Genetic Change in Bacteria

Genetic Change in Bacteria

1. Mutation

• Concept: Mutation is a sudden change in a character of an individual, inherited to the next generation, creating a new line (variant).

• Characteristics: Mutations are random, rare, stable, specific and independent.

• Mechanism: Due to changes in the gene structure in the DNA molecule.

• Result:

• Morphology: For example, S colony changes to R colony.

• Biochemical properties: Changes in the antigen structure, pathogenicity,…

2. Transformation

• Concept: Transformation is the process of transferring a segment of bacterial DNA into a recipient bacterium.

• Avery: Proved that the transforming agent is DNA.

• Conditions:

• Donor bacteria are broken down to release small DNA fragments.

• Recipient bacteria allow DNA to enter the cell.

• Stages:

• Stage 1: Receive DNA fragments.

• Stage 2: Integrate the DNA fragments into the chromosome through recombination.

• Results: Changes in the ability to synthesize amino acids, sugar metabolism, pathogenicity, resistance, antigen structure.

• Features: Only occurs in bacteria of the same species or closely related species (with gene homology).

3. Conjugation

• Concept: Conjugation is the process of transferring genetic material from a male bacterium to a female bacterium.

• Lederberg and Tatum: Discovered the phenomenon of conjugation.

• Hays: Discovered sex in conjugation.

• F factor:

• F+: The F factor is located in the cytoplasm.

• Hfr: The F factor is integrated into the chromosome.

• F’: The F factor carries a segment of gene out of the cytoplasm.

• Stages:

• Stage 1: Male and female bacteria form F pili conjugation bridges.

• Stage 2: Transfer genetic material from male to female.

• Stage 3: Integrate the transferred gene segment into the chromosome.

• Results:

• F+ and F-: The F factor is transferred, the DNA segment is not transferred. The frequency of receiving the trait is high, the frequency of recombination is low.

• Hfr and F-: The F factor is not transferred, the DNA segment is transferred. The frequency of receiving the trait is low, the frequency of recombination is high.

• Conditions: Bacteria of the same species, if different species then the frequency of recombination is low.

4. Transduction

• Concept: Transduction is the process of transferring genetic material through temperate phages.

• Zinder and Lederberg: Discovered transduction.

• Types of transduction:

• Restricted, specific transduction: A certain phage only transfers a specific segment of gene.

• Generalized incomplete transduction, non-specific: No chromosomal integration, no replication, so only one daughter cell receives the new trait.

• Generalized complete transduction, non-specific: Integrate the chromosome, replicate, so both daughter cells receive the new trait.

5. Plasmid

• Concept: A plasmid is a closed circular DNA molecule located outside the chromosome and capable of self-replication.

• Structure: Includes 3 genes.

• R-plasmid: Carries genes for resistance to antibiotics and heavy metals.

• Characteristics: F factor, toxin production, containing virulence factors (adhesion, cell invasion).

• Episome: Plasmid integrated into the chromosome.

• Transfer of resistance genes: The resistance transfer factor (RTF) of the R-plasmid creates a conjugation bridge to transfer the drug resistance genes.

6. Transposon

• Concept: DNA segments that attach to DNA at various locations through irregular recombination (no homology required).

• Role: Carry genes for antibiotic resistance and virulence.