DNA and Mutation

Lecture 1.3: DNA and Mutation

Chapter 1: Sections 1.11 – 1.21

Mutations

• Definition of Mutation: A heritable change in the genetic material.

• Role of Mutations:

  • Allelic Variation: Mutations provide the genetic variability necessary for evolution.

  • Positive Outcomes:

  • Serve as the foundation for evolutionary change.

  • Allow organisms to adapt to changes in their environment.

  • Negative Outcomes:

  • New mutations can be harmful, leading to diseases.

  • Statistically, mutations are more likely to be harmful than beneficial.

Effects of Mutations on Gene Structure and Function

• Mutations can significantly alter both molecular and phenotypic expression of genes.

• Types of Changes:

  • Changes in Chromosome Structure

  • Changes in Chromosome Number

  • Changes in the DNA of a Single Gene

Classes of Mutations

1. Spontaneous Mutations

• Origin: Result from errors in biological processes, particularly during DNA replication.

• Characteristics:

  • Occur naturally within healthy cells, without contamination.

  • Contribute to background levels of mutations.

  • Background Mutation Rate factors:

  • Stringency of the replication system.

  • Effectiveness of repair systems.

  • Variation across species and cell types.

  • Generally occur at a slow frequency, though much higher in viruses.

2. Induced Mutations

• Origin: Caused by environmental agents known as mutagens.

• Types of Induced Mutagens:

  • Radiation: Includes ultraviolet and ionizing radiation (such as gamma rays).

  • Chemical Agents: Such as base analogs (e.g., BrdU), oxidative agents.

  • Biological Factors: Pathogens, e.g., viruses inserting their DNA into the host genome.

Types of Mutations by Molecular Nature

1. Nucleotide Substitution:

   • Known as point mutations where one nucleotide is replaced by another.

2. Nucleotide Insertions:

   • Involves the addition of one or more nucleotides to the DNA sequence.

3. Nucleotide Deletions:

   • Entails the removal of one or more nucleotides from the DNA sequence.

Types of Mutations by Molecular Nature Continued

Nucleotide Substitution Examples:

• Transitions: Substitutions where a purine is replaced by another purine or a pyrimidine by another pyrimidine.

  • Example: Change from G-C base pair to A-T leads to continuation of G-C base pairs.

• Transversions: Substitutions where a purine is replaced by a pyrimidine or vice versa.

  • Example: G to C mutation (purine to pyrimidine).

Mutations by Scale

• Single Base Pair Mutations: A point mutation affects just a single base pair and can be classified as a substitution, insertion, or deletion.

• Large Scale Mutations:

  • Insertion and Deletion: Can occur at a point mutation scale or larger.

  • Chromosome Rearrangements: Caused by double-strand DNA breaks leading to deletions, duplications, inversions, or translocations, which may affect chromosomes within or between them.

Example of Chromosome Rearrangement

• Philadelphia Chromosome:

  • Forms from a translocation between chromosome 9 and chromosome 22 resulting in a fusion gene between ABL1 and BCR genes.

  • ABL1: Normally encoded a tyrosine kinase that is self-regulated. The fusion resulted in a constitutively active form leading to chronic myelogenous leukemia (CML).

  • Mutation found in 95% of CML patients; associated with aging; no known cause aside from high doses of radiation as a minimal risk factor.

  • The associated BCR::ABL1 fusion gene acts as an oncogene, highlighting a critical mutation hotspot.

Gene Mutations Impact on Coding Sequence

Types of Coding Sequence Mutations:

• Silent Mutations:

  • Base substitutions that do not alter the amino acid sequence due to the degeneracy of the genetic code.

• Missense Mutations:

  • Base substitutions that do alter the amino acid sequence, with variable effects on protein function (example: Sickle-cell disease).

  • Sickle-cell Disease: A single amino acid change (Glu to Val at position 17) leads to altered hemoglobin structure and function.

Nonsense and Frameshift Mutations

• Nonsense Mutations:

  • Convert normal codons to stop codons, potentially resulting in truncated, nonfunctional proteins.

  • Example: Cystic fibrosis transmembrane conductance regulator (CFTR) when mutated leads to improper ion transport.

• Frameshift Mutations:

  • Caused by insertion or deletion of nucleotides that are not multiples of three, altering downstream amino acid sequences drastically.

Consequences of Point Mutations Within a Coding Sequence (Table 19.1)

Suppressor Mutations

A suppressor mutation is a second mutation that alleviates or counteracts the phenotypic effects of a first mutation. This is distinct from a direct reverse mutation (reversion) because it occurs at a different site than the original mutation.

1. Intragenic Suppressor Mutation (Reversion)

• Location: The second suppressor mutation occurs within the same gene as the first mutation.

• Mechanism: It often restores the protein's function by compensating for the structural change caused by the initial mutation.

• Example: If the first mutation is an insertion that causes a frameshift, an intragenic suppressor might be a deletion nearby that restores the original reading frame downstream ($n+1-1=0$).

• Protein Scaling: It may also involve a second amino acid change that restores the proper folding or activity of the protein that was disrupted by the first amino acid change.

2. Intergenic Suppressor Mutation

• Location: The second suppressor mutation occurs in a different gene than the first mutation.

• Mechanism: These often involve changes in the cellular machinery or metabolic pathways that bypass the original defect.

• Common Examples:

  • Redundant Pathways: A mutation in a second gene turns on a pathway that performs the same function as the defective gene from the first mutation.

  • Protein-Protein Interactions: If two proteins ($A$ and $B$) must bind, and a mutation in gene $A$ prevents binding, a suppressor mutation in gene $B$ might change its shape to restore the interaction.

  • Nonsense Suppressors: A mutation in a tRNA gene (e.g., a change in the anticodon) allows the tRNA to recognize a stop codon produced by a nonsense mutation in the first gene, allowing full-length protein synthesis to continue.