7-BACTERIAL RECOMBINATION

Bacterial Recombination

Overview

• Bacterial recombination refers to the process by which bacteria increase genetic variability, impacting evolution and survival.

Function

• Introduces variation within a species

• Aids in the process of evolution ("Survival of the fittest")

Types of Bacterial Recombination

• Conjugation

• Transformation

• Transduction

Conjugation

Definition

• Transfer of DNA from a donor cell to a recipient cell through direct contact.

Mechanisms

• Gram-negative cells: Produce a sex pilus that facilitates contact between cells.

• Gram-positive cells: Produce sticky molecules, creating a mating bridge for cell contact.

Visual Representation

• Diagram representation of the conjugation pilus and mating bridge.

Summary of Bacterial Conjugation

1. Pilus Formation: Donor (F+) cell attaches to recipient (F- cell) using its pilus, drawing cells together.

2. Cell Contact: Direct contact allows for DNA transfer.

3. Plasmid Replication: The F plasmid replicates its strand, transferring it to the recipient.

4. Recipient Conversion: The recipient synthesizes a complementary strand, resulting in conversion to F+ cell.

Hfr Cells and Recombination

Integration Process

• When an F plasmid integrates into a bacterial chromosome, the cell becomes a high-frequency recombination (Hfr) cell.

• Steps of Recombination:

  1. F plasmid integrates into the donor chromosome.

  2. Upon conjugation, portion of donor chromosome is transferred to the recipient.

  3. Recipient becomes recombinant through homologous recombination.

Transformation

Definition

• Uptake of naked DNA by competent cells; DNA alterations allow uptake.

• Proven that DNA serves as the primary genetic material.

Griffith’s Experiment

• Used Streptococcus pneumoniae to demonstrate transformation:

  • Live encapsulated bacteria killed mice.

  • Heat-killed encapsulated bacteria + live non-encapsulated bacteria killed mice.

  • Resulted in living encapsulated bacteria from dead mice.

Steps of Transformation

1. Uptake: Recipient cell absorbs donor DNA fragments.

2. Alignment: Donor DNA aligns with complementary bases in recipient DNA.

3. Recombination: Homologous recombination occurs, creating genetically transformed cells.

Transduction

Overview

• Transfer of DNA by bacteriophages (viruses infecting bacteria).

• Generalized transduction: Random segments of DNA are transferred.

• Specialized transduction: Only specific donor DNA sequences are transferred.

Bacteriophage Structure

• Components: Head, neck, collar, sheath, tail fiber, and base plate.

Bacteriophage Replication Cycle

1. Phage attaches to host cell and injects DNA.

2. Phage DNA circularizes, entering lytic or lysogenic cycle.

3. New phage particles are produced.

4. Cell lyses, releasing phage virions.

Plasmids

Types

• Conjugative plasmids: Carry genes for sex pilus and plasmid transfer.

• Dissimilation plasmids: Encode enzymes for catabolism of unusual compounds.

• R factors: Carry antibiotic resistance genes.

Transposons

Definition

• Mobile DNA segments that can move within and between DNA molecules.

Importance

• Often carry antibiotic resistance genes.

Types of Transposons

• Simple transposons: Insertion sequences with transposase gene and inverted repeat sequences.

• Complex transposons: Include additional genes unrelated to transposition, often antibiotic resistance.

Transposition Mechanism

1. Transposase cuts DNA, leaving sticky ends.

2. The transposon inserts into the target DNA (e.g., resistance genes into plasmids).