Genetics of Bacteria
The bacterial genome is small:
 Usually one circular bacterial chromosome (double stranded
DNA).
 Simple with fewer related proteins than eukaryotes.
 Found in nucleoid region.
 Many bacteria contain extrachromosomal plasmids.
Plasmid – small double stranded ring of DNA.
 have only a few genes
 not required
 replicate independently from the chromosome (at own pace).
 R plasmid (“r” for resistance)
o Carry genes for resistance to antibiotics
o Not species specific
Most bacteria reproduce by binary fission:
 Asexual reproduction that results in 2 clones (genetically
identical with the exception of rare mutations).
 Replication starts on a single place on the chromosome and
moves in opposite directions until the forks meet.
 Under optimal conditions bacteria will divide every 20
minutes.
There are 3 major sources for genetic variation in bacteria:
1. Transformation – bacteria takes up foreign DNA from the
environment.
o DNA is often in the form of a plasmid.
o DNA can become part of the bacterial chromosome by
recombination.
o Bacteria can be artificially induced to take in DNA.
o This is a method used to introduce genes of interest into
bacteria.
2. Transduction – gene transfer from one bacteria to another
using a phage as a vector.
o vector- a vehicle used to transfer DNA into a cell.
o Generalized transduction – random pieces of bacterial
DNA is packaged into a viral particle by accident
during the lytic cycle.
o Specialized transduction – when a propahage carries an
adjacent gene when moving from the lysogenic cycle to
the lytic cycle.
3. Conjugation – the direct transfer of genetic material between
cells (similar to sexual reproduction).
o A DNA donatic cell extends “sex pili” (an appendage)
and attaches to a receiving cell.
o A cytoplasmic bridge is made and a plasmid is
exchanged.
o The presence of the “F plasmid” (F for fertility) allows
the formation of sex pili.
 Cells with the F plasmid are called F+
 Cells without the F plasmid are called F Conjugation is between F+ and F- cells. The Fwill become F+ if the F plasmid is exchanged.
Control of gene expression in prokaryotes
 Genes can switch on and off under varying conditions
 Many of the same mechanisms for regulation already
discussed still apply
o activation and inhibition
o feedback inhibition
 Many bacteria control their gene expression through operon
units.
o Operon – a cluster of adjacent genes with related
function.
o Structural gene – gene that codes for a polypeptide.
o Operator – a segment of DNA that controls the operon.
o Repressor – a protein that binds to the operator and
stops mRNA transcription.
o Activator – a protein that stimulates the transcription of
mRNA by binding to the operator.
o Regulatory genes – genes that code for the production
of repressors.
o Repressible operon – operons which have their
transcription inhibited.
o Inducible operon – operons which have their
transcription stimulated.
 The Lac operon of E.coli is a widely studied operon.