DNA repair mechanisms

DNA repair mechanisms are grouped into which major categories

Direct repair, excision repair, strand break repair, interstrand crosslink repair, and translesion synthesis

What does direct DNA repair mean

Repair that directly reverses DNA damage without removing nucleotides

What DNA ends are required for DNA ligase to repair a break

A 3’ OH and a 5’ phosphate

What type of damage does photoreactivation repair

UV induced pyrimidine dimers

What enzyme performs photoreactivation in E coli

Photolyase

How does photolyase repair pyrimidine dimers

Uses visible light energy to split the dimer

What lesion is repaired by alkyltransferase

O6 methylguanine

Why is alkyltransferase called a suicide enzyme

It transfers the alkyl group to itself and is not recycled

What is the human equivalent of Ada protein

MGMT

What is excision repair

Removal of damaged DNA followed by resynthesis using the intact strand

What does mismatch repair fix

Replication errors missed by proofreading

Loss of mismatch repair is associated with which disease

Hereditary non polyposis colorectal cancer

What type of DNA damage is repaired by NER

Bulky helix distorting lesions

What are classic examples of NER substrates

Pyrimidine dimers and 6 4 photoproducts

What enzyme complex recognises bulky damage in E coli NER

UvrABC endonuclease

What feature of DNA damage is recognised by NER

Helix distortion not the specific lesion

How many nucleotides are removed in E coli NER

About 12 to 13 nucleotides

What enzyme unwinds DNA during bacterial NER

UvrD helicase

Why is NER energy expensive

It requires ATP for damage recognition unwinding and excision

What human disease results from defective NER

Xeroderma pigmentosum

Why do XP patients have high skin cancer risk

Inability to repair UV induced DNA damage

Which XP proteins act as helicases

XPB and XPD

Which XP proteins make the incisions

XPF ERCC1 makes the 5’ cut and XPG makes the 3’ cut

What is base excision repair used for

Repair of small non bulky base lesions

What enzyme initiates BER

DNA glycosylase

What does a DNA glycosylase remove

The damaged base only

What is an AP site

A sugar phosphate without a base

What enzyme cuts DNA at an AP site

AP endonuclease

Which DNA polymerase fills the gap in human BER

DNA polymerase beta

Why is BER especially important

Many base lesions arise spontaneously

Why excision repair is considered error free

The intact strand is used as a template

Why excision repair cannot fix double strand breaks

No intact complementary strand is available

What are the two main DSB repair pathways

NHEJ and HRR

Which DSB repair pathway is error prone

Non homologous end joining

Which DSB repair pathway is high fidelity

Homologous recombination repair

What is the SOS response

An inducible bacterial DNA damage tolerance response

What triggers the SOS response

Stalled replication forks and single stranded DNA

Which protein senses ssDNA during SOS response

RecA

What happens to LexA during SOS induction

It self cleaves and releases repression

What is the role of LexA

Transcriptional repressor of SOS genes

What is the benefit of the SOS response

Increased survival after DNA damage

What is the cost of the SOS response

Increased mutation rate

What is translesion synthesis

DNA replication across damaged templates

Why is TLS mutagenic

Polymerases insert bases without correct template reading

Does TLS remove the original DNA lesion

No the damage remains

Which polymerase is encoded by dinB

DNA polymerase IV

Why TLS polymerases are tightly regulated

To limit mutagenesis in undamaged cells

Why NER cannot act at replication forks

The DNA is single stranded

What happens if stalled replication is not bypassed

Cell death