Cell Cycle I

Question 1

Run through the cell cycle with cyclin/CDK specifics involved: Start at Initial stages of G1, and stop with S phase.

Answer 1

1. Initially in G1, Cyclin D is in a complex with either CDK4 or CDK6.
2. cyclin D-CDK4,6 phosphorylates Rb (transcription repressor). This frees up E2F (a transcr. factor)
3. E2F allows cyclin E to be made, as well as more cyclin D, and also cyclin A to be made. Also DNA replication enzymes.
4. In G1, Cyclin E complexes with CDK2. Cyclin E-CDK2 assist cyclin D-CDK4,6 in phosphorylating Rb.
5. Cyclin E-CDK2 helps in forming the pre-replication complex.
6. During transition from G1 to S phase, cyclin E levels drop and CDK2 then complexes with cyclin A.
7. Cyclin A-CDK2 triggers DNA replication.
8. Now in the S phase, DNA replication/synthesis takes place.

Question 2

Which cyclin is increased in response to growth factor, and starts the whole process?

Answer 2

Cyclin D

Question 3

How do cyclins activate CDKs?

Answer 3

Causes conformational change allowing ATP binding and exposure of phosphorylation site on the T-loop. CAK phosphorylates it, and then the complex is active.

Question 4

What marks the passing of the G1 restriction point?

Answer 4

The positive feedback loop of cyclin D promoting its own expression via EF2, via the cyclin D-CDK4/6 complexes. Cell is then COMMITTED to divide

Question 5

What is the pre-replicative complex made of? What does it do?

Answer 5

Made of ORC, CDC6, CDT1, and mcm helicase. It “licenses” origins for replication.

Question 6

which complex triggers replication as well as mcm helicase?

Answer 6

cyclin A-CDK2.

Question 7

Two main purposes of G2?

Answer 7

Cell growth, as well as making sure DNA was all copied correctly in the S phase.

Question 8

Explain the regulation of cyclin B-CDK1 (MPF). How are these enzymes controlled?

Answer 8

wee1- adds inhibitory phosphate
cdc25 phosphatase- removes that inhibitory phosphate
They are controlled by the size of the cell

Question 9

What is the order of the stages of interphase?

Answer 9

G0, G1, S, G2.

Question 10

Difference between prophase and prometaphase?

Answer 10

prophase- chromosomes condense and mitotic spindle begins to form.
prometaphase- Dissolution of the nuclear envelope starts it. spindle assembly completes. Chromosomes begin migration.

Question 11

Continue cell cycle, following S phase.

Answer 11

9. Now in G2 phase, Cyclin B-CDK1 (**MPFcomplex) forms. Wee kinase phosphorylates it to be inactive, but then cdc25 (phosphatase) dephosphorylates it so it can be active. —size of cell also dictates moving into M phase.
10. Now in the M phase, Cyclin B CDK-1 then phosphorylates lamin proteins, disrupting the nuclear envelope.
11. Cyclin B CDK1 phosphorylates condensins to allow chromosomes to condense.
12. cyclin B CDK1 phosphorylates Microtubule Associated proteins (MAPs), which allow the mitotic spindle to form.
13. It then also phosphorylates the anaphase promoting complex/cytosome (APC/C, a ubiquitin ligase), activating it.
14. APC/C then binds to cdc20, which allows it to cause the degradation of securin, which is an anaphase inhibitor that holds in separase.
15. Separase then cleaves linkage holding chromosomes together.
16. Later, APC/C binds to cdh1, which allows it to ubiquitinate cyclin B, for degradation. this allows for cytokinesis, and the entering again into G1.

Question 12

What cyclin complex induces mitosis?

Answer 12

cyclin B CDK-1 (MPF)

Question 13

Cyclin and CDK order, in phases?

Answer 13

cyclin D Cyclin E cyclin A cyclin B
4/6 2 2/1 1
G1 G1/S S/G2 (G2)/M