Question

The semi-conservative replication of DNA was first demonstrated in-
A) Salmonella typhimurium
B) Drosophila melanogaster
C) Escherichia coli
D) Streptococcus pneumonia

Answer 1

Hint:  In the process of semi-conservative replication, the double-helix DNA must be split such that the new template strand may be attached to the adjacent base pairs.

Complete answer:

To solve this question, first, we should know about the semi conservative replication of DNA. Watson and Crick suggested a semi-conservative replication of DNA. This is a method of replication in which one daughter duplex strand is extracted from the parent, while the other strand is created from. The two strands of a single DNA molecule, and each strand, act as a template for a new yet complementary strand formation.

Now lets us find the solution from the given options-

1) Meselson and Stahl produced E. Coli in a heavy density ¹⁵N solution to obtain the DNA. Salmonella typhimurium is a pathogenic Gram negative bacteria located primarily in the lumen of the intestine. The toxicity is due to an exterior membrane mainly composed of lipopolysaccharides (LPS) that defends the bacteria from the environment. So, Salmonella typhimurium is an incorrect answer. Thus, option A is an incorrect option.

2) The experiment performed by Messelson and Stahl in E. Coli proved to be semi-conservative replication of DNA. Drosophila melanogaster are also known as fruit flies and their main distinctive character is to remain ripped or rotted on fruits. So, Drosophila melanogaster is an incorrect answer. Thus, option B is an incorrect option.

3) DNA replication is the mechanism by which two similar replicas are formed from one initial DNA molecule. This evolutionary cycle takes place in all living species and forms the origin of biological inheritance.  E. Coli are grew in a medium containing ¹⁵NH₄Cl and the ¹⁵N gets incorporated in the newly synthesised DNA along with other nitrogen containing compounds. Then they are transferred in a medium containing  ¹⁴NH₄Cl and the extracted DNA after 1st generation shows intermediate density upon centrifugation in cesium chloride density gradient. So, Escherichia coli is the incorrect answer. 

4) The experiments carried out by Messelson and Stahl in E.coli have proved semi-conservative replication of DNA. Streptococcus pneumoniae, is a spherical Gram-positive bacterium. So, Streptococcus pneumonia is an incorrect answer. 

Thus, option C i.e Escherichia coli is the correct option.

Note: The cultured by Meselson and Stahl, E. Coli in a high density ¹⁵N medium to provide DNA. Bacteria were then shifted to a ¹⁴N medium, replication cycle DNA was isolated. After one replication process the DNA was all of moderate density and ruled out the conservative replication model, which assumes the existence of both heavy density DNA and medium density DNA.