dna damage

Question 1

depurination

Answer 1

loss of purines

due to hydrolysis of glycosidic bond between deoxyribose and the purine

Question 2

deamination

Answer 2

loss of amino group from base due to hydrolysis

results in cytosine —–> uracil (change in base)

Question 3

pyrimidine dimers

Answer 3

formation of c-c bonds between carbon atoms of pyrimidines –> distorts structure so the pyrimidines can no longer pair with their purine counterparts

Question 4

which form of dna damage is caused by exposure to uv light?

Answer 4

pyrimidine dimers

Question 5

which form of dna damage is caused by exposure to ionising radiation?

Answer 5

double stranded breaks

Question 6

what is base excision repair used for?

Answer 6

repairing deaminated + depurinated bases + pyrimidine dimers

Question 7

describe base excision repair step by step

Answer 7

1. dna glycosylase recognises altered base and cleaves it from dna
2. apurinic/apyrimidic endonuclease cleaves sugar
   phosphodiesterase cleaves phosphate
3. single nucleotide gap = signal for dna polymerase to add new nucleotides to the gap
4. dna ligase seals the nick

Question 8

describe nucleotide excision repair step by step

Answer 8

1. excision nuclease cleaves dna backbone at both ends of the damaged dna
2. dna helicase removes the damaged, disconnected dna segment - leaving a gap
3. dna polymerase adds new nucleotides to the gap/primer-template junction
4. dna ligase seals the nick

Question 9

what is nucleotide excision repair used for?

Answer 9

repairs pyrimidine dimers + distorted double helical structure + other types of dna damage

Question 10

how is nucleotide excision dna repair targeted to regions of the genome where repair is most needed (protein coding genes/transcribed dna sequences)?

Answer 10

physical coupling of the nucleotide excision dna repair machinery to rna polymerase

Question 11

what are the genes mutated in xeroderma pigmentosum?

Answer 11

7 different XP genes, including XPA, XPC, XPD, XPF, XPG

- involved in nucleotide excision repair

Question 12

xeroderma pigmentosum

Answer 12

• uv induced skin cancer
• back covered in nodules = tumours induced by thymine dimers
• exposure to uv light —-> XP genes capacity to repair dna lesions overloaded ——> mutations

Question 13

non-homologous end joining

Answer 13

1. ends of broken double stranded dna becomes degraded because nucleotides at the site of the original break are damaged and cannot be ligated
2. end-joining of dna fragments by ligation

Question 14

describe the process of homologous recombination

Answer 14

1. exonuclease resects the 5’ ends of each strand to produce overhanging
2. recA promotes strand invasion of damaged 3’ dna strand into the template duplex. point where this occurs = branch point.
3. complementary base pairing occurs between the damaged 3’ dna strand and the undamaged sister chromatid
4. dna polymerase uses the undamaged sister chromatid as a template to extend 3’ end. branch point migrates leftwards.
5. newly synthesised dna dissociates from its template and forms base pairs with the top strand
6. dna polymerase synthesises the 3’ end of the top strand using strand from damaged dna as a template
7. dna ligase repairs the pair of staggered nicks

Question 15

which genes are involved in homologous recombination and when mutated, may result in cancer?

Answer 15

BRCA2
ATM
FANC

Question 16

Lynparza

Answer 16

poly ADP ribose polymerase inhibitor

• inhibits base excision repair
• cancer treatment

Question 17

describe the process of homologous recombination in meiotic cells

Answer 17

1. spo11 endonuclease cleaves both strands of dna and creates double stranded staggered break. spo11 remains bound.
2. mre11 exonuclease digests the 5’ staggered ends, creating overhangs + removes spo11
3. recA-like protein promotes strand invasion
4. dna polymerase extends damaged 3’ end using sister chromatid as a template
5. double holliday junction - 3’ end of damaged dna strand joins with its 5’ end to form a 2nd branch point
6. dna strands are broken and rejoined at each junction