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Questions 1. Why is a nitrogen label a good tool for studying DNA? 2. What other molecules in a cell have nitrogen in them? 3. What's the difference
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Questions1. Why is a nitrogen label a good tool for studying DNA?

2. What other molecules in a cell have nitrogen in them?

3. What's the difference between 14N and 15N at the atomic level?

4. What's the term for two atoms of the same element with different molecular masses?

5. Give an example of another element that has atoms of more than one molecular mass.


Bacteria in the laboratory can grow on plates or in broth cultures composed of precisely defined mixtures of chemicals.

Meselson and Stahl grew bacteria in cultures containing only 15N nitrogen for many generations so that the DNA was

almost entirely composed of 15N-containing nucleotides. Th ese chromosomes are more dense than 14N chromosomes,

and by spinning chromosomes at very high speeds in a density gradient tube, Meselson & Stahl could tell the

difference between the two kinds of chromosomes. Heavy chromosomes sank farther to the bottom of the tube, where

the liquid was more dense. Lighter chromosomes floated in the less dense liquid toward the top of the tube. Th is

can be represented by the diagram below. Th e blue lines represent the location of chromosomes in the tubes after

centrifugation.

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