Transcript bacterial genetics and control of gene expression

Bacterial Genetics
No Mitosis in prokaryotes
• Binary fission
– Asexual reproduction
• High reproductive
rates/short generation
spans
• Variation in bacteria
mainly caused by
mutation
– Our variation mainly
caused by
recombination of
existing alleles
Although there is no meiosis,
recombination of alleles can occur
Sources of genetic recombination:
• Transduction
• Transformation
• Conjugation
Transduction
• Viruses may transfer (act as vector) DNA
from one bacteria to another
• New bacterial DNA may line up with a
homologous section of the bacterial
chromosome and recombination (crossing
over) may occur
• May be GENERALIZED TRANSDUCTION
or SPECIALIZED TRANSDUCTION
Generalized transduction
• Virus is in the lytic cycle
• Bacterial DNA that was degraded
accidentally is incorporated into viral
capsid
• Capsid injects bacterial DNA into new
bacteria
• The genes transferred are random
Specialized transduction
• Occurs from temperate viruses (in
lysogenic cycle)
• Prophage comes out of bacterial
chromosome and take adjacent genes
with it
• Only specific genes are transferred in this
way
Conjugation
• Bacterial “Sex”
• Many bacteria have
plasmids
– Small circular pieces of DNA
separate from chromosome
– Contain only a few genes
– Not necessary to bacteria’s
survival
– May give an advantage under
certain environmental
conditions
F plasmids
• Fertility plasmids
• Have genes that allow bacteria to form sex pilli
F plasmid
• Plasmids are
transferred through
the sex pilus during
conjugation
• A copy of plasmid is
first made then
transferred
• F+ transfers to F-
R plasmid
• Plasmid
containing
genes that
make bacterial
RESISTANT to
antibiotics
• May have up to
10 antibiotic
resistance
genes on one
plasmid
Regulation of gene expression in prokaryotic cells
Operons
Set of structural genes that are have one promoter
and are controlled by one operator
Operator determines whether transcription occurs
Regulatory gene  codes for repressor
trp operon
This is a repressible operon
The repressor is made in its inactive form
Operon is naturally “on”
A corepressor is required to activate the repressor and turn the operon
“off”
Anatomy of the lac operon
The regulatory gene codes for the
regulatory protein which is a repressor
molecule
Active repressor prevents transcription
The repressor is MADE in its ACTIVE form
The operon is in its “OFF” position
Effect of lactose on the lac operon
The operator determines whether
transcription will occur by being able
to bind with a regulatory protein
Energy preference of E.coli
The concentration
of glucose is
inversely
proportional to the
concentration of
cyclic AMP
Cyclic AMP binds with an
activator protein and helps
RNA polymerase to bind to
the promoter