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.