Messelson and Sthal's explained the model of semi conservative nature of DNA replication. According to this model, the newly synthesized DNA molecule contains one newly synthesized strand and one parental strand.
Firstly, bacteria grown in 15N media is transferred to 14N media. The isolated DNA is centrifuged and intermediate density DNA band is observed that contains one strand of 15N (parental strand) and one strand of 14N (newly synthesized strand).
