Semi conservative replication of DNA was proved by work of Meselson and Stahl (1958). They grew E.coli for many generations in medium with N15 till the bacterial DNA became completely labelled with N15. Then they transferred it to N14 containing medium and first generation showed DNA with both N14 and N15 in equal amount. It proved semi conservative nature of replication i.e., the two resulting double strands are identical, and each of them consists of one original and one newly synthesized strand.