In Hershey-Chase experiments, which radiolabel was used to track DNA?

• A. 35S
• B. 3H
• C. 14C
• D. 32P

Answer: (D)

DNA carries genetic information, so to see what actually enters the bacterial cell during infection, scientists needed a label that would mark DNA specifically. Phosphorus-32 was used because DNA's backbone is made of sugar-phosphate units, giving it phosphate groups that can be tagged with radioactive phosphorus. When the phage infected bacteria, tracking 32P showed the labeled DNA, not the protein coat, inside the cells, proving DNA is the genetic material. In contrast, sulfur-35 labels proteins (sulfur in amino acids), so it marks the protein coat rather than DNA. Other labels like 3H or 14C can label various molecules, but the key choice for tracking DNA in this experiment is 32P.