04/18/23

The Genetics of Bacteria

04/18/23

The Genetics of BacteriaThe 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.

04/18/23

The Genetics of BacteriaFission 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.

04/18/23

The Genetics of BacteriaThe 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).

04/18/23

Replication of the bacterial

chromosomeFrom 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.

04/18/23

The Genetics of Bacteria

Rod-Shaped Bacterium, E. coli, dividing by binary fission (TEM x92,750).

04/18/23

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

04/18/23

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.

04/18/23

TransformationProcess 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).

04/18/23

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.

04/18/23

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.

04/18/23

Genetic recombination arg+ trp- strain arg- trp+ strainMixture

arg+ trp+ colonies resulting from genetic recombination

No colonies (control)

No colonies (control)

MM MM MM

04/18/23

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

04/18/23

A+ B+Generelized Transduction

A+ B+

A- B-

A+

A+ B-

Phage DNA

Phage infects bacterial cell

Host DNA is hydrolysed 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 (crossing over) can occur

The recombinants have genotypes (A+B-) different from either the donor (A+B+) or recipient (A-B-)

Recombinant bacteria

Crossing over

04/18/23

TransductionSpecialized 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

04/18/23

Specialised Transduction

A+ B+

A+

B+

A- B-

A+

A+

B+

A+

A+ B-

Bacterial DNA

Prophage DNA

Transducing phages infect new host cells, where recombination (crossing over) can occur

The recombinants have genotypes (A+B-) different from either the donor (A+B+) or recipient (A-B-)

Bacterial cell has prophage integrated between genes A and B

Occasionally, prophage DNA exits incorrectly, taking adjoining bacterial DNA with it

Phage particles carry bacterial DNA (gene A) along with phage DNA

Recombinant bacteriaCrossing over

04/18/23

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.

04/18/23

Conjugation

E. coli strains undergoing conjugation (TEM x27,700).

Direct transfer of genetic material between two bacterial cellsThe 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.

04/18/23

Conjugation

04/18/23

Conjugation

04/18/23

PlasmidsSmall, 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.

04/18/23

Plasmids

A genetic element that can replicate either as a plasmid or as part of the bacterial chromosome is called an episome (plasmids, phage ).

04/18/23

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

04/18/23

The F plasmid and conjugationThe 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).

04/18/23

R plasmids and antibiotic resistanceR 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.

04/18/23

Readings

Campbell et al. Biology. Ch. 27 (556-565)