Cell Cycle Flashcards
What cells are incapable of forming tumours?
Senescent cells. Cancer is a deregulation of the cell cycle, so the more quiescent a cell is the less likely it is to be transformed.
What are the stages of the cell cycle?
- Mitotic phase
- Gap1
- S phase
- Gap2
What occurs in G1?
Duplication of cellular contents (not chromosomes)
What occurs in S phase?
Chromosomal replication
What occurs in G2?
The cell checks the replicated chromosomes for errors and makes any necessary repairs.
How is the cell cycle controlled?
Various checkpoints prevent it from continuing and lead to cell cycle arrest in response to various signals such as DNA damage.
What are the cell cycle checkpoints?
- Anaphase – Spindle Assembly Checkpoint
- G1 - M Restriction point
- Entrance to S is blocked if genome is damage
- DNA replication is halted in S phase if DNA is damaged - G2 – DNA Damage Checkpoint
Which checkpoint is arguably the most important and why?
The G1 checkpoint is also known as the restriction point. This is the ‘point of no return’ for the cell cycle, the vinegar strokes of proliferation. Having passed this carefully regulated signalling gate the replication cannot be stopped.
What are the primary signalling components that regulate the cell cycle?
Cyclins and cyclin dependent kinases. Cyclins are structurally related proteins with a cyclin box, allowing them to interact with and regulate CDKs to produce active kinases which comprise the checkpoints.
Deregulation of CDKs is therefore unsurprisingly a cornerstone to transformation.
How are CDKs complexes regulated?
Association with cyclins
Phosphorylation state
Interaction with specific inhibitory proteins
Turnover - their concentration oscillates in time with the cell cycle
How many CDKs and cyclins are there?
There are eight of each, the cyclins denoted as A-H and the CDKs as 1-8. These can exist in different combinations to produce different effects.
Describe the cyclin-CDK complexes which are prevalent at different points of the cell cycle.
Mid G1 - CycD + CDK4/6
Late G1 + S - CycE + CDK2
G1/S boundary (and throughout S) - CycA + CDK2
Late G2 - CycA + CDK1
G2/M boundary - CycB + CDK1
D 4/6 E 2 A 2 A 1 B 1
What is the Cyc-CDK complex which controls entry into M-phase?
CycB-CDK1. This is also known as maturation promoting factor, MPF.
How is MPF regulated?
MPF is inhibited during S + G2 by phosphorylation of CDK1 at Thr-14 and Tyr-15.
It is activated in late G2 by cdc phosphatase removing these groups.
CycB must be degraded for exit from M-phase.
How are cyclin-CDK complexes regulated in general?
CDK Activating Kinase (CAK) is a major regulatory complex formed of CycH + CDK7 (or alt CDK MO15). This phosphoactivates the CDK at Threonine 160/161.
Cdc phosphatase removes inhibitory phosphorylation of CDKs at Thr-14 and Tyr-15. Cdc phosphatase is inhibited by the 14-3-3 sigma protein, a downstream target of p53
What controls the speed of the cell cycle?
Entry into G1 from G0 - controlled by contact inhibition and growth factor exposure.
The speed of the cycle itself is largely dependent on the rate of G1 and G2. G1 progression is controlled by mitogens and antiproliferative agents with the R-point acting as the terminus point for them - exposure to more will have no effect on cell cycle rate.
What is required to pass through the R point?
A CycD-CDK4/6 concentration above the required threshold.
What factors affect CycD-CDK4/6?
Both intracellular and extrinsic signals can act on them via a large number of mediators.
How is the cell cycle regulated after the R point?
It isn’t really. After that it’s all autonomous and inevitable up until the DNA damage checkpoint and doesn’t respond to mitogenic or growth supressor signalling.
How does CycD-CDK4/6 control the R point?
When activated it allows for entry into S phase. It does this by phosphorylating pRb (retinoblastoma susceptibility protein), relieving its inhibition of a dimeric TF called E2F.
E2F activates the genes requires for replication.
How does pRb repress E2F, and how is this undone?
pRb binds and inhibit E2F with its long c-terminal tail. Repression by pRb is undone by CycD-CDK4/6 which double phosphorylates the c-terminal domain of pRb, causing it to release the HDAC and lowers its affinity for E2F.
Three further phosphorylations by both the Cyclin D-CDK4/6 and CycE-CDK2 complexes causes total complex disruption leading to full activation of EF2.
What proteins are members of the pRb family? What is their role?
p105, p107, p130. All are capable of inhibiting E2F, and respond to a huge number of signals including CycD-CDK4/6 making them the primary regulatory hub of the R point.
Describe the life of E2F.
8 different types of E2F.
Upregulates genes such as dihydrofolate reductase, DNAP, histones, etc
Rapidly degraded once in S phase so’s not to overproduce the replication machinery.
How does the action/regulation of CycE-CDK2 compare to CycD-CDK4/6?
CycE-CDK has a broader specificity than cycD-CDK. For example, cycE-CDK phosphorylates the CDK inhibitor, p27, causing its degradation.
The activity of cycE-CDK is inhibited by p21, p27, and p57.
In contrast, these CDK inhibitors, at least at low concentrations, co-activate cycD-CDKs. Another class of CDK inhibitors, p15, p16, and p18, specifically inhibit cyclin D-CDKs.
