20. Bacterial conjugation was first described by Lederberg and Tatum in 1946 as a phenomenon involving the exchange of markers between closely related strains Of Escherichia coli. For experiment they used
(1) One auxotroph and one prototroph
(2) Two auxotroph and two prototroph
(3) Two auxotrophs
(4) Two prototroph

The correct answer is (3) Two auxotrophs. Lederberg and Tatum in 1946 discovered bacterial conjugation using two auxotrophic strains of Escherichia coli which were mutants requiring different nutritional supplements. They mixed these two auxotrophs and then plated them on minimal medium to select for prototrophic recombinants that had acquired genetic material from each other, allowing growth without supplements. This proved genetic exchange by conjugation between closely related bacterial strains.

Explanation of each option:

(1) One auxotroph and one prototroph: This would not work for detecting conjugation because the prototroph already grows on minimal medium, so no selection for recombinants would occur.

(2) Two auxotrophs and two prototrophs: This is incorrect and unnecessarily complicated. The experiment specifically required two auxotrophic strains to detect recombination by complementation of nutritional deficiencies.

(3) Two auxotrophs: Correct choice. Mixing two auxotrophic strains with mutations in different genes and plating on minimal medium allowed detection of rare recombinants that regained prototrophy through conjugation.

(4) Two prototrophs: This would not allow detection of genetic exchange because both strains grow on minimal medium individually without supplementation.

Introduction:
Bacterial conjugation, a process of genetic exchange between bacteria, was first demonstrated by Joshua Lederberg and Edward Tatum in 1946. They discovered this phenomenon using two auxotrophic strains of Escherichia coli, each requiring different nutritional supplements for growth. By mixing these strains and plating them on minimal medium, they observed prototrophic colonies, providing evidence of gene transfer through conjugation. This groundbreaking experiment revealed that bacteria possess a mechanism for genetic recombination similar to sexual reproduction in higher organisms.

Detailed explanation of experiment with all options:

The landmark 1946 experiment by Lederberg and Tatum involved mixing two auxotrophic E. coli strains—each mutant for different nutritional needs—and incubating them together. Auxotrophs are mutants that cannot synthesize certain essential nutrients and therefore cannot grow on minimal medium unless supplemented. When these strains were mixed, some bacteria exchanged genetic material through conjugation, restoring their ability to grow on minimal medium as prototrophs. This genetic exchange was only detectable because both strains required different nutrients (auxotrophs), making option (3) the correct experimental condition.

Using one auxotroph and one prototroph (option 1) or two prototrophs (option 4) would not allow selection of recombinants on minimal medium because prototrophs grow without supplements. Option (2) involving two auxotrophs and two prototrophs is not consistent with the original experiment design, which focused on two auxotrophic strains to directly detect conjugation-derived recombinants.

This experiment revolutionized microbiology by revealing that bacteria can exchange genetic information actively via conjugation, challenging the prior belief that bacteria are asexual clones.

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This response covers detailed explanation of all options, correct answer with historical context, and an SEO-friendly article format with title, slug, keyphrase, meta description, and introduction based on the bacterial conjugation experiment by Lederberg and Tatum in 1946.