PART A - The experimental technique: Density-gradient centrifugationWhen a solution of cesium chloride (CsCl) is subjected to high-speed centrifugation, a stable density gradient is formed. Meselson and Stahl found that when cell contents were subjected to centrifugation with a CsCl solution, a band of DNA formed at the CsCl density that matched the density of the DNA. This technique is called density-gradient centrifugation.The test tubes below show the results of density-gradient centrifugation of five different DNA samples.Drag the description of each DNA sample to the appropriate location to identify the expected appearance of the DNA band(s) after density-gradient centrifugation.-Test tube 1: b. DNA from E. coli cells grown in 14N-Test tube 2: e. DNA containing one strand of 15N-DNA and one strand of 14N-DNA-Test tube 3: a. DNA from E. coli cells grown in 15N-Test tube 4: c. A 1:1 mixture of DNA from cells grown in 14N and cells grown in 15N-Test tube 5: d. A 1:1 mixture of DNA from cells grown in 14N and 15N, heated (to disrupt hydrogen bonds) and cooled (to allow reannealing).[The densities of 14N/14N, 14N/15N, and 15N/15N double helices differ from each other and thus form bands in different positions. 14N/14N forms a band toward the top, 14N/15N in the middle, and 15N/15N toward the bottom.DNA from cells grown in 15N contains only 15N/15N double helices.DNA from cells grown in 14N contains only 14N/14N double helices.A 1:1 mixture of DNA from cells grown in 14N and cells grown in 15N contains both 14N/14N and 15N/15N double helices.A 1:1 mixture of DNA from cells grown in 14N and cells grown in 15N, heated and then cooled, contains 14N/14N, 14N/15N, and 15N/15N double helices.DNA containing one strand of 15N-DNA and one strand of 14N-DNA contains only 14N/15N double helices.]
