When 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. Assign letters a-e to the pictures above. DNA from E. coli cells grown in ^5N DNA from E. coli cells grown in ^14 N A 1: 1 mixture of DNA from cells grown in 14^N and cells grown in ^15 N A 1: 1 mixture of DNA from cells grown in 14^N and 15^N, heated (to disrupt hydrogen bonds) and cooled (to allow reannealing) DNA containing one strand of ^15 N-DNA and one strand of ^14 N-DNA

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).

Explanation:

Meselson and Strahl had used CsCl gradient centrifugation to demonstrate the semi-conservative nature of DNA replication. In a CsCl density gradient centrifugation, from a pool of light, heavy and intermediate size DNA, the heaviest one would go and settle towards the bottom of the test tube, while the lightest one would be at the top and the intermediate one would find a place in between the heavy and the lighter one.

If the DNA is labelled only with 15N, then the DNA is heavier in size and the DNA would settle towards the bottom of the tube during density gradient centrifugation.

If the DNA is labelled only with 14N, then the DNA is lighter in size and the DNA would settle towards the top of the tube during density gradient centrifugation.

If one strand of the DNA is labelled with 14N while the other one is labelled with 15N, then the DNA is intermediate of these two in size and the DNA would settle towards the middle of the tube during density gradient centrifugation.

but whenever we have a equimolar or equal ratio mixture of DNA grown in 14N and 15N both, then we expect two bands, one lighter corresponding to the size and position of 14N labelling, while the other one is heavier and corresponds to size and position of 15N labelling.

However, whenever we have a equimolar or equal ratio mixture of DNA grown in 14N and 15N both which is heated and cooled both, then we expect three bands because of denaturation and reannealing of DNA strands, which would lead to the annealing of the lighter strand with heavier starnd as well instead of the lighter strand and heavier strand alone in the pool and hence we would have three bands, one lighter corresponding to the density and position of 14N labelling, the second one that is heavier and corresponds to density and position of 15N labelling, and the third one which is the intermediate between these two densities.

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