The following points highlight the three methods of Sexual Reproduction (Para Sexuality) in Bacteria. The methods are: 1. Conjugation 2. Transformation 3. Transduction.
Method # 1. Conjugation:
It was first discovered in Escherichia coli by Lederberg and Tatum (1946). They found that two different types of auxotroph (nutritional mutants) grown together on minimal medium produced an occasional prototroph (wild type).
Cell contact was required for this change. Anderson (1957) observed conjugation between two such bacteria under electron microscope. Conjugation was later reported in a number of other bacteria. Bacteria showing conjugation are dimorphic, i.e., they have two types of cells, male (F+) or donor and female (F-) or recipient.
The male or donor cell possesses 1-4 sex pili on the surface and fertility factor (transfer factor, sex factor) in its plasmid. Fertility factor contains genes for producing sex pili and other characters needed for gene transfer. Sex pili are 1- 4 narrow protoplasmic outgrowths. Both sex pili and fertility factor are absent in female or recipient cells.
If these two types of cells happen to come nearer, a piles of male cell establishes a protoplasmic bridge or conjugation tube with the female cell. It takes 6-8 minutes. Gene exchange can occur by two methods (Fig. 2.13).
(a) Sterile Male Method (F+ x F– → F– becomes F+):
The plasmid having fertility factor replicates. A copy of it gets transferred to the recipient cell through the conjugation tube. The recipient cell also becomes donor. The phenomenon of reversibility of sex is called sexduction.
(b) Fertile Male Method (Hfr x F– →F– remains F–):
The F+ plasmid or fertility factor of the donor cell gets integrated to bacterial chromosome or DNA. The attachable plasmid is known as episome. The point at which plasmid gets integrated to bacterial chromosome differs in different strains. Integration is possible because certain nucleotide sequences present in bacterial chromosome are compatible with sequences in plasmid DNA.
The donor cell having fertility factor integrated to its chromosome is called Hfr (high frequency of recombination), meta male or super male because it has a recombination frequency of 1000 times more as compared to normal F+.
Non-integrated F+ plasmids disintegrate in her cells. The integrated F+ factor breaks the bacterial chromosome at one end of its attachment. The bacterial chromosome now undergoes replication.
A copy of the freed end of bacterial chromosome (end distal to F+ factor, also called zero end) passes into the recipient cell through the conjugation tube. Fertility factor is the last to do so.
Generally whole of bacterial chromosome does not pass into recipient cell. F+ factor is very rarely transferred as conjugation is maintained for a brief period. Only a few genes are transferred, one in seven minutes, two in nine minutes, three in ten minutes, four in eleven minutes, etc..
Conjugation produces an incompletely diploid “zygote” known as merozygote or partial zygote. The new genes may replace the genes present in the recipient cells (those of the recipient cells disintegrate) or get added to them.
Method # 2. Transformation:
It is the absorption of DNA segment from the surrounding medium by a living bacterium. The phenomenon was discovered by Griffith in 1928. Its mechanism was worked out by Avery (1944).
Receptivity for transformation is present for a brief period when the cells have reached the end period of active growth. At this time they develop specific receptor sites in the wall. Normally E. coli does not pick up foreign DNA but it can do so in the presence of calcium chloride.
Method # 3. Transduction:
It is the transfer of foreign genes by means of viruses. Transduction was first discovered by Zinder and his teacher Lederberg (1952) in Salmonella typhimurium. The process also occurs in E. coli and a number of other hosts. A virus may pick up gene of the host in place of its own gene during its multiplication in the host cell.
Such a virus is never virulent. It passes over the gene of the previous host to the new host. Transducing viruses may carry the same genes (restricted transduction) or different genes (generalized transduction) at different times.