Biochem 71

Question 1

What 3 methods are used to repair single strands of DNA?

Answer 1

1) Nucleotide excision repair
2) Base excision repair
3) Mismatch repair

Question 2

What method is used to repair double strands of DNA

Answer 2

Nonhomologous end joining

Question 3

Describe how nucleotide excision repair works. (2)

Answer 3

1) Specific endonucleases release the oligonucleotide-containing damaged bases
2) DNA Polymerase (5’-3’ exonuclease activity) and Ligase fill and reseal the gap

Question 4

Nucelotide excision repair is defective in which disorder?

Answer 4

Xeroderma Pigmentosum

Question 5

Ultraviolet light exposure forms what within DNA

Answer 5

Pyrimidine (Thymine) dimers

repaired via UV-endonucleases with nuceleotide excision repair

Question 6

How does base excision repair work? (3)

Answer 6

1) Abnormal bases are recognized and removed by specific glycosylases without disruption of phosphodiester backbone.
2) Removal produces apurinic and apyridimidinic sites that are cleaved by specific nucleases.
3) Gap is then filled by DNA polymerase and joined by Ligase.

Question 7

When is base excision repair important?

Answer 7

Repair of spontaneous / toxic deamination

Question 8

In base excision repair which enzyme cleaves the 5’ end and which cleaves 3’?

Answer 8

5’: AP-endonuclease

3’: Lyase

Question 9

Describe mismatch repair: (3)

Answer 9

1) New synthesized strand (not methylated) is recognized
2) Mismatch nucleotides removed
3) Gap is filled and resealed

note: template strand is adenine methylated (UW 1477)

Question 10

Nonhomologous end joining is mutated in which disease?

Answer 10

Ataxia Telangiectasia

Question 11

Which DNA repair mechanism is defective in HNPCC?

Answer 11

Mismatch repair