Semi-conservative replication
This is the theory that when DNA is replicated, 50% is new and 50% is old – therefore one strand is from the original DNA and the other is newly created. It was discovered by the Meselson-Stahl experiment.
E. coli was taken and grown on 15N (nitrogen that is slightly radioactive and heavier than normal nitrogen) for generations. This new nitrogen was then incorporated into the nitrogenous bases. Eventually, all ‘normal nitrogen’ (14N) was removed from the nitrogenous bases.
After this, the E. coli was grown in a medium that didn’t contain any 15N. When the DNA replicated, two slightly lighter strands were produced. As more replications occurred, the strands continued to get lighter.