DNA Repair

Lehninger, chapters 8 and 25Blackburn & Gait, Ch. 6 and 8

Know main types of DNA damageKnow repair mechanisms used for each type of damageKnow the difference between transition and transversionUnderstand why defects in repair mechanisms could lead to cancers

DNA metabolism

DNA RepairDNA damage from:1. spontaneous loss of exocyclic amino group (deamination)

C U occurs once every 107 C residues in a day (100x a day)

A G (Hyp) occurs 100x slower

2. Hydrolysis of bond between sugar and base (apurinic residue)

Occurs once every 105 purines in a day (10,000x a day)

Slower for pyrimidines

DNA metabolism

DNA RepairDNA damage from:3. UV damage causes pyrimidine dimers

DNA metabolism

DNA RepairDNA damage from:4. Reactive chemicals

Nitrous acid precursorsAlkylating agents (nitrogen mustard, DMS, SAM)

DNA metabolism

DNA RepairDNA damage from:5. Oxidative Damage

H2O2, •OH, •O2-

DNA metabolismDNA Damage & Repair

DNA metabolism

DNA Repair

DNA metabolism

DNA RepairIn bacteria

DNA metabolism

DNA Repair

DNA Mismatch RepairCorrection of mismatches increases fidelity by 100 to 1000-fold

DNA metabolism

Repairs mismatches up to 1000 bp from hemi-methylated GATC

Binds all mismatches except C-C

Binds to MutL-MutS and methylated GATC

DNA Mismatch RepairDNA metabolism

DNA metabolism

DNA Mismatch Repair

DNA Base-excision Repair

DNA glycosylases recognize deaminations and remove them leaving an apurinic/apyrimidinic site (AP site)

DNA metabolism

DNA Nucleotide-excision RepairUsed to repair DNA lesions that cause large distortions in DNA helix (pyr dimers)

DNA metabolism

Photolyase

DNA metabolism

DNA damage - mutation

DNA damage - mutation

DNA SOS RepairResponds to extensive DNA damage, coordinated induction of a variety of genes

DNA metabolism

DNA Repair

Defects in genes encoding proteins involved in mismatch repair, nucleotide-excision repair, and recombinational repair can cause cancer

Nucleotide-excision repairsole repair pathway for pyrimidine dimers

genetic defect causes XP, xeroderma pigmentosa, these individuals are extremely sensitive to sunlight and quickly develop sunlight-induced skin cancer

Mismatch repairHereditary nonpolyposis colon cancer (HNPCC) linked to defects in these genes

Defects in human MutL homolog, human MutS homolog

Recombinational repairRecombination - linear sequence of DNA altered by cleavage and rejoining of chromosome (involves RecA protein)

Repair of this type sometimes needed to reconstruct replication fork

Human breast cancer genes (BRCA1 and BRCA2) produce proteins that interact with the human homolog of RecA, therefore these are linked to recombination repair

10% of breast cancers have defects in BRCA1 or BRCA2Women with defects in these genes have a >80% chance of developing breast cancer

DNA metabolism