Bacteria are microscopic organisms with double-stranded, circular DNA in their genome. Though bacteria do not have obligate sexual reproductive stages during their life cycle, they can actively exchange genetic information among them. The exchange of genetic information in bacteria occurs in three ways: conjugation, transformation, and transduction. The direct transfer of genetic material between two bacteria is known as conjugation; it occurs through a sex pilus. Transformation is the picking up of naked, foreign DNA from the surroundings. Transduction is the carrying of bacterial DNA by bacteriophages to another bacteria.
Key Areas Covered
1. What are Bacteria
– Definition, Features, Bacterial DNA
2. How Do Bacteria Exchange Genetic Information
– Conjugation, Transformation, Transduction
Key Terms: Bacterial DNA, Conjugation, Plasmids, Sex Pilus, Transformation, Transduction
What are Bacteria
Bacteria are prokaryotes that lack membrane-bound organelles. They have a peptidoglycan cell wall. Asexual reproduction by binary fission is the main reproduction method in bacteria. Bacteria may contain two types of genetic material within a bacterium and they are bacterial genome and plasmids. Genome is a double-stranded circular chromosomal DNA and plasmids are double-stranded, small, circular DNA molecules. Bacterial chromosomal DNA is located in the nucleoid. The size of the E. coli genome is 4.6 million base pairs, and it consists of approximately 4,300 genes. Plasmids or nonchromosomal bacterial DNA are smaller than the bacterial chromosome, and several to more than thousands of plasmids can be found per bacterial cell. Bacteria in a culture is shown in figure 1.
Figure 1: Bacteria
How Do Bacteria Exchange Genetic Information
The genetic information can be transferred from one bacteria to another in the form of DNA. The transfer of genetic information between bacteria is not often considered as a true exchange since one bacterium serves as the donor while the other serves only as the receiver. Moreover, only small pieces of bacterial chromosomes are transferred.
The three modes used for the exchange of genetic information between bacteria are conjugation, transformation, and transduction.
Conjugation
The process that exchanges genetic information between two bacteria through a direct cell-to-cell contact is known as conjugation. It is mediated by conjugative plasmids that are encoded with efficient mechanisms for the mediation of DNA transfer. Conjugation is a unidirectional process, and only the donor bacterium has the conjugative plasmids. Conjugative plasmids of Gram-negative bacteria are encoded for a sex pilus that facilitates the DNA transfer to the recipient bacterium. The direct contact between two bacteria is achieved through the sex pilus, forming a conjugal bridge. The steps of bacterial conjugation are shown in figure 2.
Figure 2: Conjugation
1- Donor cell produces pilus.
2- Pilus attaches to recipient cell, brings the two cells together.
3- The mobile plasmid is nicked, and a single strand of DNA is then transferred to the recipient cell.
4- Both cells recircularize their plasmids, synthesize second strands, and reproduce pili; both cells are now viable donors.
Transformation
The process of taking the free fragments of DNA by bacteria from the surrounding medium is known as transformation. Only the competent bacterial cells can take up DNA by transformation. Limited types of bacteria can undergo transformation naturally such as Bacillus, Streptococcus, Neisseria, Haemophilus, etc. In Addition, most bacteria such as E. coli can be made competent artificially by exposing into a medium with calcium chloride. Plasmids are used as carriers of DNA in the transformation during DNA cloning experiments.
Transduction
The process of transferring bacterial DNA from one cell to another cell by means of bacteria-infecting viruses is known as transduction. It is considered as an efficient method of DNA transfer since foreign DNA is protected inside the bacteriophage. The DNA transferred by bacteriophages are permanently integrated into the recipient bacterial genome by homologous recombination.
Conclusion
The exchange of genetic material between DNA occur in three methods; conjugation, transformation, and transduction. Conjugation is the direct transfer of genetic material between two bacteria with the help of a sex pilus. Transformation is the take-up of naked DNA fragments by bacteria from the surrounding medium. Transfection is the transfer of bacterial DNA through bacteria-infecting viruses.
Reference:
1. Rogers, Kara, and Robert J. Kadner. “Bacteria.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., 22 Feb. 2018, Available here.
Image Courtesy:
1. “Bacteroides hypermegas bacteria” by CDC/ Dr. V. R. Dowell, Jr. (Public Domain) via Public Health Image Library
2. “Conjugation” By Adenosine – Own work (CC BY-SA 3.0) via Commons Wikimedia
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