Evidence from genetic recombination in bacteria or bacterial conjugation : Lederberg and Tatum (1946) discovered the genetic recombination in bacteria from two different strains through the process of conjugation. Bacterium Escherichia coli can grow in minimal culture medium containing minerals and sugar only. It can synthesize all the necessary vitamins from these raw materials. But its two mutant strains were found to lack the ability to synthesize some of the vitamins necessary for growth. These could not grow in the minimal medium till the particular vitamins were not supplied in the culture medium.

(a) Mutant strain A : It (used as male strain) had the genetic composition Met– , Bio–, Thr+, Leu+, Thi+. It lacks the ability to manufacture vitamins methionine and biotin and can grow only in a culture medium which contains these vitamins in addition to sugar and minerals.

(b) Mutant strain B : It (used as female strain or recipient) has a genetic composition Me++, Bio+, Thr–, Leu–, Thi–. It lacks the ability to manufacture threonine, leucine and thionine and can grow only when these vitamins are added to the growing medium.
These two strains of E.coli are, therefore, unable to grow in the minimal culture medium, when grow separately. But when a mixture of these two strains was allowed to grow in the same medium a number of colonies were formed. This indicates that the portion of donor DNA containing information to manufacture threonine, leucine and thionine had been transferred and incorporated in the recipient’s genotype during conjugation.

This experiment of Lederberg and Tatum shown that the conjugation results in the transfer of genetic material DNA from one bacterium to other. During conjugation a cytoplasmic bridge is formed between two conjugating bacteria.

Hershey and Chase (1952) conducted their experiment on T2 bacteriophage, which attacks on E.coli bacterium. The phage particles were prepared by using radioisotopes of 35-S and 32-P in the following steps.

(a) Few bacteriophages were grown in bacteria containing 35-S. Which was incorporated into the cystein and methionine amino acids of proteins and thus these amino acids with 35-S formed the proteins of phage.

(b) Some other bacteriophages were grown in bacteria having 32-P. Which was restricted to DNA of phage particles. These two radioactive phage preparations (one with radioactive proteins and another with radioactive DNA) were allowed to infect the culture of E.coli. The protein coats were separated from the bacterial cell walls by shaking and centrifugation.

The heavier infected bacterial cells during centrifugation pelleted to bottom. The supernatant had the lighter phage particles and other components that failed to infect bacteria. It was observed that bacteriophages with radioactive DNA gave rise to radioactive pellets with 32-P in DNA. However in the phage particles with radioactive protein (with 35-S) the bacterial pellets have almost nil radioactivity indicating that proteins have failed to migrate into bacterial cell. So, it can be safely concluded that during infection by bacteriophage T2, it was DNA, which entered the bacteria. It was followed by an eclipse period during which phage DNA replicates numerous times within the bacterial cell. Towards the end of eclipse period phage DNA directs the production of protein coats assembly of newly formed phage particles. Lysozyme (an enzyme) brings about the lysis of host cell and release, the newly formed bacteriophages. The above experiment clearly suggests that it is phage DNA and not protein, which contains the genetic information for the production of new bacteriophages. However, in some plant viruses (like TMV), RNA acts as hereditary material (being DNA absent).