Microbial genetics: Genetic transfer and recombination

Gene transfer

movement of genetic information between organisms

• In eukaryotic organisms this can occur during the fertilization of an egg

• In prokaryotic organisms, this is not an essential part of the life cycle(When it does occur, DNA is transferred from a DONOR to a RECIPIENT cell)

recombination

Combining genes (DNA) from two different cells

• the resulting cell is a recombinant (result of horizontal or vertical gene transfer)

Vertical gene transfer

Occurs during reproduction or between generations of cells.

• parent to daughter down family tree

• binary fission

Horizontal gene transfer

The transfer of genes between cells of the same generation.

• cells donating nucleic acids to completely different cell, not binary fission

What are the three ways bacteria can transfer DNA

• transformation

• conjugation

• transduction

Transformation

Naked DNA from cell to cell

• the uptake of free-floating DNA by a cell that is maintained in a heritable form and incorporated into its own genome.

• competent cells are cells that can take up the DNA

• competence can be altered in lab settings

Conjugation

transfer of DNA via cell to cell contact via a pili

• required a conjugative plasmid (F factor)

• F Factor (fertility factor) contains genes for cell attachment and plasmid transfer

• sex pilus (because it kinda looks like they’re doing it) provides channel for F factor to transfer DNA

• can be between completely unrelated cells

Plasmids (conjugation)

• replicate independently from bacterial genome

• breaks apart double helix and sends one strand to recipient, then both cells use DNA polymerase to create double helix

• F+ can transfer to F+, not always F+ to F-

Hfr cells (conjugation)

a process where a specialized donor bacterium (F+ cell) uptakes its plasmic DNA into its genome and transfers part of its main chromosome along with fertility plasmid (F factor) genes into a recipient cell through direct physical contact.

• may transfer plasmic DNA, genome DNA, or mix of both

• how most antibiotic resistance is transferred

Transduction

DNA is transferred from a donor cell to a recipient via a bacteriophage (a virus that infects bacteria by injecting DNA)

• generalized transduction: occurs with lytic viruses

• specialized transduction: occurs with prophages

What is the lytic cycle

Viruses do not replicate on their own, they infect host cells and force the cell to make copies

1. phage attaches to cell and injects viral chromosome

2. once inside, genes are transcribed and translated into protein

3. breakdown of bacterial chromosome by phage-specific enzyme

4. replication of phage chromosome using bacterial materials and phage enzymes

5. expression of phage genes to produce phage structural components

6. assembly of phage particles

7. release of phases by lysis of bacterial wall

Virulent phage:

undergoes the lytic cycle

Lysogenic cycle

• some viruses can fully incorporate their DNA into bacterial genome once they inject their DNA

• these are called prophage

• when host cell divides, new chromosome replicates viral DNA

• something clicks and virus DNA pops out of cell and goes back into lytic cycle

generalized transduction (lytic cycle)

This is a rare mistake that happens during the assembly phase of the lytic cycle. Instead of packaging viral DNA into the phage head, the virus accidentally packages a random piece of fragmented host bacterial DNA

• during lytic cycle, when viral DNA is packaged into viral proteins, sometimes it gets packaged with host DNA in phage capsid

• if this phage carrying bacterial DNA attaches to new host cell, the new host cell will take up bacterial DNA through horizontal gene transfer

• recombination can occur, producing a recombinant cell with a genotype different from both doner and recipient cells

specialized transduction (lysogenic cycle)

a process of horizontal gene transfer. It happens when the integrated prophage improperly (imprecisely) excises itself from the host chromosome, accidentally taking a small piece of adjacent bacterial DNA along with it. This combined DNA is then packaged into a new phage particle and injected into a different bacterium

• a phage infects a bacterium and incorporates itself into host genome to become prophage

• when prophage leaves bacterial chromosome, it makes a mistake and takes piece of adjacent bacterial host genome

• hybrid DNA is packaged into viral particles

• when viral particles injects into new host cell, hybrid DNA is incorporated into host genome, including the bacterial DNA