Genetics of Bacteria
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
vs. virus 1:1000)
- Densely packed, it forms the region called nucleoid.
- Bacterial cells divide by fission (20 min per division in optimal condition) proceeded by
replication of the chromosome.
- Single cell- 12 hr- 10 -10 cells
- Fission is asexual process-most of the bacteria in a colony are genetically identical to the parent
- Spontaneous mutation rate of 10 per cell division result in 2000 E.coli mutants in a human
colon where 2x10 cells are produced per day.
- Thus new mutations, although rare, can have a significant impact on genetic diversity when
reproductive rates are very high
- 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 beings 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 spontaneous mutation)
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
- Though mutations are rare events, they can impact genetic diversity in bacteria because of their
- Though mutation can be a major source of genetic variation in bacteria, it is not a major source
of genetic variation in bacteria, it is not a major source in more slowly reproducing organisms
- In higher organisms, genetic recombination from sexual reproduction is responsible for most of
the genetic diversity between populations.
Genetic recombination produces several new bacterial strains
- Thee natural process of genetic recombination in bacteria:
o 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.
- Process of gene transfer during which bacterial cell assimilates foreign DNA from surroundings
- Many bacteria have surface proteins that recognize and import naked DNA from closely related
- 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 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)
- Certain cell types of bacteria can “donate” a piece of their DNA to a recipient cell.
- The recombination is the bacterial equivalent of sexual reproduction in eukaryotes.
- Note that the entire DNA is not usually transferred, only a small piece.
- By this mechanism, harmless Streptococcus pneumonia can be transformed by pneumonia-
causing cells and become pathogenic.
- Gene transfer from one 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.
o This process can transfer almost any host gene and a little or no phage genes
Phage infects bacterial cell
Host DNA is hydrolyzed into pieces, phage DNA and proteins are made
Occasionally a bacterial DNA fragment is packaged in a phage capsid
Transducing phages infect new host cells, where recombination can occur
+ - + +
The recombinants have genotypes (A B) different from either the donor (A B )
or recipient (AB) -
- Specialized transduction- occurs when macrophage excises from the bacterial chromosome and
carries with it only certain host genes adjacent to the excision site (restricted transduction)
o Carried out by only temperate phages
Bacterial cells had prophage integrated between genes A and B
Occasionally, prophage DNA