Definition :
The unidirectional transfer of DNA from our cell to another through the cytoplasmic bridge is called Conjugation.
Introduction :
Conjugation was discovered by J. Lederbery and E.L. Tautum in e. Coli During conjugation DNA is transferred from a donor cell to a recipient cell through specialized conjugation tube ( an intercellular connection), that forms between them. The transfer of genetic information is, therefore, “One-way transfer” during conjugation.
(The Donor and recipient cells are sometimes referred to as male and female cell respectively) Bacterial cell are differenciated, as being donor by the puexnce of specialized cell surface appendages called F Pilli.
Synthesis of F Pilli is controlled by small circular molecule of DNA or “minichromosome” called F factor [ Also named as F plasmid/sex factor]
Subtitle 1 : Donar cells/male bacterium carrying F factor (called F+ cells) from conjugation tubes and initiate DNA transfer after making contact with cells not carrying on F factor called F- cells (recipient cells)
Subtitle 2 : F factor can exist in two different states
(1) The autonomous state : Here, F – factor present in cytoplasm replicates independently of the host chromosome.
(2) Integrated State : F – factor is covalently inserted into the host chromosome and replicated along with the host chromosome like any other set of chromosome genes.
Subtitle 3 : Fig: showing autonomous state :
A donor cell containing F factor in an autonomous state is called F+ cell. When F+ donor cell conjugates with an F– recipient cells, only the autonomous F– factor is transferred. Then, both enconjugants (cells which are involved in conjugation) become F+ cells ( because F – Factor replicates during transfer)
Consequently When a population of F+ cells is mixed with F– cells then it results in virtually all cells becoming F+ cells.
Subtitle 4 : A cell carrying an integrated factor called Hfr cell (for high frequency recombination) (Hfr cell is male bacterium) donor cell). In integrated state the F– factor mediates the transfer of chromosome of Hfr cell the donor cell to recipient (F–) cell usually, a portion of Hfr chromosome is
Fig : Showing F factor transfer b/w Hfr & F- is transferred before cells separate, thus breaking the chromosome. ( very rearely, entire Hfr chromosome is transferred the recipient cell)
Result : In mating between Hfr (male donor cell) and F- (female, recipient cell), Hfr cell will remain cell will usually remain F- . It happens because during transfer of chromosome from Hfr to F-, F- factor is the last segment of DNA to be transferred and chromosome usually break spontaneously before entire chromosome is transferred ( as, in rare case Hfr Chromosome is transferred the recipient cell)