Sexual Reproduction:
Sexual reproduction involves the fusion of two different cells and a transfer of hereditary factors occurs in bacteria although infrequently. Genetic recombination occurs in those bacteria that have been carefully studied and presumably occurs in other species as well.
The usual vegetative bacterial cells are haploid and in sexual reproduction part of all of the chromosomes pass from the male cell to the female cell, yielding a cell, partly or completely diploid. Crossing over then occurs between the female chromosome and the male chromosome or fragment followed by a process of segregation that yields haploid progeny cells.
1. Bacterial Transformation:
The genetic transfer in bacteria also occurs by transformation, in which the DNA molecules of the donor cell, when liberated by its disintegration are taken up by another recipient cell and its offspring inherit some characteristics of the donor cell. When different strains of bacteria are found in a mixed state either in culture or in nature, some of the resultant offspring possess a combination of characteristics of the parent strain. This phenomenon is known as recombination.
This phenomenon of transformation was first recorded by Griffith (1928). Avery, Macleod, and McCarty (1944) demonstrated that the transforming principle is DNA in the sequence of events in bacterial transformation. The lines of inquiry that led to an understanding of the chemical nature of genetic material arose from a study of the pestilent organism Diplococcus pneumoniae. This bacterium causes pneumonia in males. Fredrick Griffith found that there are two strains of D. pneumonia, one that forms smooth colonies protected by a capsule and the other one that formed irregular or rough colonies without a capsule, when grown on a suitable medium in Petri dishes.
2. Bacterial Transduction:
The genetic transfer in bacteria is achieved by a process known as transduction. Lederberg and Zinder’s (1952) experiment in U-tube Salmonella typhimurium indicated that bacterial viruses or phages are responsible for the transfer of genetic material from one lysogenic and lytic phages. Thus the host acquires a new genotype. Transduction has been demonstrated in many bacteria.
In this process, the DNA molecule that carries the hereditary characteristics of the donor bacterium is transferred to the recipient cell through the agency of the phage particle. In this process, very few closely linked characters can be transferred by each particle. Thus the bacteriophage brings about genetic changes in those bacteria which survive the phage attack.
3. Bacterial Conjugation:
Wollman and Jacob (1956) have described conjugation in which two bacteria lie side by side for as much as half an hour. During this period of time, a portion of genetic material is slowly passed from one bacterium which is designated as a male to a recipient designated as female. This was the establishment that the male material entered the female in a linear series.
In bacterial conjugation, the transfer of genetic material (DNA) takes place through cell-to-cell contact between donor and recipient cells. During the process of conjugation, a large portion of the genome is transferred, while in transformation and transduction, only a small fragment of DNA is transferred. The process of conjugation was discovered by Lederberg and Tatum (1944) in a single strain of Escherichia coil. Conjugation has also been demonstrated in Salmonella, Pseudomonas, and Vibrio.