Transcript Bacterial Genetic

T4 bacteriophage infecting an
E. coli cell
0.5 m
Comparing the size of a virus,
a bacterium, and an animal cell
Virus
Bacterium
Animal
cell
Animal cell nucleus
0.25 m
Basic shapes of bacteria
• Bacillus  rod-like
• Coccus  round
• Spirillium  spiral
Roles of non-pathogenic bacteria
• Some examples
– Decomposition
– Intestinal mutualistic relationship
– Food prep
Genetics of Bacteria
• Bacterial genome 
Genetics of Bacteria
• Bacterial genome  One circular DNA
molecule
• E. coli chromosome has 100 times more
DNA than in a typical virus, but much
less than a eukaryotic cell.
• Packed into nucleoid region of cell
• Plasmid 
Genetics of Bacteria
• Bacterial genome  One circular DNA
molecule
• E. coli chromosome has 100 times more
DNA than in a typical virus, but much
less than a eukaryotic cell.
• Packed into nucleoid region of cell
• Plasmid  small circular extra piece of
DNA
Bacterial Genetic
Recombination
• What is the main source of genetic
recombination in bacteria?
– Mutations
• What are the other sources of
recombination?
– Transformation
– Transduction
– Conjugation
General steps to
transformation
• Isolate gene of interest using restriction
enzymes
• Expose recipient bacterium to same
restriction enzyme, temperature shock,
ions, and DNA binding protein
• Combine gene of interest with recipient
bacterium
Transformation  uptake of naked,
foreign DNA
Transduction:
bacterial genes
moved from one
host to another
What is the
vector of
transduction?
A phage
Bacterial conjugation
Sex pilus
1 m
Conjugation
• Defined as the direct transfer of genetic material
between 2 bacterial cells that are temporarily joined
• “male” bacterium uses a sex pilus to pull “female”
bacterium towards it creating a mating bridge…serves
as the avenue for DNA transfer
• There needs to be a “fertility” (F) gene present either
as part of the bacterial genome or as a plasmid…an F
plasmid is an episome:
 genetic element that can replicate independently or as
part of the bacterial genome
Conjugation
• Plasmid genes are advantageous to the
bacteria that have them
• Plasmids that confer resistance to
antibiotics are called R plasmids
Transposons
• Jumping genes (do not exist
independently…either a part of a plasmid or
the bacterial chromosome)
• Does not depend on complementary base
pairing between homologous regions of the
chromosome.
• Transposons move to regions that the gene
has never been (ex. plasmid  chromosome)
Transposase recognizes the inverted
repeats
Targeted
inverted
repeats are cut,
and the target
is cut, then the
transposon is
inserted
Composite transposons move extra genes
along with the inserted sequence, and are
very beneficial to the bacteria
Operons
• Regulatory systems in E.coli
• 2 Types: Repressible or Inducible
• 5 components
– Regulatory gene (codes for mRNA to be translated into repressor
protein)
– Promoter (site on gene where RNA pol. binds to begin
transcription)
– Operator (on/off switch)
– Repressor (binds to the operator to turn operon gene off)
– Corepressor (allosterically binds to repressor to change shape of
repressor to turn the operon gene off)
OR
– Inducer (allosterically binds to the repressor to change the shape of
the repressor to turn the gene on)
Regulation of Gene
Expression
Structural Genes
Repressible operons
• Repressible operons have structural genes
that code for the production of the substrate.
(anabolic pathways)
• The repressor protein is produced in an
inactive form, leaving the operator open and
the genes on
• In the presence of the substrate, the substrate
will allosterically bind to the repressor protein
(is a co-repressor) and activate the repressor
protein causing it to bind to the operator to
turn the genes off
Inducible operons
• Inducible operons have structural genes that
produce enzymes that break down the
substrate. (catabolic pathways)
• The repressor is translated into its active
configuration and will bind to the operator in
the absence of the substrate to keep the gene
off.
• If the substrate is present, it binds to the
repressor protein and de-activates it, thereby
opening up the operator and turning the gene
on.
Glucose and its affects on the lac
operon
• E.coli would prefer to use glucose as its fuel
• If glucose is scarce, cyclic AMP is abundant and
serves as an allosteric activator to a regulatory
protein called CAP  stimulates RNA pol and
transcription of enzymes that metabolize lactose
• If glucose is availabe, cyclic AMP (cAMP) is absent 
CAP detaches and transcription of the enzymes to
metabolize lactose occurs at a very low level
• Lac repressor molecule turns operon genes on or off,
CAP controls the rate of transcription