Bacterial DNA Transfer Methods

DNA Transfer between Bacterial Cells

• DNA transfer occurs through three methods: conjugation, transduction, and transformation.
• Conjugation: the mating of two bacterial cells, where DNA is transferred from the donor to the recipient cell, controlled by an F (fertility) plasmid.

Conjugation

• F plasmid carries genes for proteins required for conjugation, including pilin, which forms the sex pilus (conjugation tube).
• High-frequency recombination (Hfr) cells: F+ cells with integrated F plasmid into bacterial DNA, allowing for higher frequency of recombination.
• Hfr plasmid conjugation: increases probability of recombination by incorporating directly into the host genome.

Transduction

• Virus (bacteriophage) acts as a genetic vector, passing DNA from donor to recipient.
• Donor DNA incorporates into recipient DNA, and the bacteriophage containing the bacterial DNA is called transduced DNA.
• Two types of transduction: generalized transduction (random) and specialized transduction (specific genes).
• Transduction can occur in various prokaryotes, including Escherichia coli, Pseudomonas, Salmonella, and Staphylococcus.

Lysogenic Conversion

• The phage DNA can integrate into the cell DNA, and the cell can acquire a new trait, changing a nonpathogenic organism into a pathogenic one.
• Examples of toxins encoded by bacteriophages and transferred by transduction: diphtheria toxin, botulinum toxin, cholera toxin, and erythrogenic toxin.

Transformation

• The transfer of DNA itself from one cell to another, where naked DNA is transferred from a dead donor into a competent recipient.
• Competent recipient: young, actively dividing, or has receptors; donor and recipient are usually closely related.
• Donor DNA recombines with recipient, conferring traits such as virulence factors and antibiotic resistance.

Mutations

• Base substitution: when one base is replaced by another, resulting in a different amino acid being inserted.
• Missense mutation: when the base substitution results in a codon that causes a different amino acid to be inserted.
• Nonsense mutation: when the base substitution generates a termination codon that stops protein synthesis prematurely.
• Frameshift mutation: when one or more base pairs are added or deleted, shifting the reading frame and resulting in the incorporation of wrong amino acids.

Gene Transfer

• Genetic recombination: the process of combining genetic information from two different sources.
• Vertical gene transfer: occurs during reproduction between generations of cells.
• Horizontal gene transfer: the transfer of genes between cells of the same generation.

Plasmids

• Can be integrated with chromosomal DNA, forming episomes.
• Types of plasmids: F (fertility) factor, R (resistance) factor, bacteriocin factor, and virulence factor.

Transposons

• Small DNA segments that can move within the same chromosome or to different chromosomes or plasmids.
• Copy their DNA and insert the copy at another site in the bacterial chromosome or plasmid.

Bacterial Genetics

• Gene: a segment of DNA coding for a functional product.
• Chromosome: a structure made of DNA that contains the genes, carrying hereditary information.
• Genome: all the genetic information in a cell.
• Genotype: the genes of an organism.
• Phenotype: the expression of the genes.

Bacterial Chromosomes

• Bacteria are haploid, with a single chromosome and a single copy of each gene.
• Eukaryotic cells are diploid, with a pair of each chromosome and two copies of each gene.
• The genetic material of a typical bacterium, such as Escherichia coli, consists of a single circular DNA molecule with a molecular weight of about 2 x 10^9 and approximately 5 x 10^6 base pairs.