Biology · Molecular Basis of Inheritance · NEET
They needed a label that marks only DNA and another that marks only protein. DNA contains phosphorus but no sulfur, so radioactive phosphorus (32P) tags only DNA. Protein contains sulfur (in amino acids like cysteine and methionine) but no phosphorus, so radioactive sulfur (35S) tags only protein. This clean separation is the whole point of the design. NEET loves this exact logic.
Carbon and nitrogen are present in BOTH DNA and protein. If they had used labelled C or N, both molecules would become radioactive and you could not tell which one entered the bacteria. Phosphorus and sulfur work because each is unique to one molecule. This is a very common NEET reasoning trap.
After the radioactive phages attached to E. coli and infection began, the empty viral coats were stuck to the outside of the bacteria. Agitating in a blender shook these coats off the bacterial surface. Then spinning in a centrifuge separated the lighter viral particles (supernatant) from the heavier bacteria (pellet). This let them check which label went inside the bacteria.
Avery, MacLeod and McCarty (using Griffith's transforming principle in Streptococcus pneumoniae) gave BIOCHEMICAL evidence that DNA is the transforming material by destroying protein, RNA and DNA one at a time. Hershey and Chase used bacteriophages and radioactive tracers to give the FINAL, unequivocal proof. NEET asks which one is the 'unequivocal/final' proof, the answer is Hershey and Chase.
They used bacteriophages (viruses that infect bacteria) that attach to E. coli bacteria. A common NTA trap swaps this with TMV (Tobacco Mosaic Virus). TMV belongs to Fraenkel-Conrat's RNA experiment, not Hershey-Chase. Remember: Hershey-Chase = bacteriophage + E. coli.
Earlier experiments (Griffith, Avery) strongly suggested DNA but did not fully rule out protein for everyone. Hershey-Chase directly showed only the labelled DNA (32P) entered the bacteria and produced new viruses, while labelled protein (35S) stayed outside. There was no room for doubt, so NCERT calls it the unequivocal (conclusive) proof.
The final proof for DNA as the genetic material came from the experiments of
Which scientist conducted an experiment with 32P and 35S labelled phages for demonstrating that DNA is the genetic material?
Which scientist experimentally proved that DNA is the sole genetic material in bacteriophage?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It was performed in 1952 by Alfred Hershey and Martha Chase. NEET often tests the scientists' names and the year, so remember 1952.
It proved that DNA, not protein, is the genetic material that passes from virus to bacteria. This is why NCERT calls it the 'unequivocal proof that DNA is the genetic material.'
Both were labelled but in separate batches. DNA was labelled with radioactive phosphorus (32P) and protein was labelled with radioactive sulfur (35S), because phosphorus is found only in DNA and sulfur only in protein.
DNA contains phosphorus (in its phosphate backbone) but no sulfur, so 32P tags only DNA. Protein contains sulfur (in some amino acids) but no phosphorus, so 35S tags only protein. This makes the two molecules easy to tell apart.
No. They used bacteriophages that infect E. coli. TMV (Tobacco Mosaic Virus) belongs to Fraenkel-Conrat's experiment, which is a frequent NEET trap.
