Conjugation Mechanism in Gene Recombination

Conjugation Mechanism in Gene Recombination

	Conjugation Mechanism in Gene Recombination - Literature in bacterial morphology contains many descriptions of microscopic observations of cell pairs which were identified as indicators of mating and sexuality in bacteria. However; no confirmatory genetic evidence was available till the discovery of conjugation in E. coli by J. Lederberg and E.L. Tatum in 1946, who mixed auxotrophic mutants and selected rare recombinants. In their initial experiments, Lederberg and Tatum plated E. coli mutants having triple and complementary nutritional requirements (abcDEF X ABCdef) on minimal agar and obtained prototrophic bacteria (ABCDEF).
These recombinants were stable, could be propagated and arose at a frequency of 10-6,10-7. Further evidence to show that the development of prototrophic colonies required the cooperation of intact bacteria of both types was obtained by the 'U' tube experiments (see transduction). Neither the culture filtrates nor the cell free culture extracts were productive suggesting that actual cell contact was necessary. Lederberg also examined a large number of the prototrophic colonies to know whether the process was reciprocal. He found that most colonies contained only one class of recombinants suggesting that recombination in bacteria may be of an unorthodox kind. Also detailed analysis of prototrophs showed initial heterozygous nature but later were converted to haploids.
These studies by Lederberg and his colleagues proved that bacteria possessed sex which made them amenable to formal genetic analysis and also revealed the existence of genetic material in a chromosomal organization. Subsequent studies carried out to determine the size of the DNA fragment involved, by detecting the number of genetic markers transferred, suggested that more than one market could be transferred at a time and interestingly, linkage between certain markers was always seen.
It was concluded that in this process of conjugation (i) large fragments of DNA were transferred from one bacterium to another in a non reciprocal manner, and (ii) that transfer always occurred from a given point. It was also found that the size of the DNA transferred from one cell to another was much larger than in transformation and this technique appeared to be a more useful technique for gene mapping in bacteria. The bacteria that transfer DNA were called the "donor" bacteria, while those that receive the DNA were called the "recipient" bacteria.

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