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) Radioactive nitrogen has a half-life of 100,000 years, and the material would be too dangerous for too long.
B) There is no radioactive isotope of nitrogen.
C) Although there are more nitrogens in a nucleotide, labeled phosphates actually have 16 extra neutrons; therefore, they are more radioactive.
D) Amino acids (and thus proteins) also have nitrogen atoms; thus, the radioactivity would not distinguish between DNA and proteins.

Alfred Hershey and Martha Chase carried out an experiment by employing the sulfur labeled proteins and phosphorous labeled DNA of the phage. These phages were permitted to infect bacteria. It was noted that the DNA had entrance to the bacterial cells and was thought to be the genetic material. The offspring of the phages that were marked with radioactive phosphorus was still marked although the offspring of the phages marked with radioactive sulfur were unmarked. This proved that DNA and not protein can be labeled as genetic material. When students employed the use of radioactive nitrogen in lieu of radioactive phosphorous, and could not obtain the required results due to nitrogen being found in proteins and DNA as nitrogen bases. Therefore, the radioactivity could not differentiate between DNA and proteins.

In view of this, the right answer is

D.

'Amino acids (and thus proteins) also have nitrogen atoms. Thus, the radioactivity would not distinguish between DNA and proteins.'