Watson and Crick in 1953 proposed a scheme that DNA replication was semi-conservative. According to the scheme, the two parental strands separate and each strand acts as a template for synthesising a complementary strand over it. After completion of replication, each DNA had one parental strand and one newly synthesised strand. 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) showed an intermediate hybrid density.
• The DNA extracted from culture after two generations (i.e., after 40 minutes) showed equal amount of light DNA and hybrid DNA.