The parental DNA (depicted with one red and one green strand) labelled with 15N is replicated. Each daughter molecule of the first generation (that has been synthesized in media with normal 14N) contains one of the parental strands (red or green) and one new, 14N-strand (yellow or blue). During he following replication are the labelled strands passed on to just half of the daughter molecules (second generation). (M. MESELSOHN, F. W. STAHL, 1958)