DNA Replication & DNA Damage Repair

Question 1

What stage of the cell cycle does DNA replication occur?

Answer 1

S

Question 2

What enzymes are involved in DNA replication and what do they do?

Answer 2

• DNA Polymerase catalyses the reaction
• Primase kickstarts the process as DNA polymerase can extend from the 3’ ends only
• Helicase unwinds the strands

Question 3

Describe the process

Answer 3

1) Moving replication forks
2) Helicase unwinds the double helix
3) DNA polymerase extends 3’ ends only so the leading strand is continuous. The lagging stand requires okazaki fragments.
4) DNA ligase joins fragments
5) Termination when two replication forks meet and DNA ligase joins the final fragments

Question 4

What is replication stress?

Answer 4

inefficient replication that leads to replication fork slowing, stalling or breaking.

Question 5

What 3 factors cause replication stress (in a broad sense)?

Answer 5

• replication machinery factor has a defect
• factors hinder replication fork progression
• there are defects in response pathways

Question 6

What is slippage?

Answer 6

When a strand, either new or template loops out

Question 7

What is the effect of the new strand slipping out?

Answer 7

Extra nucleotide on new strand

Question 8

What is the effect of the template strand slipping out?

Answer 8

Nucleotide omitted on template strand

Question 9

What defect (defect in response to fork blockage) causes DNA single strand breaks to persist?

Answer 9

Base Excision Repair

Question 10

What defect (defect in response to fork blockage) causes DNA double strand breaks to persist?

Answer 10

BRCA deficiency defect

Question 11

Why are double stranded breaks so dangerous?

Answer 11

• disrupt molecular continuity
• difficult to resolve
• can lead to cell death
• associated with disease

Question 12

What are so sources of DNA damage?

Answer 12

• ionising radiation
• alkylating agents
• UV light
• anticancer agents
• DNA replication errors
• Free radicals

Question 13

In a broad sense, what are some possible outcomes of DNA repair?

Answer 13

• Senescence
• DNA Repair
• Apoptosis
• Transcription
• Stopping cell cycle transitions

Question 14

What does G1 checkpoint look for?

Answer 14

suitable environment

Question 15

What does G2 checkpoint look for?

Answer 15

if all DNA is replicated and if all DNA damage has been repaired, the cell can move to mitosis

Question 16

Why can cancers grow despite cellular mechanisms tor destroy or repair pre malignant cells?

Answer 16

Suppression/alteration of DNA damage response pathways