Question

For a science fair project, two students decided to repeat the Hershey and Chase experiment, with modifications. They decided to label the nitrogen of the DNA, rather than the phosphate. They reasoned that each nucleotide has only one phosphate and two to five nitrogens. Thus, labeling the nitrogens would provide a stronger signal than labeling the phosphates. Why won't this experiment work?
A) There is no radioactive isotope of nitrogen.
B) Radioactive nitrogen has a half-life of 100,000 years, and the material would be too dangerous for too long.
C) Avery et al. have already concluded that this experiment showed inconclusive results.
D) Although there are more nitrogens in a nucleotide, labeled phosphates actually have 16 extra neutrons; therefore, they are more radioactive.
E) Amino acids (and thus proteins) also have nitrogen atoms; thus, the radioactivity would not distinguish between DNA and proteins.

Answer 1

Answer is (E) : Amino acids (and thus proteins) also have nitrogen atoms; thus, the radioactivity would not distinguish between DNA and proteins.

Let me explain you why. Hershey and Chase conducted their experiment to show what is the genetic material that passes from one generation to another generation. Is it protein or DNA? For explaining this, they allowed the virus to grow in the presence of two different radio active elements.

In one set they grew viruses in the presence of P-32 radio-active element  

In another set they grew viruses in the presence of S-35 radio active element.

The reason why they choose these two radio-active element was, phosphorus is present in DNA but not in proteins and similarly sulphur is found in proteins but not DNA.

These viruses were allowed to infect bacteria and the later was observed to be radio-actively labelled with P-32 but not S-35. This clearly showed that DNA is the genetic material which was passed from virus to bacteria.  

Now if you choose Nitrogen for labeling, it won't help you in getting any conclusion since nitrogen is present in both proteins and DNA.