Cell cycle control Flashcards
In the yeast model, what is the name for the cdk controlling entry into mitosis?
Cdc2
In the yeast model, what is the name for the cyclin controlling entry into mitosis?
Cdc13
In the yeast model, which enzyme phosphorylates the Cdk (Cdc2) at its inhibitory site?
Wee1
Which enzyme phosphorylates Cdk (Cdc2) at its activating site?
CAK (Cdk-activating kinase)
Which enzyme removes the inhibitory phosphate group from Cdk (Cdc2)?
Phosphorylated (i.e. ACTIVE) Cdc25 phosphatase
How is the phosphatase Cdc25 activated?
Positive feedback: the active Cdc2/Cdc13 (i.e. Cdk/Cyclin) complex phosphorylates Cdc25
How are the levels of cyclin controlled during the cell cycle?
Proteasomal destruction
What are CKIs?
Cdk inhibitor proteins
Give two examples of CKIs
p16
p21/27
Give an example of a CKI for:
Cdk4/6
Cdk2/Cyclin E
1) p16
2) p21/27
How are levels of cyclin D regulated throughout the cell cycle?
Activated by growth factors/ mitogens
Inhibited by growth inhibitors/ anti-mitogens
How are levels of Cdk-inhibitors regulated throughout the cell cycle?
Inhibited by growth factors/mitogens
Activated by growth inhibitors/anti-mitogens
Give 7 ways in which Cdk/Cyclins can be regulated
- CKIs binding to Cdk along
- Proteosomal degradation of cyclins
- Phosphorylation of Cdk
- Binding of cyclin
- Binding of CKI to Cdk/cyclin
etc
Which is the G1/S cyclin?
Cyclin D
Which are the two cyclins involved in the S phase? Which is involved in transition into the S phase, and which is involved in continuing the S phase?
Cyclin E and Cyclin A
Which cyclin is involved in the S/G2 transition?
Cyclin A
Which cyclin is involved in the G2/M transition?
Cyclin B
In what order are the cyclins synthesised?
Cyclin D
Cyclin E
Cyclin A
Cyclin B
When are each of the cyclins synthesised?
Cyclins D, E and A are synthesised during G1 phase.
Cyclin B is synthesised during the S phase
When are each of the cyclins broken down? How are they broken down?
They are ubiquitinylated and thus targeted for degradation.
Cyclin E is degraded in S phase
Cyclins A, B and D are degraded in mitosis
In what order are the cyclins degraded?
EABD
What are the four main cell cycle checkpoints? When do they occur?
- Restriction point checkpoint - G1
- DNA damage checkpoint - G1/S transition
- DNA replication stress response - G2/S
- Mitotic spindle assembly checkpoint - Mitosis
Cancer arises through loss of cell cycle checkpoints. What are these, and what is the result of loss of each?
- Loss of restriction point checkpoint: continuous proliferation of cells
- Loss of DNA damage checkpoint: accumulation of genetic damage (mutations), chromosome breakage and fusion
- Loss of DNA replication stress response: loss of genetic material, mutations, breakage of chromosomes
- Loss of mitotic spindle assembly checkpoint: chromosome segregation errors, aneuploidy and chromosome instability
Traditional cancer treatments target all dividing cells including both cancer and normal cells. Give some examples of such treatments, and where they affect the cell cycle.
1) Anti-metabolites - stop cells making the building blocks of DNA during G1 - e.g. methotrexate, 5-FU
2) Agents binding to DNA - stop DNA synthesis during S phase - e.g. alkylating agents
3) Microtubule inhibitors - stop cells making components needed to separate during G2 - include vincristine etc.
Give an example of a microtubule inhibitor
Vincristine
Give an example of an anti-metabolite
5-FU / methoxatrine
Give an example of an agent that binds to DNA to prevent DNA synthesis
Alkylating agents/ platinum compounds
Give three examples of drugs used as traditional cancer treatments
Microtubule inhibitors (e.g. vincristine)
Anti-metabolites (e.g. 5-FU)
Agents that bind to DNA to prevent DNA synthesis (e.g. alkylating agents)
Why have traditional cancer treatments that are aimed at killing dividing cells effective in adults but not children?
Because in an adult, most cells are not dividing
What are the serious side effects that result from treating cancer by killing off all dividing cells? List 4.
1) Nausea, vomiting, diarrhoea (gut epithelia)
2) Immune suppression (immune cells)
3) Anaemia (erythrocyte precursors)
4) Hair loss (hair follicle cells)
Why can traditional cancer treatments that target ALL dividing cells have serious side effects such as nausea, immune suppression, anaemia and hair loss?
Because some normal adult cells continue to divide, e.g. gut epithelia, immune cells, erythrocyte precursors and hair follicle cells
Traditional cancer treatments cause some serious side effects. How could treatment be changed to avoid these?
By finding specific target molecules that are absent or present at much lower levels in normal cells compared to tumour cells
OR
Exploit the fact that cancer cells have defects in checkpoint pathways
What is a tumour?
Large mass of cells that has lost control over proliferation
Which cell cycle checkpoint must be lost in order for a tumour to develop?
The restriction point checkpoint during G1
Cancer involves loss of control over cell proliferation. What else does it depend on?
Accumulation of genetic damage
The integrity of which cell cycle checkpoints must be lost in order for a cell to accumulate genetic damage?
1) DNA damage checkpoint
2) mitotic spindle assembly checkpoint
Rapidly dividing cells suffer from replication stress. What can a failure to deal with this lead to?
Increased genetic damage
Which cell cycle checkpoint controls cell proliferation?
The restriction point
Which kinases control passage through the restriction point?
Cyclin D-dependent kinases (Cdk4 and Cdk6)
Yeast genetics identified a network of genes required for mitotic entry. What were these, and which transition did they control?
G2-M
Cdc13 (the mitotic cyclin) binds to active Cdc2 (a cyclin dependent kinase). Cdc2 is phosphorylated at both its activating site and its inhibitory site by Cdk-activating kinase (CAK) and Cdk-inhibitory kinase (wee1). The inhibitory phosphate is then removed by the phosphatase Cdc25, and the Cdk/Cyclin (Cdc2/Cdc13) complex becomes active
