DNA Repair

Question 1

Why are there so little mutations?

Answer 1

1. DNA repair mechanisms require double stranded DNA (as one is being replicated complementary strand is acting as a wild type for repair)
2. DNA has built in redundancy (repaired by many repair systems)

Question 2

What does Mismatch Repair (MMR) fix?

Answer 2

replication errors, mispaired bases, strand slippage

Question 3

How does MMR differentiate between old and new strand?

Answer 3

old strand has methyl groups attached to it

Question 4

Where does MMR degrade strand?

Answer 4

between the nick and mismatched bases

Question 5

Hows does polly and ligase fill the gap that was degraded?

Answer 5

using the old strand as a template

Question 6

What happens if there’s a mutation in the MMR?

Answer 6

Cancer

Question 7

Direct Repair

Answer 7

Pyrimidine Dimers *fixes not removing, no cutting!!

Question 8

Direct Repair - What breaks the covalent bond in dimers?

Answer 8

Photoreactivation of UV induced pyrimidine dimers

Question 9

Direct Repair - What removes the methyl group restoring it to guanine (G->A transition)

Answer 9

O6 methylguanine DNA transferase

Question 10

Base Excision repair

Answer 10

abnormal bases

Question 11

Base excision repair - What removes damaged bases creating an apyrimidinic/apurinic site?

Answer 11

DNA glycosylase

Question 12

Base excision repair - what cleaves the phosphodiester bond on 5’ side of AP site?

Answer 12

AP endonuclease

Question 13

Base excision repair - What adds new nucleotides to the exposed 3’OH group?

Answer 13

DNA polly

Question 14

Base excision repair - DNA ligase seals…restoring the original sequence?

Answer 14

the nick in the sugar phosphate backbone

Question 15

Nucleotide excision repair

Answer 15

distorts double helix

Question 16

Nucleotide excision repair - What do enzymes do?

Answer 16

scan the DNA for distortions and the the DNA is separated

Question 17

Nucleotide excision repair - What bonds are cleaved on either side of the mutation?

Answer 17

Phosphodiester bonds

Question 18

Nucleotide excision repair - What is the damaged strand removed by? What is the gap filled by? What are the ends sealed by?

Answer 18

helicase, DNA polly, ligase

Question 19

Double stranded breaks

Answer 19

Stalls DNA replication and leads to chromosome rearrangements

Question 20

Double stranded breaks - HDR

Answer 20

homologous recombination uses genetic info contained in sister chromatid resulting in 2 holliday junctions

Question 21

Double stranded breaks - NHEJ

Answer 21

during G1 phase of cell (no sister chromatid) - important role in somatic recombination and generating antibody diversity