Cell Division

Question 1

What are the phases of the cell cycle

Answer 1

Interphase (G1 phase, S phase and G2 phase) and the M phase (Mitosis and Cytokinesis)

Question 2

The cell cycle is critically controlled by what?

Answer 2

Cyclin dependant Kinase. The activity of these enzymes are cyclical

Question 3

How are cyclin dependant kinases activated?

Answer 3

Inactive cyclin-Cdk complex forms
and the complex is phosphorylated by protein kinases (still inactive). Phosphate is removed by activating phosphotase (Cdc 25) activating the cylin-Cdk complex

Question 4

How are active cyclin-Cdk complexes inactivated

Answer 4

First there is the Ubiquitination of cyclin and this leads to the destruction of cyclin resulting in the inactive Cdk.

Question 5

Describe the process of ubiquitination

Answer 5

Ubiquitin (large protein) binds to E1 enzyme and then is transfered to the E2 enzyme. The E2 enzyme (Ubiquitin conjugating enzyme) is joined by an E3 enzyme (ubiquitin ligases) allowing the ubiquitin to transfer to the substrate. The presence of ubiquitin on the substrate signals for destruction of substrate

Question 6

Explain how proteasomes work

Answer 6

The protein complex recognises the polyubiquitin signal attached to the protein that is wants to degrade. The protein is unfolded and fed through the core particle where is gets broken up

Question 7

What can occur at checkpoints of the cell cycle?

Answer 7

If the environment is not right then there will be a suppression of Cdk by inducing transcription of Cdk inhibitors

Question 8

Name an example of a Cdk inhibitor and how it works

Answer 8

p21 - works by holding the Cdk in an inactive position

Question 9

What is the function of Wee1?

Answer 9

It is an inhibitory kinase so prevents cell division

Question 10

What is the R-point

Answer 10

It is where the cell transitions from being in the G1 phase to the S phase

Question 11

At the R-point how can the cell be prevented from proliferating?

Answer 11

Cdk is held in an inactive position meaning Rb (retinoblastoma) protein is activated and the expression of S-phase genes is inhibited.

Question 12

At the R-point how can the cell continue to proliferate?

Answer 12

p16 is absent or inactive meaning the cyclin-Cdk complex is activated allowing it to phosphorylate the Rb protein inactivating it. Therefore it cannot prevent the transcription of genes necessary for the S phase.

Question 13

Describe how DNA replication is allowed when transitioning from the G1 phase to the S phase

Answer 13

In the G1 phase there is a pre-replicative complex sat on the origin of replication, preventing DNA synthesis occuring. In the S phase, the Cdc6 molecule is a target for Cdk and becomes phosphroylated and then destroyed, leaving the pre replicative complex. The assembily of the replication fork can begin and DNA replication can then be completed.

Question 14

What occurs if DNA becomes damages?

Answer 14

The response depends on the stage of cell cycle. p53 causes the transcription of cdk-inhibitors. CHK2 is activated causing the inhibition of cdc25

Question 15

What are the stages of mitosis?

Answer 15

Prophase, Prometaphase, metaphase, anaphase, telophase and cytokinesis.

Question 16

What occurs in the prophase?

Answer 16

DNA is condensed and held together by condensins. Spindle fibres form from the centrosomes. Phosphorylated lamins trigger the breakdown of the nuclear envelope and microtubules attach to the kinetochore.

Question 17

Describe what occurs in the metaphase?

Answer 17

Chromosomes align on the metaphase plate. Mitosis cannot proceed until chromosomes are properly attached and under tension - spindle check point ensures this.

Question 18

What holds sister chromatids together and when is this important

Answer 18

Cohesion complexes. This is very important during the transition between metaphase and anaphase

Question 19

Defects in cohesion complexs can cause what?

Answer 19

A very rare disease called Roberts Syndrome. In this disease the chromosomes are not held tightly together so they move away from eachother.

Question 20

What allows the cell cycle to move from metaphase to anaphase?

Answer 20

Separase as they cut the cohesion complexes allowing the chromosomes to move to opposite polls

Question 21

How are separases activated?

Answer 21

Separases are kept in the inactive form by securin. APC (anaphase promotor complex) becomes activated and targets the securin molecule for Ubiquitin, securin is then degraded activating separase

Question 22

Defects in the spindle check point can cause what?

Answer 22

Aneuploidy, this is a cell with abnormal chromosomes.

Question 23

What is essential for mitotic exit?

Answer 23

Degredation of cyclins and securins

Question 24

In cytokinesis what allows for the cell to separate into two daughter cells?

Answer 24

A ring of actin and myosin