MolBio6 - 24

Question 1

List 4 forms of DNA damage

Answer 1

Deamination, depurination, pyrimidine dimers and DNA breaks

Question 2

List 3 forms of DNA repair mechanism

Answer 2

Base excision repair, nucleotide excision repair, homologous recombination

Question 3

What does deamination and depurination cause?

Answer 3

DNA mutations

Question 4

How are instance of deamination and depurination repaired?

Answer 4

Base excision repair

Question 5

Outline base excision repair

Answer 5

Damaged base removed by DNA glycosylase, sugar removed by apurinic/apyrimidinic endonuclease, phosphate removed by phosphodiesterase, DNA polymerase extends primer:template junction, DNA ligase seals nick

Question 6

What causes thymine dimers?

Answer 6

UV radiation

Question 7

What can thymine dimers cause?

Answer 7

DNA replication arrest or mis-reading by polymerase

Question 8

Outline nucleotide excision repair

Answer 8

Excision nuclease cleaves single-stranded DNA segment containing defect, DNA helicase removes damaged segment, DNA polymerase extends primer:template junction, DNA ligase seals nick

Question 9

What can defective nucleotide excision repair machinery cause?

Answer 9

Xeroderma pigmentosum (XP)

Question 10

What is XP?

Answer 10

Xeroderma pigmentosum - extremely sensitive to sunlight-induced skin cancer

Question 11

Where is DNA repair targetted?

Answer 11

The most important DNA sequences - the genes

Question 12

How is DNA repair targeted?

Answer 12

Repair machinery is physically coupled to RNA polymerase

Question 13

What is the most dangerous DNA damage?

Answer 13

Double stranded breaks - large fragments of DNA can be lost!

Question 14

How are double stranded breaks usually caused?

Answer 14

Ionizing radiation

Question 15

How are double stranded breaks repaired?

Answer 15

Non-homologous end joining, or homologous recombination

Question 16

Why is non-homolougous end-joining labelled ‘quick and dirty’?

Answer 16

Ends are redendered flush, with loss of bases, then ligated together - DNA is lost

Question 17

Outline homologous recombination

Answer 17

Exonuclease degrades 5’ ends, RecA promotes strand invasion of undamaged chromatid, this forms heteroduplex which facilitates templated DNA synthesis of one strand only by DNA polymerase, this strand then dissociates and re-anneals to original partner, allowing second strand synthesis, DNA ligase then seals nicks

Question 18

What can mutations in the homologous recombination machinery cause?

Answer 18

Cancer

Question 19

Give 3 examples of homologous recombination mutations causing cancer

Answer 19

BRCA2 - breast, ovarian, prostate; ATM - leukaemia, lymphoma; FANC - fanconi anaemia (leukaemia)

Question 20

How does base excision repair relate to cancer?

Answer 20

Can be achilles heel of BRCA2-mutated cancers - inhibition can destroy cancer because DNA cannot be repaired at all - synthetic lethality

Question 21

Outline homologous recombination during meiosis

Answer 21

Spo11 endonuclease cleaves, Mre11 endonuclease resets 5’ ends, resection and crossover as in repair, but DNA synthesis occurs on BOTH strands, forming a double holliday junction

Question 22

How can a double holliday junction be resolved?

Answer 22

In one of two ways - with or without crossover

Question 23

Draw how a double holliday junction can be resolved WITHOUT crossover

Answer 23

Question 24

Draw how a double holliday junction can be resolved WITH crossover

Answer 24