The Cell Division Cycle

Question 1

What are the requirements for division?

Answer 1

1. An internal or external signal, 2. replication of the genome, 3. segregation of the replicated DNA, 4. separation of the two new cells (cytokinesis).

Question 2

Which cells in a multicellular organism aren’t likely to divide?

Answer 2

Nerve cells

Question 3

Can cells in the immune system divide?

Answer 3

Yes, cells in the immune system can be triggered to divide in order to make more antibodies.

Question 4

In the eukaryotic cell cycle, what is Restriction Point (R)?

Answer 4

The point where cells are blocked from dividing if the right signal isn’t given.

Question 5

What happens when you fuse a G1 stage and an S stage Hela cell together?

Answer 5

The G1 nucleus initiated replication early (must be an S Phase Promotion Factor, SPF)

Question 6

What happens when you fuse either S and G2 stage Hela cells or G1 and G2 Hela cells together?

Answer 6

The G2 nucleus waits for the other nucleus to catch up (G2 delay)

Question 7

What happens when you fuse an M stage cell with a cell from any other stage?

Answer 7

both nuclei enter mitosis at the same time, even if replication is incomplete in the other cell, this is know an the MPF.

Question 8

What is the M Phase Promotion Factor?

Answer 8

It is a cyclin dependent protein kinase that is able to drive any cells through mitosis.

Question 9

What is Protein Phosphorylation?

Answer 9

The addition of a phosphate group. It is carried out by a protein kinase.

Question 10

What is MPF made up of?

Answer 10

it is a selective protein kinase with two subunits; a catalytic subunit (inactive alone) and a cyclin (activator) subunit.

Question 11

What cycle to cyclins follow in the process of cell division?

Answer 11

They accumulate in interphase, peak in mitosis and then rapidly degrade at the end of mitosis.

Question 12

What CDKs do at different stages of cell division?

Answer 12

G1- they trigger a wave of gene expression needed to support DNA replication
S - trigger replication origin firing
M - nuclear envelope breakdown, reorganisation of microtubules into a mitotic spindle and chromosome condensation.

Question 13

What do SPF and MPF both do?

Answer 13

They ensure alternation of the S and M phases.

Question 14

Why do cells need to organise their replicated DNA?

Answer 14

It must be in a form that can be segregated into the two daughter cells

Question 15

What is the Mitotic Spindle and why is it important?

Answer 15

it is made up of bundles of microtubules that interact with the chromosomes via protein complexes called kinetochores

Question 16

What is chromosome biorientation?

Answer 16

The state of sister chromatids attached to microtubules from opposite spindle poles

Question 17

Why is chromosome biorientation important?

Answer 17

all chromosomes must be orientated this way before segregation is attempted.

Question 18

How are sister chromatids held together?

Answer 18

circular protein complexes (‘cohesin’) encircle the two DNA duplexes to hold them together until metaphase.

Question 19

How is anaphase triggered?

Answer 19

Biorientation of the last chromosome triggers the destruction of ‘Securin’ which liberates Separase, which then cleaves the remaining centromeric cohesion.

Question 20

What is ‘Securin’?

Answer 20

an inhibitor of separase

Question 21

What is Mircotubule disassembly?

Answer 21

depolymerisation of microtubules at the kinetochore coupled with microtubule binding kinetochore proteins allows the kinetochore to follow the depolarising microtubule end

Question 22

What types of problems are detected by checkpoints?

Answer 22

DNA damage, incomplete spindle assembly, failure of chromosome biorientation, incomplete DNA replication

Question 23

What happens if there is DNA damage in G1?

Answer 23

There is p53-mediated cell cycle delay. Damaged DNA activates p53, which triggers expression of protein p21, which blocks G1 CDK activity, which allows time for the DNA to be repaired.