The sexual reproduction in bacteria does not involve the formation of gametes (the specialized sexual reproductive units). However, genetic recombination similar to sexual reproduction occurs in many forms of bacteria. These may occur by transformation, transduction or conjugation.
(i) Transformation
This process was first demonstrated by Griffith in 1928 in certain Pnemococci strains. The initial steps in the transformation are adsorption and penetration of the Plasmid transfer transforming principle (exogenous DXA) into living bacterial cell which is assumed to possess adsorption sites. After this, subsequent steps are
(i) incorporation of the transforming principle (DNA) into the genetic material (chromosomes) of the recipient cell
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(ii) Replication of the modified chromosome with new information
(iii) Formation of a transformed bacterial cell population.
(ii) Transduction
Here the genetic transfer is carried out by the agency of a bacterial virus or bacteriophage. Lederberg and Tatum (1944) furnished a clear experimental evidence of this genetic recombination mechanism.
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Bacteriophage has an outer protein coat and inner DNA core. The phage attaches itself to the bacterial cell and DXA of the phage particle is transferred to the bacterial cell. Once inside the bacterial cell, the phage DNA synthesizes new DNA of its kind.
During this time, the bacterial DNA fragments and this fragmented DNA gets incorporated to phage DNA. When these phages infect other bacteria, transfer of genetic material carried from earlier bacterium to the new bacterium occurs. Thus, there the transduction of genetic materials.
(iii) Conjugation
It occurs generally in mating types of bacillus (Escherichia coli) One of this is donor or fertile cell (F+) and the other is recipient or receptor cell (F–). The cells of these contrasting characters come closer in pairs. The donor has a special kind of hair like structures called pili or sex pili. This helps in attaching it to the wall of the recipient cell. A conjugation tube or bridge is formed between the two conjugant at the point of contact. Through this DNA strand gets transferred.
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The conjugant stay joined together for a very short time and then they separate from each other. After sometime, the ex- conjugant, discard the donor chromosomes and revert to its original state. Thus, it differs from true sexual reproduction in two aspects (i) absence of meiosis (ii) absence of true zygote formation.