The Search for Genetic Material — Griffith, Avery, Hershey-Chase

NCERT grounding

NCERT Class 12 Biology, Chapter 5 (Molecular Basis of Inheritance), devotes Section 5.2, "The Search for Genetic Material", to this exact story. The text notes that although Friedrich Meischer discovered nuclein and Mendel proposed the principles of inheritance at almost the same time, "that the DNA acts as a genetic material took long to be discovered and proven." By 1926 the work of Mendel, Sutton, Morgan and others had narrowed the search to the chromosomes in the nucleus — but the question of which molecule was the genetic material remained open.

The chapter then presents the answer in three stages: Griffith's transforming principle (1928), its biochemical characterisation by Avery, MacLeod and McCarty (1933–44), and the unequivocal proof from Hershey and Chase (1952). The NIOS Biology supplement (Chapter 23) reinforces the same sequence, adding that Hershey and Chase used the T2 bacteriophage and labelled its protein coat with ³⁵S and its DNA with ³²P.

"The unequivocal proof that DNA is the genetic material came from the experiments of Alfred Hershey and Martha Chase (1952)." — NCERT Class 12 Biology, Section 5.2.1

The three experiments that found the gene

The discovery that DNA carries heredity was not a single flash of insight but a relay race run over a quarter-century. Each experiment answered a sharper question than the one before it. Griffith showed that something could be transferred between bacteria; Avery and his colleagues identified that something as DNA; and Hershey and Chase removed the last reasonable doubt. Understanding each step — including the controls that made the conclusions trustworthy — is the core skill NEET tests here.

Griffith's transformation experiment (1928)

Frederick Griffith worked with Streptococcus pneumoniae, the bacterium that causes pneumonia. On a culture plate this bacterium grows as two visibly different colony types. The S (smooth) strain has a mucous polysaccharide capsule, forms shiny colonies, and is virulent — mice injected with it die of pneumonia. The R (rough) strain lacks the capsule, forms rough colonies, and is avirulent — mice injected with it survive. The capsule protects the bacterium from the host's immune system, which is why it determines virulence.

Griffith could kill bacteria with heat. His experiment ran as a set of four injections into mice, and the fourth result was the surprise that launched molecular genetics.

The fourth outcome is the heart of the experiment. Heat-killed S alone is harmless. Live R alone is harmless. Yet the mixture of heat-killed S and live R killed the mice — and Griffith recovered living S bacteria from the dead animals. Because the R strain cannot make a capsule on its own, those living S cells could only have arisen if the R bacteria had been permanently changed. Griffith concluded that some "transforming principle" had passed from the dead S cells to the live R cells, enabling them to synthesise a smooth polysaccharide coat and become virulent. He recognised this as a transfer of genetic material, but his experiment could not reveal its chemical identity.

Two features make Griffith's design rigorous and exam-relevant. First, the heat-killed S and live R controls prove that the lethal effect is not simply leftover virulence — each component is confirmed harmless on its own. Second, the recovery of living S from the dead mouse shows the change is heritable: the transformed bacteria pass the smooth, virulent phenotype to their daughter cells. NCERT also draws a deeper lesson from this — the genetic material survived heat that killed the cell, an early hint that the hereditary molecule is chemically stable.

Avery, MacLeod and McCarty — naming the transforming principle

Griffith left a clear question: what is the transforming principle? Over 1933–44, Oswald Avery, Colin MacLeod and Maclyn McCarty set out to answer it biochemically. NCERT records the prevailing bias of the time plainly: "Prior to the work of Oswald Avery, Colin MacLeod and Maclyn McCarty, the genetic material was thought to be a protein." Their approach was direct — they purified the major classes of biochemicals (proteins, DNA, RNA and others) from heat-killed S cells and tested each, one at a time, for the ability to transform live R cells into S cells.

The decisive part of their work was a set of enzyme elimination tests. Rather than relying only on which purified fraction worked, they asked which enzyme could destroy the transforming activity. Treating the active S extract with a protein-digesting enzyme or an RNA-digesting enzyme did not stop transformation — so the principle was neither protein nor RNA. Only a DNA-digesting enzyme abolished it.

Avery, MacLeod and McCarty therefore concluded that DNA is the hereditary material. Their evidence was strong, but as NCERT notes, "not all biologists were convinced." The objection was technical: no biochemical purification is ever perfectly clean, so a sceptic could argue that a trace of protein contaminating the DNA preparation was the true transforming agent. The protein-as-genetic-material prejudice — driven by the belief that 20 amino acids offered more "information capacity" than four nucleotides — kept the debate alive. Settling it required an experiment with no purification step at all.

Hershey and Chase — the unequivocal proof (1952)

Alfred Hershey and Martha Chase delivered that experiment. They worked with bacteriophages — viruses that infect bacteria — using the T2 phage that infects Escherichia coli. A bacteriophage is structurally simple: it is essentially a protein coat enclosing a core of DNA. When a phage infects a bacterium, it attaches to the cell surface and injects its genetic material inside; the bacterium then treats that material as its own and manufactures hundreds of new virus particles. The question Hershey and Chase asked was sharp and answerable: which component of the phage — its protein or its DNA — actually enters the bacterium?

Their elegance lay in the labelling strategy. The two molecules differ in their atomic composition. DNA contains phosphorus in its sugar-phosphate backbone but contains no sulphur. Protein contains sulphur (in cysteine and methionine) but contains essentially no phosphorus. This complementary chemistry let the two molecules be tagged independently.

With two separate batches of labelled phage in hand, the experiment proceeded mechanically. Radioactive phages were allowed to attach to E. coli and begin infection. The mixture was then spun in a kitchen-style blender, whose shearing force stripped the empty viral protein coats off the bacterial surface. Finally the mixture was centrifuged: the heavier bacteria sedimented into a pellet at the bottom of the tube, while the lighter phage coats remained suspended in the supernatant. Measuring where the radioactivity ended up answered the question directly.

The result was clean and symmetric. Bacteria infected with phages carrying radioactive DNA were themselves radioactive — the DNA had passed from virus into bacterium. Bacteria infected with phages carrying radioactive protein were not radioactive — the protein had stayed outside, sheared off as empty "ghost" coats. Because the genetic material is by definition the part of the virus that enters and directs the production of new viruses, the conclusion was inescapable: DNA, not protein, is the genetic material passed from virus to bacteria.

What makes this the unequivocal proof — the word NEET repeatedly attaches to it — is that the experiment never relied on a biochemical purification. There was no DNA preparation to be contaminated by a trace of protein, the loophole that had dogged the Avery result. The separation was purely physical: the blender sheared, the centrifuge sorted by density, and the radioactive label simply reported where each molecule went. With no purification artefact possible, the protein-versus-DNA debate, in NCERT's words, "was unequivocally resolved."

Why earlier work pointed to protein

It is worth understanding why scientists resisted the DNA conclusion for so long, because NEET sometimes frames a statement question around it. Proteins are polymers of 20 different amino acids, which seemed to offer the combinatorial richness needed to encode the enormous variety of inherited traits. DNA, by contrast, was viewed as a chemically monotonous molecule of just four nucleotides — apparently too simple to be an information store. This "tetranucleotide" prejudice is why the Avery result met scepticism and why the field waited for Hershey and Chase. The eventual proof did not just identify a molecule; it overturned a deeply held assumption about what a genetic material had to look like.

Worked examples

Common confusion & NEET traps

This subtopic is a favourite source of matching and statement questions, and most errors come from confusing which scientist did what, or from mixing up the labelled molecules. The traps below address the patterns NEET exploits most often.