The Genetics of Bacteria
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The Genetics of Bacteria
The major component of the bacterial genome is one
double-stranded, circular DNA molecule.
E. coli genome consists of 4.6 million nt pairs, representing
4300 genes (size of genome is 100:1 versus virus; 1:1000
versus an average eukaryote).
Densely packed, it forms the region called nucleoid.
Bacterial cells divide by fission (20 min per division in
optimal condition) preceded by replication of the
chromosome.
Single cell – 12 hr – 107-108 cells.
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The Genetics of Bacteria
Fission is asexual process – most of the bacteria in a colony
are genetically identical to the parent cell.
Spontaneous mutation rate of 10-7 per cell division results in
2000 E. coli mutants in a human colon where 2x1010 cells are
produced per day.
Thus, new mutations, although they are rare, can have a
significant impact on genetic diversity when reproductive
rates very high.
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The Genetics of Bacteria
The prokaryotic chromosome is a single DNA molecule that
first replicates, then attaches each copy to a different part of
the cell membrane.
When the cell begins to pull apart, the replicate and original
chromosomes are separated.
Following cell splitting (cytokinesis), there are then two cells
of identical genetic composition (except for the rare chance
of a spontaneous mutation).
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Replication of the
bacterial chromosome
From one origin, DNA
replication progresses in
both directions around the
circular chromosome until
the entire chromosome
has been reproduced.
Enzymes that cut, twirl
(magenta arrow), and
reseal the double helix
prevent the DNA from
knotting.
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The Genetics of Bacteria
Rod-Shaped Bacterium, E. coli, dividing by binary
fission (TEM x92,750).
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Replication of the bacterial chromosome
Though mutations are rare events, they can impact genetic
diversity in bacteria because of their reproductive rate
Though mutation can be a major source of genetic variation
in bacteria, it is not a major source in more slowly
reproducing organisms (e.g., humans).
In most higher organisms, genetic recombination from
sexual reproduction is responsible for most of the genetic
diversity within populations
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Genetic recombination produces new bacterial
strains
Three natural processes of genetic recombination in
bacteria: transformation, transduction, and conjugation.
These mechanisms of gene transfer occur separately from
bacterial reproduction, and in addition to mutation, are
another major source of genetic variation in bacteria.
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Transformation
Process of gene transfer during which a bacterial cell
assimilates foreign DNA from surroundings
Many bacteria have surface proteins that recognize and
import naked DNA from closely related bacterial species
Lacking such proteins, E. coli can be artificially induced to
take up foreign DNA by incubating them in a culture medium
that has a high concentration of calcium ions.
This technique is used by the biotechnology industry to
introduce foreign genes into bacterial genomes, so that
bacterial cells can produce proteins characteristic of other
species (human insulin and growth hormone).
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Transformation
Certain types of bacteria can "donate" a piece of the their
DNA to a recipient cell.
The recombination is the bacterial equivalent of sexual
reproduction in eukaryotes.
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Transformation
Note that the entire DNA is not usually transferred, only a
small piece.
By this mechanism, harmless Streptococcus pneumoniae
can be transformed by pneumonia-causing cells and
become pathogenic.
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Genetic recombination
arg+ trp- strain
MM
MM
No colonies
(control)
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Mixture
arg+ trp+ colonies
resulting from
genetic
recombination
arg- trp+ strain
MM
No colonies
(control)
Transduction
Gene transfer from bacteria to another by a bacteriophage.
Generalized transduction – occurs when random pieces of
host cell DNA are packaged within a phage capsid during
the lytic cycle of a phage.
This process can transfer almost any host gene and little
or no phage genes
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Phage DNA
A+ B+
Phage infects
bacterial cell
Generelized Transduction
Host DNA is hydrolysed
into pieces, phage DNA
and proteins are made
A+ B+
Occasionally a
bacterial DNA
fragment is
packaged in a
phage capsid
The recombinants have
Crossing over
genotypes (A+B-) different from
either the donor (A+B+) or
A+
recipient (A-B-)
A- BA+ BRecombinant bacteria
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Transducing phages infect
new host cells, where
recombination (crossing over)
can occur
Transduction
Specialized transduction – occurs when a prophage excises
from the bacterial chromosome and carries with it only
certain host genes adjacent to the excision site (restricted
transduction).
•Carried out only by template phages
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Specialised
Transduction
Bacterial cell has
prophage integrated
between genes A
and B
Occasionally, prophage DNA
exits incorrectly, taking
adjoining bacterial DNA with it
A+
Prophage
DNA
B+
Bacterial
DNA
A+
B+
A+
B+
The recombinants have
genotypes (A+B-) different
from either the donor
(A+B+) or recipient (A-B-)
A+ BRecombinant bacteria
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Transducing phages infect
new host cells, where
recombination (crossing over)
can occur
A+
A- BCrossing over
A+
Phage particles carry
bacterial DNA (gene A)
along with phage DNA
Transduction
•Differ from general transduction in that:
•Specific host genes and most phage genes are packed
into the same virion
•Transduced bacterial genes are restricted to specific
genes adjacent to the prophage insertion site.
•In general transduction, host genes are randomly
selected and almost any host gene can be transferred.
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Conjugation
Direct transfer of genetic material between two bacterial
cells
The DNA transfer is one-way: one cell donating DNA, and
its “mate” receiving it.
The ability to form sex pili
and donate DNA during
conjugation, results from
the F factor – special piece
of DNA.
E. coli strains undergoing conjugation (TEM x27,700).
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Conjugation
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Conjugation
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Plasmids
Small, circular, self-replicating DNA molecules are known
from almost all bacterial cells.
Plasmids carry between 2 and 30 genes. Some seem to
have the ability to move in and out of the bacterial
chromosome.
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Plasmids
A genetic element that can
replicate either as
a plasmid or
as part of the bacterial
chromosome
is called an episome (plasmids,
phage ).
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Plasmids – viruses: differences
Plasmids, unlike viruses, lack protein coats and usually do
not exit outside the cell.
Plasmids are generally beneficial to the bacterial cell, while
viruses are parasites that usually harm their hosts.
A plasmid has small number of genes which are not required
for the survival and reproduction of the bacterium
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The F plasmid and conjugation
The F plasmid (factor) consists of 25 genes, most
required for the production of sex pili.
The F plasmid replicated synchronically with
chromosomal DNA giving rise to offspring that are both
F+.
Upon conjugation F factor is transferred to female: F
“condition” is contagious.
A cell with the integrated F factor is called an Hfr cell (for
high frequency of recombination).
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R plasmids and antibiotic resistance
R plasmids (R for resistance) – plasmids that carry
resistance genes.
The theory of natural selection predicts that, under the
antibiotic pressure an increasing number of bacteria will
inherit genes for antibiotic resistance.
The problem: R plasmids, like F plasmids, can be
transferred from one bacterial cell to another.
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Readings
Campbell et al. Biology. Ch. 27 (556-565)
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