The semiconservative mode of DNA replication in prokaryotes was exper…

The correct answer is Option (3) → Matthew Meselson and Franklin Stahl

In 1958, Meselson and Stahl grew E. coli for many generations in a medium with heavy nitrogen (¹⁵N), and then moved it to normal nitrogen (¹⁴N). They separated the DNA by density in a caesium chloride gradient. After one generation the DNA was of intermediate density, and after two generations half was intermediate and half was light. This proved that DNA replicates semiconservatively, and that each new molecule keeps one old strand.

(1) Oswald Avery, Colin MacLeod and Maclyn McCarty — Incorrect. They showed that DNA, and not protein, is the transforming principle, the genetic material.

(2) Watson and Crick — Incorrect. They gave the double helix model and proposed semiconservative replication. They did not prove it by experiment.

(3) Matthew Meselson and Franklin Stahl — Correct. They proved it in E. coli, a prokaryote.

(4) Alfred Hershey and Martha Chase — Incorrect. Their work with bacteriophages showed that DNA is the genetic material.

NCERT Extract:
Replication – “… The scheme suggested that the two strands would separate and act as a template for the synthesis of new complementary strands. After the completion of replication, each DNA molecule would have one parental and one newly synthesised strand. This scheme was termed as semiconservative DNA replication (Figure 5.6).”
The Experimental Proof – “It is now proven that DNA replicates semiconservatively. It was shown first in Escherichia coli and subsequently in higher organisms, such as plants and human cells. Matthew Meselson and Franklin Stahl performed the following experiment in 1958: (i) They grew E. coli in a medium containing ¹⁵NH₄Cl (¹⁵N is the heavy isotope of nitrogen) as the only nitrogen source for many generations.”