KN

L10

DNA Repair Overview

  • Importance of DNA Repair

    • Prevents permanent changes (mutations) in DNA

    • Mutations can lead to hereditary diseases if in germ cells and somatic diseases (e.g., cancer) if in somatic cells

    • Maintains fidelity of the DNA code through multiple repair mechanisms

DNA Repair Mechanisms

  • DNA is repaired during and after replication

  • Two types of DNA repair enzyme systems

    • Constitutive: Always active

    • Damage-inducible: Activated in response to damage

Types of DNA Repair Mechanisms

  1. Proofreading or 3’–5’ Editing

    • Performed by DNA polymerase

    • Corrects wrongly incorporated bases

    • Essential for improving fidelity during replication

  2. Mismatch Repair (MMR)

    • Fixes errors that escape proofreading

    • Enhances accuracy by 100-1000x

    • Rapid process that must occur before the next round of replication

    • Utilizes proteins like MutS and MutL in bacteria for error identification

  3. Direct Reversal

    • Specifically repairs thymine dimers caused by UV light

    • Carried out by photolyase enzyme in some organisms

  4. Base Excision Repair (BER)

    • Involves removal of damaged bases

    • Specific glycosylases recognize and remove damaged bases

    • DNA polymerase and ligase restore DNA integrity

  5. Nucleotide Excision Repair (NER)

    • Not lesion-specific; removes broader regions of DNA

    • Cleavage on both sides of damage allows for strand removal and replacement

  6. Recombination Repair (DSBR)

    • Activated by double-strand breaks during replication

    • Retrieves sequences from homologous dsDNA to fill gaps

    • Similar processes involved in genetic recombination

  7. Error-prone Repairs

    • Non-homologous End Joining (NHEJ): Ligates broken DNA ends directly, highly error-prone

    • SOS Translesion Repair: Bypass DNA polymerases used during stress, introduces mutations but allows for replication to continue

Mismatch Repair System (MMR) Detailed

  • Components: Mut proteins (MutS, MutL, and MutH)

    • MutS scans for distortions, binds to mismatches

    • MutL recruited by MutS, stays at the distortion, activates MutH

    • MutH nicks the unmethylated strand for degradation

  • Mechanism involves methylation status; newly synthesized DNA remains hemimethylated which aids in identifying the error

Specifics of Direct Reversal of Damage

  • Thymine Dimers:

    • Formed by UV light, distorts DNA

  • Repair Mechanism: Photoreactivation using photolyase; not found in humans but present in some eukaryotes

Base Excision Repair Processes

  • Specific DNA glycosylases recognize and remove damaged bases

  • Followed by the replacement processes involving PCNA and ligase for bond restoration

Nucleotide Excision Repair Steps

  • Recognizes and addresses distortions broadly; involves multiple nucleases and polymerases for strand removal and replacement

Recombination Repair Mechanism

  • Addresses double-strand breaks and lesions during replication fork stalling

  • Utilizes homology to repair gaps effectively and maintain genomic integrity

Error-Prone Repair Mechanisms

  • NHEJ: Ligates broken ends; inherently error-prone

  • SOS Response: A stress-induced mechanism allowing for bypass repair, but increases mutation rates

Genetic Recombination Overview

  • Biological Roles:

    • DNA repair, genetic diversity during meiosis, gene formation,

  • Experimental Uses:

    • Gene mapping, creation of transgenic organisms

  • Types of Recombination:

    • General (homologous) recombination, site-specific recombination, and transposition.