DNA Damage and Repair Notes

DNA Damage Overview

  • DNA can be damaged through various mechanisms, potentially leading to mutations if not repaired.
  • Mutations: Changes in DNA base pairs or chromosomes that introduce variability in genetic information.

Distinction Between DNA Damage and Mutation

  • DNA Damage: Chemical alterations of DNA that can lead to mutations if left unrepaired.
  • Mutation: A final result of damage, representing a change in DNA sequence or chromosome structure.

Causes of DNA Damage

  • Endogenous Agents: Damage arising from internal cellular processes.
  • Exogenous Agents: Damage caused by external environmental factors.

Radiation Damage

Types of Radiation
  • UV Light, Gamma Rays, X-Rays: High-energy electromagnetic radiation that can severely damage DNA.
  • UV Radiation: Divided into UVA, UVB, and UVC:
    • UVA: Causes aging and skin cancer; penetrates skin and glass.
    • UVB: Induces sunburn and skin cancer; strongest between 10 AM and 4 PM.
    • UVC: Blocked by the ozone layer.
Effects of UV Light
  • Pyrimidine Dimers: Most common effect, where adjacent pyrimidine bases (e.g., thymine) bond abnormally, distorting DNA structure and affecting replication.
Cyclobutane Pyrimidine Dimers
  • Form between adjacent pyrimidines, commonly thymine-thymine. Alters base sequence during replication.

Reactive Oxygen Species and DNA Damage

  • Reactive species generated from radiation and cellular processes can lead to oxidative damage.
  • Common reactive species include:
    • H₂O•⁺ (water radical cation)
    • •OH (hydroxyl radical)
    • O₂•⁻ (superoxide anion)

Hydrolytic Cleavage

  • DNA is also subjected to hydrolytic damage:
    • Sensitive Bonds:
    1. Phosphodiester Bonds
    2. N-glycosyl Bonds
    3. Bonds linking amines to DNA bases (deamination).
Depurination Process
  • A mammalian cell loses about 10,000 purines/day due to spontaneous processes. This can result in genetic damage unless repaired.
Deamination Events
  • Conversion of cytosine to uracil can result in pairing misalignments during replication, leading to mutations:
    • G-C pairs change to A-T pairs, resulting in transition/transversion mutations.

Mutagen Screening: The Ames Test

  • The Ames Test developed by Bruce Ames assesses the mutagenic potential of compounds using strains of bacteria that cannot synthesize histidine (his-). It measures reverse mutations to determine carcinogenic risks.

Conclusion

  • Understanding DNA damage mechanisms, repair processes, and how mutations arise is crucial for genetics, cancer research, and evaluating environmental risks. Efficient repair systems are vital in maintaining genetic integrity over cell division cycles.