AP BIOLOGY

Detailed explanation-1: -The heavy isotope of nitrogen was used in Meselson and Stahl’s experiment for various reasons. The heavy DNA molecule (containing 15N) could be easily distinguished from the normal DNA by centrifugation In a Cesium chloride (CsCI) density gradient.

Detailed explanation-2: -The semi-conservative nature of DNA replication was established by Meselson and Stahl in their classic experiment with bacteria They grew bacteria in N15−NH4Cl containing medium, washed and then incubated in fresh medium with N14-containing compounds and allowed to grow for three generations.

Detailed explanation-3: -Detailed Solution They used heavy isotope (nitrogen-15) and Cairns used radioactive thymidine.

Detailed explanation-4: -They used heavy (N15) and light (N14) nitrogen, which is found in the nucleotides that make up the DNA. They aimed to differentiate between DNA samples depending on the weight or the position on a density gradient. Density-gradient centrifuge was a technique developed by Meselson himself.

Detailed explanation-5: -Diagram of the Meselson-Stahl experiment. All DNA is initially nitrogen-15-labeled. A DNA sample is taken prior to adding the bacteria to nitrogen-14 medium, and when centrifuged in a CsCl gradient, the DNA forms a single band low in the tube (indicating DNA labeled entirely with nitrogen-15).