First experimental proof for semi-conservative DNA replication was sh…

The correct answer is Option (3) → Escherichia coli

Watson and Crick proposed that DNA replicates semiconservatively: each new DNA molecule keeps one parental strand and gets one new strand. The first experimental proof came from Matthew Meselson and Franklin Stahl in 1958, who worked on Escherichia coli. They grew the bacteria in a medium with heavy nitrogen (¹⁵N), moved them to normal ¹⁴N medium, and separated the DNA by density in a caesium chloride gradient. After one generation the DNA had a hybrid density, as the model predicts. Taylor and colleagues did very similar experiments on the chromosomes of Vicia faba (faba beans).

NCERT Extract:
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.”