DNA Repair

Question 0

Strand directed mismatch repair

Answer 0

After DNA synthesis is over, newly made strands (before methylation) are looked over for this

1. MutS binds to mismatch
2. MutL binds and loops to nick in strand
3. Strand from mutation (in base pair) to nick is removed
4. Then repair DNA is synthesized

Question 1

Oxidation of benzoapyrene

Answer 1

This is a carcinogen produced by cigarette smoke. It binds to guanosine residues in DNA, causing the formation of bulky adducts, interrupting G-C base pairing and distorting the helix structure - results in G to A transition mutations!

Question 2

Base Excision Repair

Answer 2

This happens when a single base is cut out & the correct one is replaced

Question 3

Nucleotide Excision Repair

Answer 3

This is usually caused by a Thymine or Pyrimidine dimer. Then excision nuclease cuts out a chunk of DNA (including mutation) & then DNA Polymerase & DNA ligase add the new strand

Question 4

What is Xeroderma Pigmentosum (XP)?

Answer 4

XP is generally heterogeneous, autosomal recessive disease

• defective DNA repair mechanisms which cause UV radiation sensitivity
• associated with inbreeding (India)

Question 5

What is the Rad51 protein (humans)?

Answer 5

It is required for DSB (double stranded break) repair utilizing homologous recombination

• strand exchange function/DNA Synapsis/Branch migration
• requires accessory proteins BRCA1 and BRCA2
• THIS protein wraps DNA together into a three stranded structure and then pairs one of the helix strands to an invading single strand

Question 6

What does Ku protein do?

Answer 6

It is involved in non-homologous DSB (double stranded base) repair - binds to end of DNA during repair - Ku heterodimers recognize the end of the DNA strands

Question 7

What is p53?

Answer 7

It is an important G1 checkpoint control gene. Nearly 50% of cancers have defect in p53: usually acts as transcription factor, has short half life