Bacterial Genetics

Replication of Bacteria

Bacteria are haploid and have one circular chromosome of double stranded DNA. Bacteria replicate through binary fission producing genetically identical daughter cells. Each molecule of DNA in the daughter cells is composed of a strand from the parent and a newly synthesised complementary strand. This process of DNA replication is called semiconservative replication.

Plasmids

Plasmids are small pieces of genetic material found in the cytoplasm and these plasmids are able to replicate independantly of the bacterial chromosome. Most species of bacteria contain plasmids that are composed of double stranded DNA which are circular in shape. In pathogenic bacteria it is often the plasmid that encodes virulence factors and traits such as antibiotic resistance.
Replication of most plasmids is not directly related to the replication of the host bacterium and it has been found that the distribution of plasmids to daughter cells is a random process as plasmids in the cytoplasm may or may not be transferred when the cytoplasm of the cell is seperated prior to forming the daughter cells.
Bacterial plasmids can not only be transferred during bacterial replication but also via processes called conjugation and transformation (although the transformation process rarely occurs in nature).

Bacteriophages

A bacteriophage is a term used to describe a virus that is able to infect a bacterial cell and they can be either virulent or temperate depending on their method of replication. Virulent bacteriophages undergo a lytic cycle within the bacterium which eventually results in the production of bacteriophage progeny from the cell and the lysis of the bacterium.
A bacteriophage can be composed of either DNA or RNA and either single or double stranded. The capsid (outer protective layer) of the bacteriophage often remains outside the bacterial cell after the viral nucleic material has entered the cell cytoplasm. The host specificity of bacteriophages is related to the chemical affinity between attachment structures on the surface of the bacteriophage capsid and the receptors on the surface of the bacterium.

Genetic Variation

Genetic variation can occur in a number of ways and the genotype of the bacteria determines its inheritable potential. Below are the main ways that genetic mutation can occur in bacteria;

Mutation
A mutation is a stable inheritable alteration in the bacterial genome. This means that base pairs within a gene are altered. Genes with altered base pairs may, or may not, depending on the mutation be functional or can incorrectly code for an amino acid in a protein resulting in a phenotypic change rather than simply a gene alteration. The type of mutations occurring in bacteria are silent, non-sense, mis-sense, frame shift, deletion of base pairs, insertions, translocations and inversions.

Genetic Recombination
Genetic recombination occurs when sequences of DNA from seperate sources become integrated. This new genetic material can be introduced via conjugation, transduction and transformation.

Conjugation
Conjugation represents the process whereby DNA is transferred from a donor cell to a recipient cell. The donor cell synthesises a modified "pilus" which the donor cell inserts into the recipient cell. This is often called a sex pilus. Genetic material is then transferred through the pilus to the recipient. During conjugation, plasmid genetic material is mostly transferred, although chromosomal DNA can also be transferred via this process. Conjugation is most frequently associated with gram negative bacteria, but can occur in some gram positive bacteria. A sex pilus is not formed in gram positive bacteria and instead plasmid DNA is transferred when the bacteria are in close physical contact.

Tranduction
During transduction, DNA from a donor bacterium is incorporated into the nucleic acid of a bacteriophage and it is the progeny of the bacteriophage infecting another bacterium that allows the transfer of the genetic material.

Transformation
The process of transformation involves the transfer of genes from a segment of chromosomal DNA from a lysed donor bacterium to a fully functional recipient. Natural transformation is uncommon and is usually restricted to propcedures carried out in the lab.