DNA Mutations and Repair Mechanisms

Mutations and DNA Repair

Chemically Induced Mutations

  • Definition of Mutagen: Any factor that induces mutation above the spontaneous rate.

  • Types of Mutagens:

  • Base Analogs: Mimic normal bases and can be mistakenly incorporated into DNA.

  • Alkylating Agents: Add alkyl groups (e.g., methyl and ethyl) to DNA, altering base pairing.

    • Example: Ethyl methanesulfonate (EMS) can ethylate guanine, allowing it to pair with thymine.

  • Deamination: The removal of an amine group from a base, which can alter base pairing.

  • Oxidative Reactions: Highly reactive oxygen species can modify bases, leading to mutations.

  • Intercalating Agents: Insert between base pairs, causing insertions and deletions during replication (e.g., acridine orange, ethidium bromide).

Radiation-Induced Mutations

  • Types of Radiation:

  • UV Radiation: Causes formation of pyrimidine dimers which link adjacent thymidine or cytosine bases together.

    • Consequences: Disrupts base pairing and DNA replication, potentially leading to apoptosis.

  • Ionizing Radiation: (e.g., X-rays) generates free radicals that can break DNA strands.

DNA Repair Mechanisms

  1. Mismatch Repair:

  • Corrects DNA replication errors that DNA polymerase does not fix.

  • Mismatched bases create bubbles in the DNA, guiding repair systems to recognize the error.

  • Methylation at GATC sequences helps identify which strand is the correct template (unmethylated = new strand).

  • Process:

    • Recognize mismatch

    • Excision of incorrect base by exonuclease

    • Replacement with correct base by DNA polymerase

    • Nick sealing by DNA ligase

  1. Direct Repair:

  • Repairs certain mutations without removing or replacing bases.

  • Example: Photolyase enzyme uses light energy to reverse pyrimidine dimers directly.

  1. Base Excision Repair (BER):

  • Targets specific damaged bases. Steps include:

    • Recognition and removal of the damaged base by DNA glycosylase (creates an AP site).

    • AP endonuclease cleaves the DNA strand at the AP site, leading to DNA polymerase filling in the gap.

    • Ligase seals the final strand.

  • Error-prone, especially in eukaryotes where polymerase beta lacks proofreading.

  1. Nucleotide Excision Repair (NER):

  • Removes and replaces a short segment of DNA containing the damage. Steps include:

    • Identify damaged region

    • Excision of several nucleotides surrounding the damage

    • Replacement with correct nucleotides

    • Sealing by ligase

  1. Repairing Double-Stranded Breaks:

  • Non-homologous End Joining (NHEJ): Directly joins broken ends, often resulting in deletions.

  • Homologous Recombination: Uses the homologous chromosome as a template to accurately repair the break.

Genetic Diseases Associated with DNA Repair Defects

  • Xeroderma Pigmentosum: Defects in NER, leads to sensitivity to sunlight and skin cancer.

  • Cockayne Syndrome: Defects in NER, results in sensitivity to sunlight and premature aging.

  • Trichothiodystrophy: Also linked to NER defects, affects hair and skin development.

  • Hereditary Nonpolyposis Colon Cancer: Due to mismatch repair defects, causing increased cancer risk.

  • Fanconi Anemia: Involves defects in the repair of interstrand cross-links.

  • Ataxia Telangiectasia: Associated with defects in DNA damage response mechanisms.

  • Li-Fraumeni Syndrome: Increases cancer predisposition due to defective DNA damage response.