Meselson and Stahl carried out centrifugation in CsCl density gradient to separate the normal DNA from 15N-DNA. Meselson- Stahl experiment showed the semiconservative nature of DNA replication. E.coli growing in presence of ammonium chloride with heavy isotope of nitrogen (15 N) was transferred to 14N normal isotope containing medium. One distinct band of 15N-14N was obtained after 20 mins. 2nd generation have half 15N-14N and other 14N-14N DNA. In this manner after 80 mins large fraction of extracted DNA is 14N-14N.