Sexual reproduction in bacteria takes place by three methods:
According to Lederberg and Tatum (1946) in two different strains of some bacterial species, recombination of genetic material takes place. They furnished a clear experimental evidence of genetic recombination mechanism. In the beginning, a conjugation bridge or tube develops between two mating type bacterial cells. Through the chromatin, genetic material of the one cell enters into the other cell. This type of reproduction has been studied in Escherichia coli. The two cells taking part in this bacterium are haploid.
One of them is donor cell and the other is called a recipient cell. The donor cell has a special kind of pili known as sex-pili, which help to attach it to the cell wall of the recipient cell. After this, a conjugation tube develops between the two conjugants at the point of contact. Through this tube, the genetic material (DNA strand) of donor cell enters into the recipient cell.
The transformation study was carried out for the time with Pneumococcus bacteria, which cause pneumonia. This bacterium is oval in shape and is surrounded by a capsule. Occasionally, this bacterium pneumococcus does not secrete capsule around it. This inability is passed into all its progeny. Griffith (1928) and Avery, McLeoid and McCarty (1944) proved by their experiments, that if capsulated bacteria are injected into the body of a mouse, it develops pneumonia, but when non-capsulated bacteria are injected into the body of mouse then they could not cause pneumonia. These scientists then observed that if cell-free extract of capsulated bacteria in which DNA is also present is mixed with the culture of non-capsulated, non-pathogenic bacteria, capsulated bacteria are produced in the culture after some time, and they also acquire the capacity to cause pneumonia. It could be concluded from the experiment that non-capsulated bacteria by absorbing DNA converted themselves into capsulated type of bacteria. This phenomenon of genetic transfer is called as transformation.
It differs from transformation in the mode of genetic transfer. In this method, the genetic transfer is carried out by the agency of a bacterial virus known as bacteriophage. Bacteriophage contains nucleoproteinaceous material, which remains in the protoplasm of bacteria. Occasionally, the DNA of bacteriophage combines with the DNA bacterial cell; this may result in appearance of new characters
in bacterial daughter cells. In this way, the transfer of genetic material through bacteriophage is termed as transduction. The transduction is described as the 'unilateral recombination in bacteria'. This was studied by Lederberg and Zinder in Salmonella bacteria.