AP BIOLOGY

Detailed explanation-1: -N (heavy) and 14N (normal) are two isotopes of nitrogen, which can be distinguished based on their densities by centrifugation in Ca, esium chloride (CsCl). Meselson and Stahl cultured E.coli in a medium constituting 15NH4Cl over many generations. As a result, 15N was integrated into the bacterial DNA.

Detailed explanation-2: -Before beginning their analysis of DNA replication, Meselson and Stahl first showed that DNA made with regular 14N could be separated from DNA containing heavy 15N. They accomplished this by growing two separate batches of Escherichia coli bacteria, feeding each batch a different nitrogen isotope.

Detailed explanation-3: -Messelson & Stahl used 15N for their classical experiment because it is a heavy isotope and can be separated from 14N on the basis of density using CsCl based density gradient centrifugation.

Detailed explanation-4: -Meselson and Stahl used density-gradient centrifugation to separate different molecules in a solution, a method they later used to separate DNA molecules in a solution.

Detailed explanation-5: -Just prior to the addition of 14N and periodically thereafter, as the bacterial cells grew and replicated, Meselson and Stahl sampled DNA for use in equilibrium density gradient centrifugation to determine how much 15N (from the original or old DNA) versus 14N (from the new DNA) was present.