1. Genetic Recombination in Bacteria

Bacteria, being prokaryotic organisms, lack a true nucleus and reproduce asexually via binary fission. However, they still exchange genetic material through genetic recombination, ensuring adaptability and evolution. This process allows genetic material to be incorporated into a bacterial genome from another bacterium or external sources.

2. Transformation

Definition: Transformation is the uptake of free, naked DNA fragments from the environment by a bacterium.

How it Happens:

1. Source of DNA: When a bacterium dies, its DNA fragments are released into the environment.
2. Competence: Some bacteria develop a special state called competence, which allows them to bind and take up DNA.
3. DNA Uptake: The competent bacterium binds to the DNA fragments on its surface and transports them inside.
4. Integration: The incoming DNA aligns with the homologous region of the bacterial genome and is incorporated through a process called homologous recombination.

Examples:

• Griffith’s Experiment (1928): Demonstrated transformation in Streptococcus pneumoniae. A non-virulent strain became virulent when mixed with heat-killed virulent bacteria, due to the uptake of DNA coding for the capsule.

Applications:

• Genetic engineering (e.g., introducing new traits into bacteria in the lab).
• Natural adaptation (e.g., acquiring antibiotic resistance genes).

3. Conjugation

Definition: Conjugation is the direct transfer of DNA between two bacterial cells via physical contact, typically mediated by a pilus.

How it Happens:

Donor and Recipient:

1. • The donor cell possesses an F plasmid (fertility factor), making it an F+ cell.
   • The recipient does not have the plasmid, making it an F- cell.

Pilus Formation: The donor cell forms a sex pilus, a tubular structure, which attaches to the recipient cell.

DNA Transfer:

1. • A copy of the F plasmid is transferred from the donor to the recipient via the pilus.
   • If the F plasmid integrates into the donor’s chromosome, it becomes an Hfr (high-frequency recombination) cell.
   • Hfr cells transfer chromosomal genes along with the plasmid genes to the recipient.

Result: The recipient becomes an F+ cell (with the plasmid) or receives new genetic material if chromosomal DNA was transferred.

Examples:

• Spread of antibiotic resistance genes among bacterial populations.
• Transfer of virulence factors in pathogenic bacteria.

4. Transduction

Definition: Transduction is the transfer of bacterial DNA from one cell to another via a bacteriophage (a virus that infects bacteria).

Types of Transduction:

Generalized Transduction:

1. • Occurs during the lytic cycle of a bacteriophage.
   • The phage accidentally packages fragments of bacterial DNA instead of its own.
   • When this phage infects another bacterium, the carried DNA is introduced and can integrate into the recipient’s genome.

Specialized Transduction:

1. • Occurs during the lysogenic cycle.
   • The phage integrates its DNA into the host chromosome as a prophage.
   • When the prophage excises, it may carry adjacent bacterial genes with it.
   • These genes are transferred to another bacterium during infection.

Key Features:

• Virus-mediated: Transduction relies on phages for gene transfer.
• Gene Integration: Bacterial DNA transferred by the phage may integrate into the recipient’s genome via homologous recombination.

Example:

• Transfer of toxin genes, such as those encoding the diphtheria toxin, by bacteriophage Corynebacterium diphtheriae.

5. Plasmids

Definition: Plasmids are small, circular, double-stranded DNA molecules separate from the bacterial chromosome.

Key Characteristics:

• Replication: Plasmids replicate independently of the bacterial chromosome.
• Size: Typically 1-200 kilobases.
• Genes: Carry non-essential but advantageous genes (e.g., antibiotic resistance, toxin production, or metabolic enzymes).

Types of Plasmids:

1. F Plasmids (Fertility): Involved in conjugation (e.g., F plasmid in E. coli).
2. R Plasmids (Resistance): Carry genes for antibiotic resistance.
3. Virulence Plasmids: Contain genes that enhance bacterial pathogenicity (e.g., Yersinia pestis plasmid for toxins).
4. Metabolic Plasmids: Encode enzymes for unusual metabolic pathways.

Applications:

• Genetic engineering (e.g., cloning vectors in biotechnology).
• Spread of antibiotic resistance in bacterial populations.

6. Transposons (Jumping Genes)

Definition: Transposons are DNA sequences that can move from one location to another within the genome.

Key Features:

• Structure: Composed of inverted repeat sequences at both ends and a central region encoding enzymes like transposase (needed for movement).
• Movement:
  • Cut and Paste: Transposon is excised from one location and inserted into another.
  • Copy and Paste: A copy of the transposon is inserted elsewhere while the original remains in place.

Impact:

• Cause mutations by disrupting genes.
• Carry genes for antibiotic resistance, spreading them among bacteria.

Examples:

• Tn3: A transposon carrying beta-lactamase genes for ampicillin resistance.
• Insertion Sequences (IS elements): Simplest transposons containing only the transposase gene.

Summary of Mechanisms