Which feature best distinguishes mismatch repair (MMR) from direct reversal repair in prokaryotes with respect to the timing of action and the nature of the error corrected?

Explanation

## Distinguishing MMR from Direct Reversal Repair ### Timing and Lesion Type **Key Point:** The timing of action and the nature of the error corrected are the key discriminators between mismatch repair and direct reversal repair. | Feature | Mismatch Repair (MMR) | Direct Reversal Repair | |---------|----------------------|------------------------| | **Timing** | Post-replication (after DNA polymerase has passed) | Immediate, on damaged bases | | **Error type** | Base mispairing (e.g., G–T, A–C) from polymerase slippage or errors | Specific chemical modifications: thymine dimers, methylated bases, alkylated bases | | **Mechanism** | Recognizes mismatch → endonuclease nicks unmethylated strand → exonuclease removes segment → polymerase resynthesizes | Enzyme directly reverses the damage without removing nucleotides | | **Strand discrimination** | Uses GATC methylation in prokaryotes; PCNA in eukaryotes | No strand discrimination needed | | **Examples of repair** | Corrects base-pair errors from replication fidelity failure | Photolyase reverses thymine dimers; AlkB reverses alkylation | ### Mechanism Overview **High-Yield:** MMR is the "proofreader" for replication errors; direct reversal is the "eraser" for specific chemical damage. 1. **Mismatch Repair (MMR):** - MutS recognizes mismatched base pair - MutL recruits MutH endonuclease - MutH nicks the unmethylated (newly synthesized) strand at GATC - Exonuclease I removes the segment containing the mismatch - DNA polymerase III resynthesizes; DNA ligase seals 2. **Direct Reversal Repair:** - Photolyase directly reverses thymine dimers (using light energy) - AlkB oxidizes alkyl groups on DNA bases - No nucleotide excision; the base is chemically restored in situ **Clinical Pearl:** Lynch syndrome (hereditary nonpolyposis colorectal cancer, HNPCC) results from mutations in MMR genes (MLH1, MSH2, MSH6, PMS2). Patients develop microsatellite instability and early-onset colorectal cancer because they cannot correct replication errors. Direct reversal defects are rare in humans because we have limited direct reversal capacity (e.g., no photolyase). **Mnemonic:** **MMR = Mismatch (post-replication, remove & replace) / Direct Reversal = Damage (immediate, reverse in place)** ```mermaid flowchart TD A[DNA Damage]:::outcome --> B{Type of lesion?}:::decision B -->|Base mismatch from replication| C[Mismatch Repair]:::action B -->|Chemical modification: dimer, alkylation| D[Direct Reversal]:::action C --> E[MutS recognizes mismatch]:::action E --> F[MutH nicks unmethylated strand]:::action F --> G[Exonuclease removes segment]:::action G --> H[Polymerase resynthesizes]:::action D --> I[Enzyme reverses damage in situ]:::action H --> J[Ligase seals]:::action I --> K[Base restored, no excision]:::action J --> L[Mismatch corrected]:::outcome K --> M[Damage reversed]:::outcome ``` [cite:Alberts Molecular Biology of the Cell 6e Ch 5]