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Bacterial Genetics &
Operons
The Bacterial Genome
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Because bacteria have simple genomes, they
are used most often in molecular genetics
studies
Most of what we know about bacterial
genetics comes from the study of Escherichia
coli
Bacteria have one double-stranded circular
DNA molecule, with a little bit of associated
protein
Bacterial vs. Eukaryotic
Genomes
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E. coli
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Typical eukaryote:
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base pairs
E. coli’s DNA is tightly coiled so it
will fit inside the cell
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4.6 million nucleotide pairs
4,400 genes
most of the DNA is found in the
nucleoid region
Most bacteria also have a number
of plasmids
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Plasmids are much smaller circles
of DNA, each with only a few
genes
Bacterial Reproduction
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Bacteria divide by binary fission
Before the cell can divide, the bacterial chromosome
must be replicated
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DNA replication begins at a single origin of replication, but
goes in both directions around the circle
Asexual process:
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The offspring is produced from a single parent, so they
have the same DNA
HOWEVER, even though random mutations are rare in
individual bacteria, because bacteria reproduce so quickly,
there is a lot of genetic diversity in a population of bacteria
Sources of Genetic Diversity in
Bacteria
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In eukaryotes, meiosis & fertilization provide for
genetic recombination (new combinations of genes)
However, meiosis and fertilization do not occur in
bacteria
Instead, 3 processes are responsible for combining
DNA from different individuals
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Transformation
Transduction
Conjugation & Plasmids
Transformation
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Transformation is the uptake of naked, foreign DNA
from the surrounding environment
The foreign DNA is incorporated into the DNA of the
bacterium
The cell is now considered a recombinant because
its genome contains DNA from 2 different sources
Transduction
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In transduction, phages (viruses that infect
bacteria) transfer DNA from one bacterium to
another
Sometimes this can happen as a mistake
during the lytic cycle of a virus
Conjugation & Plasmids
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Conjugation is the transfer of genetic material
between two bacterial cells that are temporarily joined,
via sex pili
The DNA that is transferred is usually a plasmid
(small, circular, self-replicating piece of DNA that
contains only a few genes, separate from the bacterial
chromosome)
 This DNA can give a “genetic advantage” to the cell
that contains it
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The F plasmid is required for conjugation to occur
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The cell that gives the F plasmid is F+; the cell that receives
the F plasmid is FBoth cells are F+ after conjugation because the plasmid is
replicated before the plasmid is passed on
R Plasmids & Antibiotic
Resistance
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Sometimes, antibiotics that used to work to “kill”
certain bacteria become ineffective
WHY??
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Some bacteria have R (resistance) plasmids, which carry
genes that cause antibiotics to be ineffective for a variety of
specific reasons
Why does this matter?
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Antibiotics will kill bacteria that DO NOT have R plasmids,
while those that are resistant to antibiotics will continue to
live and reproduce
Therefore, the bacteria who have R plasmids and are
antibiotic resistant will become more common
Transposons- “jumping
genes”
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Genetic material can also be moved
(transposed) within a single cell
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A transposable element may move within the
chromosome, from a plasmid to the chromosome
(or vice versa), or from one plasmid to another
Sometimes transposable elements are called
“jumping genes”
Regulation of Gene
Expression
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Bacteria respond to their environment by regulating
their gene expression
Bacteria require tryptophan to survive
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If there is not enough tryptophan in the environment, the
bacterium responds by activating a metabolic pathway that
makes tryptophan from another compound
If, later, however, there is enough tryptophan in the
environment, the bacterium switches “off” that metabolic
pathway to conserve resources
Light switch analogy
Regulation of Gene
Expression
The metabolic pathway that controls the
production of tryptophan can be controlled in 2
ways:
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1.
2.
The first enzyme in the pathway can be turned “off” by the
presence of enough tryptophan
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FAST response
The expression of the genes that produce the enzymes
that make the tryptophan can be turned “off” by the
presence of enough tryptophan
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SLOWER response
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OPERON model
Operons
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In E. coli, all 5 genes that code for the production of
the enzymes that make tryptophan (when
necessary) are all located together
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Benefit:
 A single “on/off” switch can control the whole group of
genes
This on/off switch, which is part of the DNA, is known as an
operator
The operator controls the access of RNA polymerase to the
promoter, and therefore controls whether or not
transcription occurs
The promoter, operator, and the genes they control are
known all together as AN OPERON
trp Operon- Repressible
operon
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The trp operon is turned on normally
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Therefore, RNA polymerase can attach to the promoter
and transcribe the genes
However, if there is enough tryptophan, a protein
called a trp repressor binds to the operator and
blocks the way so that RNA polymerase cannot
attach and can’t transcribe the genes
This is known as a repressible operon because it’s
usually on, but it can be turned off
Trp Operon
Trp Operon
lac Operon-Inducible Operon
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The lac operon differs from the trp operon because it
is an inducible operon
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The enzyme B-galactosidase is necessary to break
down lactose (disaccharide) into glucose and
galactose
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It is usually off but can be turned on
If there isn’t much lactose, there aren’t many enzymes
In the normal state, the lac repressor is bound to the
operator, causing the gene to be “off”
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When lactose is added to the environment, however, an
inducer inactivates the repressor, turning the gene “on”
Lac Operon
Lac Operon