer 1

Existence of a master governor that makes major decision regarding cell fate = cell cycle clock, which operates in the nucleus

Question 2

Cell cycle clock

Answer 2

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

Question 3

Proliferation

Answer 3

Proliferation -> cell cycle of growth and division

Question 4

Quiescence

Answer 4

Quiescence -> non-proliferative state imposed on the cell

Question 5

Interphase - G1 phase

Answer 5

• Cell increases in size
• Ribosomes, RNA produced
• Preparation for DNA synthesis

Question 6

Interphase - S phase

Answer 6

• DNA synthesised (chromosome duplicated)

Question 7

Interphase - G2 phase

Answer 7

• Cell checks fidelity of DNA

- Preparation for nuclear division

Question 8

MITOSIS: cell division

Answer 8

Sub-phases: - prophase, prometaphase, metaphase, anaphase, telophase
- cytokinesis

Question 9

Remember you have 2 major control factors

Answer 9

The normal control by different cyclins – this drives the cycle forward

Mechanisms to stop the cycle (and correct) if there are problems ie Checkpoints

Question 10

R = restriction point

Answer 10

– Beyond this you no longer need eternal signals to drive the cell cycle

Question 11

Pairing of cyclins with CDKs G1:

Answer 11

CDK4 and CDK6 depend on the association with cyclin Ds (D1,D2 &D3 = D-type cyclins)

Question 12

Pairing of cyclins with CDKs

After the R point:

Answer 12

E-type cyclins associate with CDK2 -> phosphorylation of substrates required for entry in S phase

Question 13

Pairing of cyclins with CDKs

S phase:

Answer 13

: A-type cyclins replace E cyclins in complex with CDK2 -> S phase progression. Later in S-phase, A-type cyclins associate with CDC2 (also called CDK1)

Question 14

Pairing of cyclins with CDKs

G2:

Answer 14

B-type cyclins replace A-type ones in the complex with CDC2

Question 15

Pairing of cyclins with CDKs

M phase:

Answer 15

B-type cyclins/CDK1(CDC2) -> mitosis triggering

Question 16

Pairing of cyclins with CDKs

G0 to G1:

Answer 16

Mediated by cyclin C/CDK3 complex

Question 17

Cyclin levels fluctuate during the cell cycle

Cyclin E:

Answer 17

Low levels throughout most of G1, rapid increase after the R point

Question 18

Cyclin levels fluctuate during the cell cycle

Cyclin A:

Answer 18

Levels increase in concert with the entrance in S phase

Question 19

Cyclin levels fluctuate during the cell cycle

Cyclin B:

Answer 19

Levels increase in anticipation of mitosis

Question 20

Cyclin levels fluctuate during the cell cycle

Answer 20

Collapse of cyclin levels as the cell progresses through the cell cycle -> degradation (ubiquitination-dependent)
The cell cycle can only progress in one direction

Question 21

Exception: D-type cyclins

Answer 21

D-type cyclins (D1 is the most studied) are controlled by extracellular signals: growth factors + integrin-mediated ECM attachment
Removal of GFs -> rapid collapse of cyclin D1 levels
D-type cyclins convey messages from the extracellular environment to the cell cycle clock in the nucleus
synthesised in the cytoplasm & transported in the nucleus

Question 22

Control of cyclin levels during the cell cycle

Answer 22

• D-type cyclins -> extracellular signals
• Other cyclins -> intracellular signals & coordinated with cell cycle advance
  cyclin/CDKs activate complexes of the subsequent phase & inhibit those active in the previous phase

Question 23

Cyclin/CDKs are regulated by CDK inhibitors (CKIs)

Answer 23

CKI = CDK inhibitors -> 7 proteins antagonising the activity of cyclin/CDKs

Question 24

Cell Cycle Checkpoints

G2 Checkpoint:

Answer 24

Is all DNA replicated?
Is cell big enough?
Is environment favourable?

Question 25

Cell Cycle Checkpoints

Metaphase Checkpoint:

Answer 25

Are all chromosomes aligned on spindle?

Question 26

Cell Cycle Checkpoints

G1 Checkpoint:

Answer 26

Is cell big enough?
Is environment favourable?
DNA damage?

Question 27

G1/S restriction point progression

Answer 27

• Cyclin D has a high turnover its levels can only be maintained under continuous mitogen signalling
• As levels of CyclinD/CDK4 are maintained -> Rb is phosphorylated (actually HYPO phosphorylated).
• This allows some E2F transcription. E2F causes CyclinE/CDK2 to accumulate and this HYPER phosphorylates Rb to fully release E2F transcription and enter S-phase

Question 28

There are 3 places where DNA damage is detected and acted upon to STOP the cell cycle

Answer 28

(i) G1
(ii) Entry to S-phase
(iii) Entry into mitosis

Question 29

What happens if the DNA gets damaged?

Answer 29

+ check for chromosome non-disjunction
G1 repair = non-homologous end joining
G2 repair = homologous recombination

Question 30

DNA Damage

Answer 30

• ATM/ATR get activated and associate with the site of DNA damage
• ATM/ATR will activate other kinases to block the cell cycle
• p53 is stabilised and turns on p21, (p21 is a CKI)

Question 31

DNA Damage response in G1

Answer 31

• p21 renders the G1/S-CDK and S-CDK complexes INACTIVE. Thus preventing cycle progression
• DNA is then repaired

Question 32

What happens if repair is not possible?

Answer 32

• Apoptosis

Question 33

G1: growth v/s quiescence decision

Answer 33

Discrete window to consult the extracellular environment: from the onset of G1 phase to an hour or 2 before the G1-to-S transition.

Question 34

G1 decision making machinery apparent in the responses of cultured cells to extracellular signals:

Answer 34

• Serum and growth factors removed before the cells have completed 80-90% of G1 -> fail to proceed further and revert to G0 state
• Serum and growth factors removed in the final hr of G1 -> proceed to S, G2 and M phase
• Restriction point (R point)

Question 35

S-phase: The cell’s problems

Answer 35

• It has to copy very large arrays of DNA ie chromosomes and repackage the DNA for the next G1 phase
• S-phase is the central event where DNA is replicated.
  Two problems
• The DNA has to be replicated accurately to prevent mutations
• The DNA must only be copied ONCE

Question 36

G1

Answer 36

• INACTIVE Helicases are loaded onto replication origins, forming a PreRC (pre-replication complex)
• This is called licensing

Question 37

S

Answer 37

DNA is unwound.

DNA is replicated (forks move away from each other)

Question 38

M

Answer 38

M-CDK trigger chromosome segregation

Question 39

If the forks stall you can also get a DNA damage response.

Answer 39

• This can occur, for example when nucleotides are depleted in a cell.
• This response prevents cells segregating partially replicated chromosomes
  [ You can also get DNA repair in S-phase where a mistake is detected and the DNA around it is resected and filled back in ]