DNA repair 2

DNA Repair Mechanisms

Introduction to DNA Repair

  • DNA repair involves correcting sequence issues and ensuring the integrity of the genetic material.

  • Scanning mechanisms are essential as damage locations cannot be predicted.

  • Two primary types of issues:

    • Non-standard nucleotides: Incorrect or exotic bases that should not be present.

    • Mismatch pairs: Normal bases that are paired incorrectly (e.g., G with T).

Repair Strategies

  • Specialized repair proteins are responsible for correcting errors:

    • Direct reversal: Straightforward repair of minor modifications.

    • Excision repair: More involved, involves removing segments of DNA and re-synthesis.

  • Time-sensitive: Repairs must occur before DNA replication to prevent mutations.

Importance of Tumor Suppressors

  • P53: A crucial protein acting as a guardian of the genome.

    • Detects damaged DNA and arrests the cell cycle to allow for repair.

    • Activates DNA repair proteins in response to damage.

    • Triggers apoptosis or senescence if damage is irreparable, preventing tumor formation.

  • Mutations in p53 are frequent in cancers, leading to defective repair systems and increased mutation rates.

Types of DNA Repair Mechanisms

  1. Direct Reversal: Restores normal nucleotides with minimal alteration.

    • Example: Removal of alkylation modifications (demethylation).

    • Methyl transferase enzyme restores guanine by transferring unwanted methyl groups.

    • Special enzymes (e.g., DNA photolyase) can repair cross-links induced by UV light.

  2. Base Excision Repair:

    • Targets specific damaged bases without affecting the entire nucleotide.

    • Glycosylases recognize and remove faulty bases, creating abasic sites.

    • AP endonuclease cleaves the DNA backbone, allowing for restoration via DNA polymerase.

    • Discrimination between correct bases (T) and incorrect bases (U) ensures proper repair.

  3. Nucleotide Excision Repair (NER):

    • Removes distorted sections of DNA due to bulky adducts (e.g., benzopyrene).

    • A protein complex (UvrABC) scans the DNA, binds to distortions, and removes a segment for repair.

    • Involves specialized helicase to eject the damaged fragment and prepare for replication.

  4. Mismatched Repair:

    • Identifies and corrects incorrect base pairs (e.g., A pairing with G incorrectly).

    • Utilizes enzymes like MutS and MutL to distinguish between newly synthesized and template strands.

    • The MutH protein cleaves the daughter strand for correction, often removing significant DNA segments.

Translesion Synthesis

  • Specialized polymerases (e.g., pol IV, pol V) can bypass DNA lesions.

  • They allow for incomplete replication where matching bases might not occur, leading to potential mutations but ensuring DNA synthesis is completed.

Implications of Defective Repair Systems

  • Disease conditions arising from repair deficiencies include:

    • Xeroderma Pigmentosum: Genetic defect leads to severe UV sensitivity and skin cancer due to NER failure.

    • Colon Cancer: Mutations in mismatch repair genes (MutS, MutL) lead to increased mutation rates and cancer risk.

    • Lung Cancer: Significant damage observed from smoking overwhelms repair mechanisms, resulting in extensive mutations.

Conclusion

  • DNA repair systems are crucial for genome stability; their impairment can lead to severe health consequences.

  • Despite repair mechanisms reducing mutation rates significantly, environmental factors like tobacco can vastly increase mutation occurrences, leading to severe diseases such as cancer.