Chapter 14: Gene Mutation, DNA Repair, and Transposition Notes

Chapter 14: Gene Mutation, DNA Repair, and Transposition

  • Introduction
    • The DNA molecule is crucial for storing, replicating, transmitting, and decoding genetic information.
    • Changes in DNA sequences can lead to variations, impacting phenotypic diversity, adaptation, and evolution.
    • Mutations are essential for genetic analysis, acting as markers for gene identity.

14.1 Gene Mutations are Classified in Various Ways

  • Definition of Mutation
    • Alteration in the nucleotide sequence of a genome.
    • Can be single base-pair substitutions, deletions, insertions, or major chromosomal alterations.
    • Can occur in coding/noncoding regions, including regulatory sequences (promoters, enhancers).

Types of Mutations

  • Classified by molecular change:
    • Point Mutation (Base Substitution): Change from one base pair to another.
    • Missense Mutation: Changes the amino acid produced by a codon.
    • Nonsense Mutation: Introduces a stop codon, terminating translation prematurely.
    • Silent Mutation: Changes in codon that do not change the amino acid.
    • Neutral Mutation: Occurs in non-coding regions.

Types of Base Substitutions

  • Transitions: A pyrimidine replaces another pyrimidine, or a purine replaces another purine.
  • Transversions: A purine is replaced by a pyrimidine and vice versa.

Frameshift Mutations

  • Caused by insertions or deletions of nucleotides.
  • Alters the reading frame of translation, potentially leading to premature stop codons.

Classification Based on Effect on Function

  • Loss-of-Function Mutation: Reduces or eliminates function of the gene product.
  • Dominant Mutation: Results in a mutant phenotype in a diploid organism.
  • Recessive Mutation: Causes a loss of function that requires two copies to exhibit a phenotype.
  • Dominant Negative Mutation: An inactive gene product from one allele interferes with the normal product from the other.
  • Gain-of-Function Mutation: Produces a new or enhanced function, often dominant.

Classification Based on Location

  • Somatic Mutations: Occur in non-germ cells, not heritable.
  • Germ-Line Mutations: Occur in gametes, hereditary.
  • Mutations can also be autosomal or X/Y-linked based on their chromosomal location.

14.2 Spontaneous and Induced Mutations

  • Spontaneous Mutations: Naturally occurring changes in sequences due to replication errors.
  • Induced Mutations: Result from external factors like radiation or chemicals.

Mutation Rates

  • Low rates for all organisms; varies by gene.
  • Viral and bacterial mutations occur at approximately 1 in 100 million.
  • In humans, an average newborn carries about 60 new mutations compared to parents.

14.3 Spontaneous Mutations Arise from Replication Errors

  • Replication Errors: DNA polymerase may insert incorrect nucleotides.
  • Tautomeric Shifts: Bases can exist in alternative forms, increasing mispairing chances during replication.
  • Depurination: Loss of purines leads to apurinic sites (missing base).
  • Deamination: Conversion of amino groups to keto groups, resulting in incorrect base pairing.

14.4 Induced Mutations from Chemical/Radiation Damage

  • Mutagens: Natural or artificial agents that induce mutations (Example: UV light, X-rays, chemicals).
  • Base Analogs: Chemicals mimicking nucleotides, causing increased mutation rates.
  • Alkylating Agents: Modify nucleotides and lead to transition mutations.
  • Intercalating Agents: Distort DNA and disrupt replication/repair.

Effects of Radiation

  • UV Radiation: Causes the formation of pyrimidine dimers, distorting DNA structure.
  • Ionizing Radiation: Produces free radicals that can affect genetic material, causing significant mutations.

14.5 Single-Gene Mutations Cause Human Diseases

  • Most diseases are polygenic; however, single-gene mutations can cause specific conditions (e.g., cystic fibrosis, Marfan syndrome).
  • Diseases are often categorized by the type of mutation.

14.6 DNA Repair Systems

  • Essential for maintaining genetic integrity against mutations.

Repair Mechanisms

  • Proofreading: DNA polymerase corrects misinserted nucleotides.
  • Mismatch Repair (MMR): Activated for additional correction if proofreading fails.
  • Postreplication Repair: Handles cases of escaped damage during replication.
  • SOS Repair System: Error-prone repair that allows survival under DNA damage, can lead to further mutations.
  • Photoreactivation: Reverses UV radiation damage (not present in humans).
  • Base and Nucleotide Excision Repair: Mechanisms that cut out damaged sections and replace them.

14.7 Ames Test for Mutagenicity

  • Uses strains of Salmonella typhimurium to detect mutations and assess the impact of chemicals.
  • Widely used in industry for screening potential carcinogens.

14.8 Transposable Elements (TEs) and Mutation

  • TEs are segments of DNA that can move within the genome, potentially causing mutations.
  • Can have significant evolutionary impacts due to their ability to insert and disrupt genes.

Classes of Transposable Elements

  • DNA Transposons: Move without RNA intermediates.
  • Retrotransposons: Use RNA intermediates to replicate and insert into the genome.
  • Implications in human genetics and evolutionary biology, affecting gene expression and causing diseases.
  • Examples of TEs' impact include hemophilia caused by insertions in specific genes.
  • The significant presence of TEs in the human genome contributes to genetic diversity and function.