Bacterial genetics

Bacteria have a single circular DNA which codes for about 2000 proteins. They are haploid meaning they have just one copy for each gene. Transfer of genetic material happens both within the same cell as well as between different bacteria.

Following are the methods by which genetic material is transferred in bacteria.

Transposons

“Jumping genes” transfer DNA from one site on the chromosome to another site and also to and from plasmids. They have a critical role in spreading antibiotic resistance among bacteria. In a process called “Programmed Rearrangement” there is movement of a gene from a silent site where it is not expressed to an active site where it is transcribed and translated. It is important in antigenic variations seen in bacteria such as Neisseria gonorrhoeae, Borrelia recurrentis and Trypanosomes. It helps the bacteria to evade the immune response and cause recurring infections.

Conjugation

DNA is transferred from a donor cell to a recipient cell via a sex pilus/ conjugation tube. F (fertility ) plasmid or F factor carries all the genes needed for conjugation. The bacterial cell carrying the F plasmid is called F+ or Donor Male while the cell which does not have the F plasmid is called F - or Recipient Female. After transfer of DNA via the conjugation tube, the recipient cell becomes a F+ cell. Sometimes not only the F factor but a part of the bacterial chromosome is also transferred to the recipient via the conjugation tube in a process called High Frequency Recombination (Hfr).

Transduction

Transfer of genetic material between bacteria via a bacteriophage is called Transduction. A bacteriophage is a virus that infects a bacterium. The bacteriophage incorporates bacterial DNA into its own genome. When such a phage infects another bacterium it may transfer some of the previous host cell DNA into the new host which will now acquire a new trait. This is called Lysogenic Conversion. The genes for Diphtheria Toxin, Botulinum Toxin, Cholera Toxin and Erythrogenic Toxin of Streptococcus pyogenes is encoded in this manner. There are two types of transduction- Generalized and Specialized.

Generalized transduction

Initially a phage infects a bacterium, and incorporates any of the bacterial genes at random. After that when the phage infects another bacterium, it transfers this gene to the new cell which gets incorporated into the bacterial genome by recombination. As any gene can be transferred in this manner, it is called as generalised transduction. Seen in a lytic cycle.

Specialized Transduction

In this type, when the phage DNA is excised from the bacterial chromosome, it carries a part of the bacterial DNA which was adjacent to it. Seen in a lysogenic cycle.

Transformation

When a bacteria incorporates free DNA from a medium into its own genome, the process is called transformation. Whereas 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.