Final answer:
DNA strands' densities vary based on their G=C content, with denser DNA having more G=C pairs. CsCl density gradient centrifugation is used to separate DNA based on density, demonstrating semi-conservative replication by showing intermediate density bands after DNA replication.
Step-by-step explanation:
The densities that should be observed in the DNA strands depend on the guanine (G) and cytosine (C) content of the DNA. During cesium chloride (CsCl) density gradient centrifugation, DNA with a higher G=C content will have a greater buoyant density than DNA with a lower G=C content. The classic Meselson and Stahl experiment used this principle to demonstrate that DNA replication is semi-conservative. They found that after one generation of growth in 14N (after shifting from 15N), DNA had an intermediate density. After a second generation, two bands were observed: one at the intermediate density and another at the 14N density.