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Very adaptable both in natural selection and
adaptation to changes in the environment.
Made of one double-stranded DNA molecule arranged in
The bacterial chromosome (not to be confused with a
eukaryotic chromosome) is so tightly packed that it does not even fill the whole
cell but forms a structure something like a loop of yarn tangled into a ball.
This dense region is called the nucleoid and is not bounded by a
Some bacteria also have Plasmids that have a small
number of genes in them.
Bacterial cells divide by binary fission, which is
preceded by replication of the bacterial chromosome.
From a single point of replication, the copying of
DNA progresses in both directions around the circular chromosome.
Bacteria may proliferate very rapidly in a favorable
Since fission is asexual, most of the bacteria are
genetically identical to the parent cell.
As a result of mutation, some of the offspring do
differ slightly in makeup.
Some new mutations can have a significant impact on
genetic diversity when reproduction rates are high and this diversity affects
the evolution of the populations.
Genetic recombination generates diversity within
Recombination means combining genetic material from
two individuals into the genome of one individual.
Genetic recombination in bacteria is different from
recombination in eukaryotes.
The mechanisms for genetic recombination in bacteria
are transformation, transduction, and conjugation.
The alteration of a bacterial cellís genotype by
the uptake of naked, foreign DNA from the environment.
The bacteria takes in this DNA and then incorporates
it into its genome. The process is
like crossing over in eukaryotes.
The transformed cell now has a chromosome containing
DNA derived from two different cells.
Scientists have learned that many bacterial species
possess proteins on their surfaces that are specialized for the uptake of DNA
from the surroundings.
These proteins specifically recognize and transport
only DNA from closely related species of bacteria. E.coli does not seem to have this ability.
Viruses that infect bacteria called phages transfer
bacterial genes from one host cell to another.
Generalized transduction takes place during the
lytic cycle. When the phage DNA is
introduced, the host DNA is hydrolyzed and breaks apart.
Then when the DNA is packaged for the new phages, sometimes the hostís
DNA is packaged as well. When the released phages then attach to another bacterial
cell, the hostís DNA is released into the new host and incorporated and
replaces the hostís homologous region of DNA.
The cellís chromosome now has a combination of genetic material derived
from two cells.
Specialized transduction requires infection by a
temperate phage from the lysogenic cycle. When
the prophage is integrated into the bacteriumís chromosomes at a specific site
and then later excised, it takes with it small regions of the bacterial DNA that
were adjacent to the prophage. A virus carrying the bacterial DNA infects another cell and
passes on the original hostís DNA. This
is specific because the temperate phageís DNA is incorporated into a specific
site and the genes surrounding it are then excised with the prophage.
The direct transfer of genetic material between two
bacterial cells that are temporarily joined.
This is referred to as the bacterial version of sex.
The DNA donor is referred to as the male and it uses
an appendage called a sex pilli to attach to the DNA recipient (female).
A cytoplasmic bridge is formed between the two
Maleness is the ability to form the sex pilli and
donate DNA during conjugation. It
requires the presence of a specialized plasmid called an F plasmid.
A small circular DNA molecule separate from the
Replicate independently but do so in synchrony with
Certain Plasmids can undergo reversible
incorporation into the cellís chromosome, therefore, it can replicate as an
extrachromosomal molecule or as a part of the main bacterial chromosome called
They lack and extracellular stage and are beneficial
to the bacterial cell.
Each has only a few genes and these genes are not
required for the survival and reproduction under normal conditions but they also
have advantages to bacterial in stressful conditions.
The F Plasmid and
The F plasmid consists of about 25 genes most
required for the formation of the sex pilli. (Also seen as F+)
The F plasmid is inheritable and replicates in
synchrony with the chromosomal DNA.
Division of an F+ cell usually gives rise to two
offspring that are both F+.
Cells that lack the F+ are referred to as F- and act
as the females during conjugation.
The F+ plasmid replicates in the male and then is
transferred to the female converting it from an F- to an F+ cell.
The F plasmid is also an episome and can incorporate
itself into the main chromosome. This
is now called an Hfr cell (high frequency of recombination).
An Hfr cell continues to function as a male during
conjugation and transfers the F genes to its F- partner along with some to the
The Hfr cells chromosome is replicated as the DNA is
transferred so that it does not lose any DNA.
Recombination only occurs when the newly acquired
DNA aligns with the homologous region of the cellís own chromosome and a
crossover exchanges DNA. Binary
fission gives rise to a colony of recombinant bacteria.
If cells are interrupted during conjugation,
scientists are able to sequence the genes around the episome because only parts
of the DNA are recombined.
R Plasmid and
Noticed in the 1950ís that certain strains of
bacteria were resistant to different antibiotics.
This resistance is not carried in the bacterial
chromosome but in a plasmid called the R plasmid. (R for resistance)
Exposure of a bacterial population to a specific
antibiotic will kill antibiotic sensitive bacteria but not those that have R
Plasmids that counter the antibiotic will live.
Natural selection predicts that an increasing number
of bacteria will inherit genes for antibiotic resistance and those that are
resistant will become more common.
The problem is compounded by the F plasmid because
the R plasmid can be transferred during conjugation.
Transposable genetic elements are pieces of DNA that
con move from one location to another in a cells genome.
In bacteria a transposon may move from one locus to
another within the chromosome, from a plasmid to a chromosome or from one
plasmid to another.
Often referred to as jumping genes.
The transposonís genes are not replicated before
moving so the number of copies is conserved.
In a replicative transposition, the transposon
replicates at its original site and a copy inserts at some other location in the
- A transposon does not have a single specific target in the genome.
Insertion sequences are the simplest.
They consist of only the DNA necessary for the act
The one gene found in an insertion sequence codes
for transponase, which is an enzyme that catalyzes the transposition.
The transponase gene is bracketed by a pair of DNA
sequences called inverted repeats; noncoding sequences about 20 to 40
They are inverted because these two regions of DNA
on the ends of an insertion sequence are upside down, backward versions of each
Transponase recognizes these inverted repeats as the
boundaries of the transposon.
The enzyme binds to the two regions brings them
close together and catalyzes the cutting and resealing required for
DNA polymerase helps form identical regions of DNA,
which flack the transposon in its new target site.
The insertion causes a mutation that can increase or
decrease the transcription rates.
and R Plasmids
Transposons are larger and more complex than
Complex transposons include other genes that go
along for the ride like the genes for antibiotic resistance.
These genes are sandwiched between two insertion
These complex transposons may help bacteria adapt to
Transposable genetic elements are not unique to
bacteria but are important in eukaryotic genomes as well.
The first evidence was seen in Barbara
McClintockís breeding of Indian corn that changed the colors of the kernels.
The Control of Gene
Individual bacteria are able to adjust their
metabolism based on environmental change. For
instance, if an E.coli bacteria does not receive tryptophan from the host, it
enables an enzymatic pathway that allows it to make its own tryptophan until the
host intakes a meal with tryptophan.
Metabolic control occurs on two levels
Cells vary the numbers of specific enzyme molecules
meaning they can control gene expression.
Cells can vary the activities of enzymes already
present. This mode is more
immediate and depends on the sensitivity of many enzymes to chemical cues that
increase or decrease their catalytic activity.
Many genes of the bacterial genome are switched on
or off by changes int the metabolic status of the cell.
The method for this was first describe in 1961 by
Francois Jacob and Jacques Monod. They
described this as the operon model.
A promoter unit is a site where RNA polymerase can
bind to DNA and begin transcribing genes.
In the tryptophan unit, five genes code for the
polypeptide chains that make up these enzymes that produce tryptophan.
In this unit, a single promoter serves all five genes.
Transcription gives rise to one long mRNA molecule
representing all five genes required. It
is then translated into five different sequences due to start and stop codons.
Genes that code for polypeptides are called
An advantage of grouping structural genes into one
transcription unit is that a single switch can control a complete cluster of
functionally related genes.
The switch is a segment of DNA called an operator.
It is located within the promoter or between the
promoter and the structural genes.
It controls the access of RNA polymerase to the
The entire stretch of DNA required for enzyme
production is referred to as an operon. This
includes the promoter, the operator, and the structural genes.
By itself, the operator is on and RNA polymerase can
bind to the promoter and transcribe the structural genes.
The operator is switched off by a protein called the
The repressor binds to the operator and blocks the
attachment of RNA polymerase to the promoter which stops transcription of the
Repressor proteins are specific to the operator and
The repressor is a product of a gene called a
regulatory gene and is located some distance away from the operon it controls.
Regulatory genes are transcribe continuously but at
a slow rate.
Binding of the repressor to the operator is
Repressors are synthesized in an inactive form.
It assumes its active conformation and attaches to
the operator only if it first binds to a molecule called a corepressor which is
a small molecule that cooperates with a repressor protein.
Repressible vs. Inducible
Repressible enzymes can have their synthesis
inhibited by a metabolic end-product
Inducible enzymes are stimulated by specific small
The most well known is the lac operon.
An inducer is used to change the active conformation
of the repressor. This may be an
isomer of the end-product.
To control how fast the enzymes are produced,
sometimes a catabolite activator protein is used (CAP)
A catabolite is a molecule that can be consumed by
the metabolic pathway.
CAP accelerates the transcription of an operon by
adhering to the promoter and facilitating the binding of RNA polymerase.
Since CAP associates directly with the DNA to
stimulate gene expression, this is referred to as positive regulation.
CAP may work on several operons.
The cellís ability to use alternative catabolites
provides backup systems that enable the cell deprived of another molecule to