Genetic Variation and Horizontal Gene Transfer in Microbiology

Genetic Variation: Mutations and Their Origins

Spontaneous and Induced Mutations

Genetic mutations are changes in the DNA sequence that can arise naturally or be induced by external factors. These mutations are a primary source of genetic variation, even in organisms that reproduce asexually.

• Spontaneous Mutations: Naturally occurring mutations resulting from errors during DNA replication.

• Genetic Variation: Mutations introduce diversity in populations, providing raw material for evolution.

• Mutant Strain: Cells that carry a mutation.

• Wild-Type Strain: Cells that do not carry the mutation (nonmutated).

Mutagens and Carcinogens

• Mutagens: Agents that increase the mutation rate. They can be chemical, physical, or biological.

• Chemical Mutagens: Organic or inorganic substances that cause DNA breaks, modify bases, or induce frameshift mutations. Examples: Arsenic, asbestos, alcohol, compounds in tobacco smoke.

• Physical Mutagens: Physical factors that damage DNA, similar to chemical mutagens. Examples: Ultraviolet (UV) light, X-rays, gamma rays.

• Biological Mutagens: Living agents that introduce genetic changes, often through genetic exchange. Examples: Transposons, certain viruses.

• Carcinogens: Mutagens that increase mutation rates to levels that promote cancer development.

Horizontal Gene Transfer (HGT) in Bacteria

Vertical vs. Horizontal Gene Transfer

Gene transfer in bacteria can occur vertically (from parent to offspring) or horizontally (between unrelated cells).

• Vertical Gene Transfer: Transmission of genetic information to the next generation via cell division (sexual or asexual).

• Horizontal Gene Transfer (HGT): Transfer of genetic material between cells independent of cell division, contributing to rapid genetic diversity.

Mechanisms of Horizontal Gene Transfer

• Plasmids

• Conjugation

• Transformation

• Transduction

• Transposons

Plasmids

• Definition: Small, extrachromosomal DNA molecules that can replicate independently and be transferred between bacteria.

• Function: Often carry genes for antibiotic resistance (R plasmids) or other advantageous traits.

• Applications: Used in biotechnology to engineer bacteria for medical or industrial purposes.

Conjugation

Conjugation is a process where genetic material is transferred between bacterial cells via direct contact.

• Fertility Plasmid (F plasmid or F factor): Encodes the machinery (pilus) for DNA transfer.

• Process:

  1. Donor cell (F+) attaches to recipient cell (F-) via a pilus.

  2. Pilus forms a bridge for transferring a copy of the F plasmid.

  3. Pilus dismantles and cells separate.

• High-Frequency Recombination (Hfr) Strains: Occur when the F plasmid integrates into the bacterial chromosome, allowing transfer of chromosomal genes during conjugation.

• Key Difference: F+ cells transfer only the plasmid, while Hfr cells can transfer chromosomal DNA as well. Additional info: See slide 120 for detailed comparison.

Transformation

Transformation involves uptake of free DNA from the environment by a bacterial cell.

• No Pilus Involved: DNA crosses the cell wall through absorption.

• Competent Cells: Cells capable of taking up exogenous DNA.

• Historical Example: Frederick Griffith (1920s) demonstrated transformation in Streptococcus pneumoniae, showing acquisition of pathogenic traits.

Transduction

Transduction is the transfer of bacterial genes by viruses (bacteriophages).

• Bacteriophages: Viruses that infect bacteria.

• Outcomes After Infection:

  • Phage DNA directs production of new phages (lytic cycle).

  • Phage DNA integrates into host genome (lysogenic cycle).

• Generalized Transduction: Random bacterial DNA is packaged into phage particles during assembly. Only one virion typically carries bacterial DNA.

• Specialized Transduction: Temperate phages integrate into the host genome and, upon excision, carry specific bacterial genes with them. Multiple virions may carry hybrid DNA.

• Prophage: The integrated viral DNA during the lysogenic cycle.

• Temperate Phage: A bacteriophage capable of both lytic and lysogenic cycles.

• Additional info: See slides 126 and 128 for diagrams and further details.

Transposons

Transposons, or "jumping genes," are DNA sequences that can move from one location to another within a genome.

• Types:

  • Retrotransposons: Move via an RNA intermediate (more common in eukaryotes).

  • DNA Transposons: Move directly as DNA (more common in prokaryotes).

• Distribution: Both types are found in prokaryotes and eukaryotes, but their prevalence differs by domain.

Summary Table: Mechanisms of Horizontal Gene Transfer

Mechanism	Key Features	Example
Conjugation	Direct cell-to-cell contact via pilus; transfer of plasmids or chromosomal DNA (Hfr)	F plasmid transfer in E. coli
Transformation	Uptake of free DNA from environment; requires competent cells	Streptococcus pneumoniae acquiring capsule genes
Transduction	Transfer of DNA via bacteriophages; can be generalized or specialized	Phage-mediated gene transfer in Salmonella
Transposons	Mobile genetic elements; can move within or between DNA molecules	Antibiotic resistance gene transfer
Plasmids	Extrachromosomal DNA; often carry beneficial genes	R plasmids in pathogenic bacteria

Key Terms and Definitions

• Mutation: A change in the nucleotide sequence of DNA.

• Mutagen: An agent that increases the frequency of mutations.

• Carcinogen: A mutagen that promotes cancer development.

• Plasmid: A small, circular DNA molecule separate from chromosomal DNA.

• Transposon: A DNA sequence that can change its position within the genome.

• Bacteriophage: A virus that infects bacteria.

• Prophage: Phage DNA integrated into the bacterial genome during lysogeny.

• Competent Cell: A bacterial cell capable of taking up DNA from its environment.