Final answer:
Hershey and Chase could not use a radioactive isotope of carbon because it would not differentiate between DNA and proteins, as carbon is present in both. Their use of 35S and 32P allowed identification of DNA as the genetic material.
Step-by-step explanation:
Hershey and Chase could not have used a radioactive isotope of carbon instead of 32P because carbon is present in both DNA and proteins. Therefore, it would not have been possible to differentiate whether it was the DNA or the proteins that entered the E. coli bacteria during infection. They selected radioactive sulfur (35S) and radioactive phosphorus (32P) precisely because these elements are found exclusively in one of the two types of biological molecules. Sulfur is found in some amino acids which make up proteins, while phosphorus is a key component of nucleic acids, specifically DNA and RNA. During their experiments, only the 32P-entered bacterial cells, indicating that it was the genetic material DNA, and not proteins, which was responsible for heredity.