Watson and Crick had proposed the semi-conservative scheme for replication of DNA. 

Experimental proof for semi-conservative mode of DNA replication 

• Matthew Meselson and Franklin Stahl in 1958 performed experiments on E. coli to prove that DNA replication is semi-conservative. 

• They grew E. coli in a medium containing ¹⁵NH₄Cl (in which ¹⁵N is the heavy isotope of nitrogen) for many generations. 

• As a result, ¹⁵N got incorporated into newly synthesised DNA. 

• This heavy DNA can be differentiated from normal DNA by centrifugation in caesium chloride (CsCl) density gradient. 

• Then they transferred the cells into a medium with normal ¹⁴NH₄Cl and took the samples at various definite time intervals as the cells multiplied. 

• The extracted DNAs were centrifuged and measured to get their densities. 

• The DNA extracted from the culture after one generation of transfer from the ¹⁵N medium to ¹⁴N medium (i.e., after 20 minutes; E. coli divides every 20 minutes) showed an intermediate hybrid density. 

• The DNA extracted from culture after two generations (i.e., after 40 minutes) showed equal amounts of light DNA and hybrid DNA. 

• Similar experiment was performed by Taylor and colleagues in 1958, on Vicia faba to prove that the DNA in chromosome also replicate semi-conservatively.