Microbial genetics variation

negative (indirect) selection

detects mutant cells that cannot grow or perform a certain function 

positive (direct) selection

mutant grow or appear different than unmutated cells 

vertical gene transfer

mother to daughter cell (binary fission) or parent to child

horizontal gene transfer

transfer of gene between cells of the same generation

mobile genetic elements

move from one chromosome to another or from one cell to another

• occurs in prokaryotic and eukaryotic organisms

transposons

segments of DNA that can “jump”

• IS codes for transposes enzyme 

• complex of these carry other genes like antibiotic resistance 

transposition

transposae recognizes the inverted repeats and curs at both ends of the MGE, releasing it from its orginial site. Transposase remains with the MGE

MGEs; Plasmids

self-replicating, extrachromosomal, circular pieces of DNA

• F factor: fertility, conjugative plasmid, genes for sex pili

• R factor: resistance

transduction

bacteriophage: virus specific to bacteria 

may capture parts of bacterial genome and spread to other bacterial cells

transduction (steps)

1. attaches to donor

2. injection of phage DNA, fragmentation of bacterial cell chromosome

3. packaging of donor DNA instead of phage DNA

4. phage attaches to recipient cell

5. injection of donor DNA into recipient

6. recombination to insert donor DNA into recipient chromosome

transduction, lytic lysogenic cycle

• lysis of host cell

• integration of phage DNA into host cell’s chromosome

transformation

• fragmented DNA in the microenvironment for the bacterial cell 

• donor DNA taken up by the cell

  • recombination of donor DNA insertion into bacterial chromosome 

  • homologous region of recipient pairs with donor strand

Some bacterial cells are…

naturally competent

competent

the ability to take up foreign DNA from its environment

• donor DNA binds receptor

• 1 strand of dsDNA degraded

conjugation

donor cell (F+ plasmid)

recipient cell (F- plasmid)

conjugation pilus, only DNA fits through

DNA replication in recipient, cross over 

HGT: conjugation

• origin of transfer (oriT): directs cleavage of one phosphodiester bond in one strand

• complex of exporter proteins form conjugation pilus between donor and recipient

• relaxosome makes ss cut on oriT

• T strand enters pilus towards recipient

• rolling circle replication