DNA Mutation and Repair

Chapter 16: DNA Mutation and Repair

  1. Introduction to DNA Mutations

    • Definition: Changes in the DNA sequence that can lead to alterations in gene function.

    • Types of mutations: Point mutations, insertions, deletions, and duplications.

  2. Causes of DNA Mutations

    • Spontaneous mutations: Naturally occurring errors during DNA replication.

    • Induced mutations: Resulting from environmental factors such as radiation and chemicals.

  3. DNA Repair Mechanisms

    • Base excision repair: Corrects small, non-helix-distorting base lesions.

    • Nucleotide excision repair: Removes bulky DNA adducts and helix-distorting lesions.

    • Mismatch repair: Fixes replication errors that escape proofreading.

  4. Consequences of DNA Mutations

    • Potential to lead to diseases, including cancer.

    • Role in evolution and genetic diversity.

Types of Base Substitutions

  • Base Substitution Classification:

    • Transition: a substitution where a purine is replaced by another purine or a pyrimidine is replaced by another pyrimidine.

    • Transversion: a substitution where a purine is replaced by a pyrimidine or vice versa.

Spontaneous vs Induced Mutations

  • Spontaneous Mutations:

    • Naturally occurring mutations arising in all cells.

    • Examples include depurination (loss of base), deamination (cytosine to uracil), and oxidation damage due to reactive oxygen species (ROS).

  • Induced Mutations:

    • Caused by exposure to external factors (mutagens) such as chemical compounds or radiation.

    • Types include base analogs, specific mispairing mutagens, intercalating agents, and direct base damage.

Point Mutations

  • Types of Point Mutations:

    • Indel (Frameshift): insertion or deletion of bases that shift the reading frame.

    • Insertion: addition of one or more nucleotides.

    • Deletion: removal of one or more nucleotides.

    • Missense: change in a single amino acid coding.

    • Nonsense: introduction of a premature stop codon.

    • Silent: change in the nucleotide sequence that does not affect the protein product.

Effects of Mutations

  • Mutations in untranslated regions can affect gene expression.

  • Many mutations in non-coding regions have no observable effect on phenotype.

    • Regulatory sequences, promoters, and splice sites may be affected.

DNA Repair Mechanisms

  • Types of DNA Repair:

    • Direct Repair: uses enzymes to undo damage (e.g., CPD photolyase).

    • Base Excision Repair: removes and replaces incorrect or damaged bases.

    • Nucleotide Excision Repair: repairs bulky DNA adducts using XP family proteins for global repair and CS family proteins for transcription repair.

    • Mismatch Repair: corrects mismatches using methylation to determine parent vs daughter strands.

Specific Examples of Damage and Repair

  • Pyrimidine Dimer Repair: corrected through nucleotide excision repair.

  • Mismatch Repair in E. coli: relies on methylation to identify the template strand.

  • Frameshift Examples: Can arise from slippage during replication or from trinucleotide-repeat diseases (e.g., Fragile X Syndrome).

Disease Associations

  • Xeroderma Pigmentosum: Autosomal recessive disorder with mutations in XP genes, leading to impaired nucleotide excision repair, increasing cancer risk.

Conclusion

  • Understanding the types of mutations and the corresponding repair mechanisms is crucial for understanding genetic stability and the implications for health and disease.