DNA Repair and Recombination

Question 1

Lesion:

Answer 1

DNA damage, if unprepared it becomes a mutation

Question 2

Silent mutation:

Answer 2

had no effect on gene function OR affects a nonessential region

Question 3

Missense mutation:

Answer 3

changes on a.a. to another, may or may not affect the PRO

Question 4

Frameshift mutation:

Answer 4

deletion or insertion of a number of base pairs that isn’t a multiple of three, changes all subsequent a.a.

Question 5

Nonsense mutation:

Answer 5

changes an a.a. to one that results in a premature STOP codon

Question 6

Mutations are either ___, ___, or ___

Answer 6

substitution/addition/deletion

Question 7

The types of damage are:

Answer 7

single-base alteration, two-base alteration, chain breaks, cross-linkages

Question 8

Base excision repair (BER)

Answer 8

a repair mechanism; works for x-rays, free radicals, alkylating agents, and spontaneous mutation

• DNA glycosylases: recognize and remove specific lesions
• uracil glycosylases: remove uracil from DNA
• other glycosylases make it so the whole nucleotide is removed, not just the base

Question 9

Nucleoside excision repair (NER)

Answer 9

a repair mechanism; works for UV light and free radicals

• repairs large distortions in DNA
• a DNA segment is remove by exinucleases by cleaving the backbone in two places
• strand is resynthesized

Question 10

Recombination repair

Answer 10

a repair mechanism; works for x-rays, free radicals, UV light, and anti-tumour agents

• either homologous (exchange between two similar DNA), non-homologous end joining (not ideal but prevents cell death, broken ends are ligated together without repairing sequence), or micro-homology-mediated end joining
• segments of DNA rearrange their location either within a single chromosome or across multiple chromosomes to correct their sequence

Question 11

Mismatch repair (MMR)

Answer 11

a repair mechanism; works for replication errors
- Relies on methylation, it works in the time before the daughter strand is methylated to identify it before cleaving it until it reaches the methylated portion. The cleaved portion is degraded, rebuilt, and sealed

Question 12

Direct repair (DR)

Answer 12

a repair mechanism; works for non-enzymatic methylation of DNA by cellular S-adenosyl-methionine
- photolyases use light energy to repair pyramiding dimers

Question 13

Some conditions that affect repair mechanisms include:

Answer 13

xeroderma pigmentosa, cockayne syndrome, trichothiodystrophy, Werner syndrome, Rothmund-Thompson syndrome, Bloom, ataxia telangiectasia

Question 14

Where there is no undamaged DNA to replicate it can cause the ___ to stall or repair using another ___ as a ___

Answer 14

replication fork/chromosome/template

Question 15

Steps of recombinational DNA repair

Answer 15

1) 5’ end is degraded
2) 3’ single strand extension is bound by recombinase
3) branch moves to create Holliday junction (X structure)
4) nuclease and ligase restore the replication fork structure