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Understanding Microbial Genetic Variations

Apr 25, 2025

Variations in Microbial Population

Spontaneous and Induced Variation

Introduction

  • Genome is dynamic, subject to heritable changes.
  • Mutation: Heritable genetic change leading to alternate gene forms.
    • Process of mutation production is called mutagenesis.
    • Organism with novel phenotype due to mutation is a mutant.
    • Mutagen: Agent increasing mutation frequency.
  • Types of mutations include chromosomal changes and chemical changes in genes.

General Characteristics and Role of Mutations

  • Source of genetic variation.
  • Provides raw material for evolution and allele formation.
  • Enables adaptability to environmental changes.
  • Generally recessive, sometimes dominant.
  • Often harmful, random, and recurrent.

Molecular Basis of Gene Mutation

Types of Mutations

  1. Spontaneous Mutations: Natural occurrence without external mutagens.
    • Result from replication errors and spontaneous lesions.
  2. Induced Mutations: Caused by mutagens altering DNA structure.

Spontaneous Mutations: Replication Errors

  • DNA bases can undergo transient rearrangements, changing base pairing properties.
  • Tautomeric shifts can lead to erroneous base pairing (A-C, G-T).
  • Replication errors may result in point mutations.

Mistakes in DNA Replication

  • DNA polymerase makes errors at a rate of 1 per 100,000 nucleotides.
  • Most errors are corrected, but some persist as mutations.

Substitution Mutation

  • One base pair is replaced by another.
    • Transition: Pyrimidine to pyrimidine, purine to purine substitution.
    • Transversion: Pyrimidine replaced by purine or vice versa.

Deletions and Duplications

  • Large deletions often occur at repeated sequences.
  • Duplications also occur at sequence repeats.

Spontaneous Mutations

Types

  • Depurination/Depyrimidination: Loss of purine/pyrimidine, forming apurinic/apyrimidinic sites.
  • Oxidative Damage: ROS cause lesions like conversion of guanine to 8-oxo-guanine.

Induced Mutations

Chemical Mutagens

  1. Base Analogues: Resemble DNA bases, causing substitutions.
  2. DNA-Modifying Agents: Alter base structures, causing mispairing.
  3. Intercalating Agents: Insert between DNA bases, causing frameshifts.

Biological Mutagens

  • Transposons: DNA sequences that can relocate, disrupting gene functionality.

Physical Mutagens

  • Radiations: Ionizing and non-ionizing, causing DNA damage.
    • UV radiation causes thymine dimers.
    • Ionizing radiations cause DNA strand breaks.

Types of Mutations

Forward, Reverse, and Suppressor Mutations

  • Forward Mutation: Wild type to mutant.
  • Reverse Mutation: Mutant to wild type.
  • Suppressor Mutation: Second mutation restoring lost function.

Mutations in Protein-Coding Genes

  1. Silent Mutations: Change codon without changing amino acid.
  2. Missense Mutations: Change codon to encode a different amino acid.
  3. Nonsense Mutations: Convert codon to stop codon, shortening protein.
  4. Frameshift Mutations: Insertions/deletions causing reading frame shifts.
  5. Conditional Mutations: Expressed only under certain conditions.

Detection and Isolation of Mutants

Importance of Mutants

  • Provide insights into DNA replication, gene regulation, and are useful in recombinant DNA procedures.

Techniques

  • Screening: Identifying mutants based on altered phenotypes.
    • E.g., Replica Plating for auxotroph detection.
  • Selection: Using conditions inhibiting wild type but allowing mutant growth.
    • E.g., antibiotic resistance screening.
  • Enrichment: Favoring growth of mutants over non-mutants.
    • E.g., penicillin enrichment for auxotrophic mutants.

References

  • Prescotts Microbiology (10th edition)
  • Life Sciences Fundamentals and Practices II (6th edition)
  • Various online resources on microbial genetics and mutations.