Q.You are repeating the Hershey-Chase experiment and are provided with two isotopes: 32P and 15N (in place of 35S in the original experiment). How do you expect your results to be different?

The Hershey-Chase experiment relied on the fact that DNA contains phosphorus but not sulfur, while proteins contain sulfur but not phosphorus. By labeling phage DNA with ³²P and proteins with ³⁵S, they could track which component entered the bacterial cell during infection—the radioactive ³²P appeared inside, proving DNA was the genetic material.

If you replace ³⁵S with ¹⁵N, you face a fundamental problem: nitrogen is present in both DNA (in the nitrogenous bases) and proteins (in amino acids). When you label the phage with ¹⁵N, both the DNA and the protein coat will incorporate the isotope. After infection and blending, you would find ¹⁵N in both the pellet (infected bacteria) and the supernatant (phage coats), because nitrogen from both molecules gets distributed. …

The Hershey-Chase experiment rested on a beautiful piece of chemical logic: DNA and protein differ in their elemental composition, so radioactive tracers can mark one molecule but not the other. Alfred Hershey and Martha Chase used 32P to label DNA (because phosphorus is abundant in the sugar-phosphate backbone but absent from protein) and 35S to label protein (because sulfur is present in the amino acids cysteine and methionine but absent from DNA). When phages infected bacteria, only the 32P entered the cells—proving DNA, not protein, was the genetic material.

Now suppose you replace 35S with 15N. Phosphorus-32 still works perfectly: it labels DNA exclusively, and you will find radioactivity inside the infected bacteria after blending, just as Hershey and Chase did. The problem lies with nitrogen-15.

Nitrogen is a component of both DNA and protein. The nitrogenous bases (adenine, guanine, cytosine, thymine) form the core of every nucleotide, while amino acids contain nitrogen in their amino groups and often in their side chains. If you grow phages in a medium containing 15N, both the viral DNA and the viral protein coat will incorporate the isotope. When these labeled phages infect bacteria, you will detect 15N both in the supernatant (from the protein coats sheared off by blending) and inside the bacterial cells (from the DNA injected during infection).

Instead of predicting the outcome from first principles, build a two-row table of what each label marks: 32P -> DNA only (unchanged from the original experiment, so it still enters bacteria); 15N -> BOTH DNA and protein (nitrogen sits in the bases AND in amino acids). Laying the two isotopes' targets side by …