DNA repair 1

Introduction to DNA Damage

• DNA replication requires intact DNA, not damaged or mutated.

• Importance of DNA repair systems alongside replication.

• Overview of causes of DNA damage and repair mechanisms to be discussed.

  • Today's focus: Causes of DNA damage.

  • Next lecture will cover DNA repair.

Types of DNA Damage

• Damage can range from subtle to significant changes in DNA structure.

• Examples of DNA Damage:

  • Abnormal base pairing (e.g., G paired with U).

  • Chemical attacks leading to interference in replication.

  • Incorrect base pairs (e.g., C with A).

  • Double-stranded breaks (serious damage requiring repair).

  • Single-stranded breaks and basic sites where bases are lost.

  • Thymine dimers caused by ultraviolet (UV) light.

  • Covalent cross-links preventing separation of DNA strands.

  • Insertion or deletion mutations altering genetic information.

Causes of DNA Damage

1. Spontaneous Damage

• Occurs naturally as part of cellular processes:

  • Errors during DNA replication (e.g., incorrect base incorporation).

  • Tautomerization: Base pairs exist in different forms which may result in mismatched pairing.

  • Deamination: Loss of amino group from cytosine leads to uracil production, causing potential mutations.

  • Depurination: Loss of purine base results in basic sites leading to random nucleotide incorporation during replication.

2. Induced Damage

• Damage caused by external factors or agents:

  • Chemical Mutagens: Substances that cause mutations, which can be tested with the Ames test.

    • Intercalating agents: Distort DNA structure (e.g., ethidium bromide).

    • Base analogs: Look like DNA bases (e.g., bromo-uracil), causing wrong base pairing during replication.

    • Alkylating agents: Add groups to DNA bases, affecting base pairing.

    • Deaminating agents: Remove amino groups, increasing the rate of induced mutations.

    • Oxidizing agents: Create radical damage to DNA bases, leading to significant alterations.

  • Ultraviolet Light: Cross-links adjacent thymines causing replication blockage.

  • Radiation: X-rays and gamma rays cause breaks in DNA strands leading to significant mutations.

Repair Mechanisms of DNA Damage

Error Rates and Proofreading

• Replication errors occur at a rate of one in every 10 million base pairs.

• Proofreading mechanism reduces errors to one in every 10 billion base pairs but still allows some mutations.

• Slippage during replication can lead to insertion mutations, especially in repetitive DNA sequences.

Mechanisms for Repair

• Repair mechanisms exist to correct various types of damage before replication occurs:

  • Homologous repair: Utilizes the intact chromosome as a template to repair damage;

    • Involves crossover events leading to accurate repair.

  • Non-homologous end joining: Joins broken ends directly but is less precise and may introduce mutations or rearrangements.

Summary of DNA Damage Context

• A human cell experiences thousands of potential DNA damaging events daily (e.g., depurination, oxidative damage).

• The body continuously engages in repair to maintain genomic integrity.

• The efficiency of repair systems significantly affects health and mutation rates.