Overview
This lecture covers the major DNA repair mechanisms in cells, explaining their processes, involved enzymes, and importance in maintaining genetic stability.
Mismatch Repair (MMR)
- Mismatch repair corrects base pairing errors missed by DNA polymerase proofreading.
- The system distinguishes new DNA strands by their lower methylation levels.
- In E. coli, MutS detects mismatches; MutL and MutH (endonuclease) help excise the error.
- Exonuclease removes a section of DNA including the mismatch; DNA polymerase and ligase restore the correct sequence.
Base Excision Repair (BER)
- BER removes chemically modified or incorrect bases (e.g., oxidized guanine, uracil in DNA).
- A glycosylase enzyme cleaves the base from the sugar, leaving an abasic (AP) site.
- AP endonuclease cuts the DNA backbone at the AP site.
- DNA polymerase replaces the missing base, and DNA ligase seals the strand.
Nucleotide Excision Repair (NER)
- NER fixes bulky lesions (like thymine dimers or other large DNA adducts).
- A nuclease cuts out a segment containing the lesion.
- DNA polymerase fills in the gap; DNA ligase seals it.
- NER involves many genes (e.g., ~30 in humans); defects cause disorders like xeroderma pigmentosum and Cockayne syndrome.
Photoreactivation (Light Repair)
- Photoreactivation uses photolyase enzyme (in bacteria) to directly reverse thymine dimers using light energy.
- Humans lack photolyase and rely on NER for thymine dimer repair.
Post-Replication Repair & Recombination Repair
- Post-replication repair uses recombination to fix gaps left opposite lesions after DNA replication.
- The undamaged sister chromatid serves as a template during recombination.
Double-Stranded DNA Break Repair
- Double-stranded breaks (from ionizing radiation) are repaired by recombination using a sister chromatid as a template.
- DNA synthesis extends from the 3’ end, using the intact sister as a template, then ligated to restore continuity.
Key Terms & Definitions
- Mismatch Repair (MMR) — fixes base-pairing mismatches after DNA replication.
- Exonuclease — enzyme that removes nucleotides from the ends or breaks in DNA.
- Endonuclease — enzyme that cuts DNA internally.
- Base Excision Repair (BER) — removes and replaces individual damaged bases.
- Glycosylase — enzyme that cleaves the bond between a damaged base and sugar.
- AP Site — a DNA site missing a base (apurinic/apyrimidinic).
- Nucleotide Excision Repair (NER) — removes bulky DNA lesions by excising oligonucleotides.
- Photolyase — enzyme that repairs thymine dimers using light (bacteria only).
- Recombination — exchange or use of homologous DNA to restore sequences.
Action Items / Next Steps
- Read Chapter 16 in the textbook, focusing on DNA repair.
- Complete questions 2–16 at the end of the chapter.
- Discuss questions with group members or post on the discussion board if needed.