DNA replication & the cell cycle

Question 1

What happens during S phase?

Answer 1

DNA replication

Question 2

What happens during M phase?

Answer 2

Mitosis chromosomes separate

Question 3

What happens during the initiation phase?

Answer 3

The origin recognition complex which interacts with Cdt1 and Cdc6. Cdt1 interacts with MCM protein and forms the pre RC.

Question 4

Why is DNA replication a semi-discontinuous process?

Answer 4

Because one strand is replicated continuously while the other is not, the lagging strand.

Question 5

What happens during the formation of the leading strand?

Answer 5

Polymerase delta/epsilon synthesise DNA. Replication protein A binds to single stranded DNA and stops it from being degraded. Proliferating cell nuclear antigen forms a ring around DNA and it turns in and out.

Question 6

What is processivity?

Answer 6

This is the amount of time the polymerase stays associated with the template in one go.

Question 7

What is the function of Pcna?

Answer 7

It increases processivity by acting like a curtain ring.

Question 8

What happens during the formation of the lagging strand?

Answer 8

DNA polymerase alpha has a primate activity and synthesises the RNA primer. Swings out and allows other polymerase subunit to come in place and synthesise DNA.

Question 9

Why is RNA used for the primer?

Answer 9

Because DNA-RNA hybrids are more stable than DNA-DNA hybrids.

Question 10

Why is polymerase alpha not very accurate?

Answer 10

Because it has no proof reading ability.

Question 11

What happens when polymerase alpha meets RPA?

Answer 11

It drops off and enzyme RFC binds to primer terminus. This allows Pcna to load by opening up the ring and closing it around DNA.

Question 12

How are the Okazaki fragments joined?

Answer 12

Primer is removed forming smaller spaces between the fragments and DNA ligase 1 puts the fragments together.

Question 13

What is thought to happen during termination?

Answer 13

Topoisomerase may be involved. MCM proteins thought to be removed.

Question 14

What are replication fork barriers?

Answer 14

Termination sites where fork stops. This stops replication from interfering with other processes.

Question 15

Does all replication take place at the same time?

Answer 15

All origins do not fire at the same time as the genome is divided into early replicating regions associated with euchromatin and late regions associated with heterochromatin.

Question 16

What is condensin?

Answer 16

Dimer of 2 identical molecules with 2 globular hands and a hinge to move. It needs ATP to function and has a role in chromosome assembly and segregation.

Question 17

What is the role of the kinetochore?

Answer 17

Complex which specifies attachment between chromosomes and microtubules of spindle.

Question 18

What are the spindle pole bodies made from?

Answer 18

2 tubulin tubes arranged at right angles to each other.

Question 19

How do spindle poles work?

Answer 19

Duplicate and the new spindle pole body moves around the edge. Spindle is generated and invades the chromosome region and looks for kinetochore. Microtubules are sent out in cytoplasm to anchor the spindle.

Question 20

What is the role of CDKs?

Answer 20

Phosphorylate key groups of proteins and drive the cell into the next stages of the cell cycle.

Question 21

How is the activity of cyclin and CDKs controlled?

Answer 21

Through transcription, proteolysis, modifications, inhibitors and cell localisations.

Question 22

How does proteolysis control cyclin B?

Answer 22

Cyclin B is ubiquitenated and allows the subunit to be broken down by proteasome.

Question 23

How are cyclins and CDK complexes controlled by localisation?

Answer 23

Cyclin B is found in the cytoplasm and the substrate is found in the nucleus and it cannot access it.