Notes on Gene Mutation and DNA Repair

Introduction to Mutations

  • Definition of Mutation: A heritable change in genetic material.
  • Allelic Variation: Mutations provide variation which is critical for evolution.
    • Positive Side: Foundation for evolutionary change, allowing species to adapt to environmental changes.
    • Negative Side: Most mutations are harmful rather than beneficial, often leading to diseases.

Effects of Mutations on Gene Structure and Function

  • Types of Mutations:
    • Changes in Chromosome Structure
    • Changes in Chromosome Number
    • Changes in DNA of a Single Gene: Affect the molecular and phenotypic expression of genes.

Gene Mutations

  • Definition: Molecular changes in the DNA sequence of a gene.
  • Point Mutation: Change in a single base pair. Examples:
    • Base Substitution: e.g., 5’ AACGCTAGATC 3’ → 5’ AACGCGAGATC 3’
    • Types of Base Substitution:
    • Transition: Pyrimidine to pyrimidine or purine to purine.
    • Transversion: Pyrimidine to purine or vice versa.
    • Point mutations can affect polypeptide function:
    • Silent Mutations: Do not alter amino acid sequence (due to degeneracy of the genetic code).
    • Missense Mutations: Change an amino acid, e.g. Sickle cell disease.
    • Nonsense Mutations: Codon changes to a stop codon.
    • Frameshift Mutations: Addition or deletion not divisible by three affects downstream amino acid sequence.

Insertion and Deletion Mutations

  • Indels: Additions or deletions of nucleotide sequences.
    • Examples:
    • 5’ AACGCTAGATC 3’ → 5’ AACGCTC 3’
    • 3’ TTGCGATCTAG 5’ → 3’ TTGCGAG 5’
  • Consequences of Point Mutations:
    • Silent: No effect on protein function.
    • Missense: Possible neutral or inhibitory effects.
    • Nonsense: Likely negative effects.
    • Frameshift: Many negative effects on protein function.

Classification of Mutations

  • Wild-type: Prevalent genotype in a population.
  • Forward Mutation: Changes wild-type to a new variant.
  • Reversion Mutation: Returns mutant allele to wild-type.
  • Deleterious: Decreases survival chances; lethal mutations are the most extreme.
  • Beneficial Mutations: Enhance survival and reproductive success.
  • Conditional Mutations: Phenotype affected only under specific conditions (e.g., temperature sensitivity).

Changes in Chromosome Structure

  • Structural Changes Impacting Gene Expression:
    • Chromosomal rearrangements can impact gene expression either by interrupting a gene or altering its position related to regulatory sequences (position effect).
  • Types of Position Effects:
    1. Next to regulatory sequences.
    2. Movement to heterochromatic regions (less transcription).

Germ-Line vs. Somatic Mutations

  • Germ-Line Mutations: Occur in gametes (eggs/sperm), passed to next generation.
  • Somatic Mutations: Occur in body cells, not inherited but may create genetically different areas (mosaics).
  • Mutation Effects:
    • Germ-line: Present in whole body.
    • Somatic: Result in patches of affected cells.

Spontaneous vs. Induced Mutations

  • Spontaneous Mutations: Internal cellular process errors (e.g., DNA replication mistakes).
  • Induced Mutations: Caused by external mutagens (environmental agents).
  • Sources of DNA Damage: Reactive Oxygen Species (ROS), e.g., hydrogen peroxide, lead to DNA damage if not neutralized by enzymes.
  • Example of DNA Damage: Guanine converted to 7,8-dihydro-8-oxoguanine, causing GC → TA mutations.

Trinucleotide Repeat Expansion (TNRE)

  • Diseases: Caused by repeat expansions (e.g., Huntington's disease).
  • Mechanism: Normal transmission of trinucleotide sequences until critical size reached, leading to disease symptoms.
  • Parental Influence: More frequent expansions depending on whether inherited from mother or father (Huntington's and myotonic dystrophy).

Induced Mutations by Mutagens

  • Types of Mutagens:
    • Chemical: Base modifiers, intercalating agents, base analogues.
    • Base Analogues: E.g., 5-bromouracil (thymine analogue). Can lead to mismatching during DNA replication.
  • Physical: Radiation (X-rays, UV) affects DNA structure.

Mutation Rates and Frequencies

  • Mutation Rate: Likelihood a gene will be altered by mutation; varies across genes.
    • Average in humans: 100-200 new mutations per generation.

DNA Repair Mechanisms

  • Importance: Repair systems are critical to address deleterious mutations. Steps in DNA repair include:
    1. Detection of DNA irregularity.
    2. Removal of abnormal DNA.
    3. Synthesis of normal DNA.
  • Direct Repair: Some chemicals can reverse nucleotide modifications directly (e.g., photolyase for thymine dimers).
  • Base Excision Repair (BER): Recognizes and removes damaged bases using specialized enzymes (e.g., DNA N-glycosylases).
  • Nucleotide Excision Repair (NER): Can remove a range of DNA damage. Key components include UvrA, UvrB, UvrC, UvrD in E. coli.
  • Mismatch Repair: Detects and corrects improperly paired bases post-replication.
  • Double-Strand Break Repair: Managed through homologous repair pathways or non-homologous end joining (NHEJ) to fix severe DNA damage.

Conclusion

  • DNA mutations are crucial in genetics but can lead to serious consequences if not repaired properly, underscoring the importance of various DNA repair mechanisms.