Bacteria [genetic variation]

What is transformation?

Uptake of naked, foreign DNA fragments from the surrounding environment (e.g. dead lysed neighbouring cells) → Change of bacterial cell’s genotype and phenotype

How do bacteria accept foreign DNA during transformation?

• Competent cells ← Some bacteria possess cell surface proteins that bind to and transport DNA into the cell

• Artificially competent cells ← Made competent through genetic engineering (immersion in a culture medium with high concentrations of CaCl₂, followed by heat shock treatment)

How is foreign DNA incorporated into bacterial chromosome during transformation?

• Homologous recombination → Crossing over at homologous regions

• Result in a recombinant cell → Permanent change in organism’s phenotype → New allele expressed

What is transduction?

Phages may carry bacterial genes from one host bacterial cell to another recipient bacterial cell

How does host cell DNA enter the phage genome for generalized transduction?

1. Phage enzymes degrade bacterial chromosome into small fragments of DNA during the lytic cycle

2. A small fragment of the host cell’s degraded DNA is packaged within a capsid during assembly of the phage genome

3. Following lysis of the host cell, the defective phage is released to infect another bacterium

How does the recipient bacteria accept foreign bacterial DNA during generalized transduction?

1. Bacterial DNA acquired from the host cell is injected into the recipient cell

2. If crossing over occurs, foreign bacterial DNA can subsequently replace the homologous region of the recipient cell’s chromosome → Recombinant cell → Express new alleles, change in phenotype

What is specialised transduction and how is it different from generalized transduction?

• Bacterial DNA that is transferred is restricted to bacterial genes adjacent to the integrated prophage

• Carried out by temperate phages

• Generalised → Random portion of host bacterial cell DNA transferred

What gives rise to specialised transduction?

1. Phage attaches to host cell and injects viral genome into host → Integrates into the bacterial chromosome → Prophage

2. Phage enters lysogenic cycle → Large population of bacteria carrying the prophage as the cell divides via binary fission

3. Induction → Prophage exits bacterial chromosome (lysogenic → lytic) but viral DNA is improperly excised, such that bacterial DNA adjacent to the prophage is also excised

4. Phage-host hybrid DNA is replicated → Packaged into capsid head during assembly of new viral progeny

How are new alleles incorporated into the bacterial genome of the recipient cell?

• Crossing over at homologous regions

• Integration of the phage-bacterial hybrid DNA into the genome of the recipient cell

What is conjugation?

Direct transfer of genetic material from one bacterial cell to another through a temporary link between the two cells

What are F⁺ and F^- cells?

• F⁺ cells = Donor cells → Bacterial cells which possess an F plasmid

  • Segment of DNA called an F factor that carries genes coding for sex pili to produce sex pili

• F^- cells = Recipient cells → Without an F plasmid

How does is a mating bridge formed during conjugation?

1. Sex pilus on the surface of an F⁺ cell may contact with an F^- cell

2. After the sex pilus attaches, it retracts, pulling the two cells closer → A temporary mating bridge / conjugation tube is formed

What happens in conjugation after the forming of the mating bridge

1. One strand of plasmid DNA passes through the bridge from the F⁺ to F^- cell

2. Replication occurs via rolling circle DNA replication

3. Single strand of plasmid DNA in recipient cell recircularises → Template for the synthesis of a complementary DNA strand

4. Both cells are now F⁺

What occurs during rolling circle DNA replication in the F⁺ cell?

1. One strand of double-stranded F plasmid is nicked by nuclease

2. Free 3’OH end of the nick is extended by DNA polymerase → Synthesis of new complementary strand using intact strand as template

3. Newly synthesized strand displaces the nicked strand at the 5’ end → Transferred concurrently across the mating bridge into the recipient cell

4. Upon completion of the unit length of plasmid DNA, another nick occurs to release the original strand → recircularises → End replication

What occurs during rolling circle DNA replication in the F^- cell?

The single strand of F plasmid DNA recircularises → serves as a template for the synthesis of a complementary DNA strand to form a double-stranded DNA plasmid