Chapter 13: Gene Mutations, Transposable Elements, and DNA Repair

Overview

• Focus on gene mutations and DNA repair.
• Importance of mutations:
  • Mutations as a cause of evolution and genetic diversity.
  • Mutations associated with genetic disorders, cancer, antibiotic resistance, and virulence in viruses.

Categories of Mutations

• Somatic Mutations: Occur in non-reproductive cells, not inheritable.
• Germline Mutations: Occur in gametes, inheritable.

Types of Gene Mutations

• Base Substitutions:
  • Transitions: Purine replaced by purine or pyrimidine by pyrimidine.
  • Transversions: Purine replaced by pyrimidine or vice versa.
• Insertions and Deletions:
  • Cause frame shift mutations unless in-frame (multiples of three nucleotides).
• Expanding Nucleotide Repeats: Increase in the number of copies of a set of nucleotides.

Effects of Gene Mutations

• Phenotypic Effects:
  • Forward Mutation: Wild type to mutant type.
  • Reverse Mutation: Mutant back to wild type.
  • Missense Mutation: Codon codes for a different amino acid.
  • Nonsense Mutation: Codes for a stop codon.
  • Silent Mutation: No change in protein.
  • Neutral Mutation: Amino acid changed to a similar one, minimal effect.
  • Loss of Function: Non-functional protein.
  • Gain of Function: New function.
  • Conditional Mutation: Expresses phenotype under certain conditions.
  • Lethal Mutation: Causes death.

Mutation Rates

• Frequency of mutation is relatively low, thanks to DNA polymerase proofreading.
• RNA viruses like flu and COVID-19 mutate quickly due to lack of proofreading in RNA polymerase.

Causes of Mutations

• Spontaneous Mutations:
  • Errors during replication, strand slippage, or unequal crossing over.
  • Chemical changes like depurination and deamination.
• Induced Mutations:
  • Caused by environmental factors such as chemicals or radiation.
  • Examples of mutagens include base analogs, alkylating agents, and intercalating agents.

DNA Repair Mechanisms

• Proofreading by DNA polymerase.
• Mismatch Repair: Corrects incorrectly inserted nucleotides.
• Direct Repair: Restores nucleotides to correct chemical structure.
• Base Excision Repair: Removes incorrect nucleotide and replaces it.
• Nucleotide Excision Repair: Removes bulky DNA like thymine dimers.

Conclusion

• Mutations are critical to life and evolution but also cause many diseases.
• Understanding mutations helps in grasping their impact on genetic disorders, evolution, and potential treatments.