Bacteria typically have a single circular DNA molecule that codes for about 2000 proteins. They’re haploid, meaning they have only one copy of each gene. Genetic material can be transferred within the same cell and also between different bacteria.
Following are the methods by which genetic material is transferred in bacteria.
Transposons (“jumping genes”) move DNA from one site on the chromosome to another, and they can also move DNA to and from plasmids. They play a key role in spreading antibiotic resistance among bacteria.
In a process called “Programmed Rearrangement,” a gene moves from a silent site (where it isn’t expressed) to an active site (where it’s transcribed and translated). This is important in antigenic variation seen in bacteria such as Neisseria gonorrhoeae and Borrelia recurrentis, and in Trypanosomes. By changing expressed antigens, the organism can evade the immune response and cause recurring infections.
In conjugation, DNA is transferred from a donor cell to a recipient cell through a sex pilus (conjugation tube). The F (fertility) plasmid, also called the F factor, carries the genes needed for conjugation.
A bacterial cell that carries the F plasmid is called F+ (Donor Male). A cell that lacks the F plasmid is called F- (Recipient Female). After DNA transfer through the conjugation tube, the recipient cell becomes an F+ cell.
Sometimes, not only the F factor but also part of the bacterial chromosome is transferred to the recipient through the conjugation tube. This is called High Frequency Recombination (Hfr).
Transduction is the transfer of genetic material between bacteria via a bacteriophage. A bacteriophage is a virus that infects a bacterium.
During transduction, the bacteriophage incorporates bacterial DNA into its own genome. When that phage infects another bacterium, it may transfer some of the previous host cell DNA into the new host. The new host can then acquire a new trait. This is called Lysogenic Conversion.
The genes for Diphtheria Toxin, Botulinum Toxin, Cholera Toxin, and the Erythrogenic Toxin of Streptococcus pyogenes are encoded in this manner. There are two types of transduction- Generalized and Specialized.
A phage infects a bacterium and incorporates bacterial genes at random. When the phage later infects another bacterium, it transfers this gene to the new cell. The transferred DNA can then be incorporated into the bacterial genome by recombination.
Because any gene can be transferred in this way, it’s called generalised transduction. Seen in a lytic cycle.
In specialized transduction, when phage DNA is excised from the bacterial chromosome, it carries with it a portion of bacterial DNA that was adjacent to the phage insertion site. Seen in a lysogenic cycle.
Transformation occurs when a bacterium incorporates free DNA from the surrounding medium into its own genome.
In contrast, when purified DNA is injected into the nucleus of a eukaryotic cell, the process is called transfection. It is a technique used for genetic engineering.