Molecular Basis of Inheritance · Lesson 3 of 13
Experiments That Identified DNA as Genetic Material
“Genes were known to reside on chromosomes before researchers identified their chemical substance. Chromosomes contain DNA and proteins, so either could initially have seemed a plausible carrier of inheritance. A decisive test must show”
• Distinguish observation, hypothesis and inference in genetic experiments. • Explain Griffith's transformation experiment and its limitations. • Interpret enzyme-treatment evidence from Avery and colleagues. • Explain the radioactive-labelling logic of the Hershey–Chase experiment. • Identify what the experiments establish about inheritance.
What needed to be demonstrated
Genes were known to reside on chromosomes before researchers identified their chemical substance. Chromosomes contain DNA and proteins, so either could initially have seemed a plausible carrier of inheritance. A decisive test must show which molecule can enter a recipient and cause a stable, heritable biological change. Researchers also need controls to distinguish a real transformation from the survival of cells already carrying the trait.
Griffith's bacterial transformation
Frederick Griffith studied two forms of Streptococcus pneumoniae. Smooth S colonies had a protective polysaccharide capsule and were virulent in his experimental mouse system; rough R colonies lacked that capsule and were comparatively nonvirulent. Live S bacteria caused disease, live R bacteria did not, and heat-killed S bacteria alone did not cause disease. Unexpectedly, a mixture of heat-killed S and living R bacteria produced disease and yielded living S bacteria. The dead S population must have supplied something that changed the hereditary behaviour of some live R bacteria.
Griffith called the phenomenon transformation but did not identify the chemical material. The result did not mean dead bacteria had come back to life: live R cells acquired genetic information enabling capsule production. This distinction matters because the experimental outcome establishes a heritable change, not the direct chemical identity of the transforming agent.
Avery, MacLeod and McCarty test candidate molecules
Avery and colleagues separated components from S bacteria and assessed whether treated material could still transform R cells. Proteases destroy proteins, RNases digest RNA and DNases digest DNA. Transformation persisted following protease or RNase treatment but was lost when DNA was destroyed with DNase. DNA, not an intact protein or RNA molecule, was therefore required for the observed transforming activity. The causal reasoning rests on comparing a control with otherwise similar samples exposed to different enzymes.
Enzyme names describe their substrates: DNA is the macromolecule, while DNase is an enzyme that cuts DNA. A positive transformation result after protein digestion is not by itself sufficient; the critical combination is preserved transformation after removing other candidates and loss of activity after specifically destroying DNA.
Hershey and Chase use labelled bacteriophages
In bacteriophages, a protein coat encloses the genetic material. Hershey and Chase grew one group of phages with radioactive phosphorus-32, which labels DNA's phosphate-containing backbone, and another with sulfur-35, which labels sulfur-containing phage proteins. The radiolabelled phages infected E. coli. After allowing attachment and injection, agitation separated most external phage coats from cells; centrifugation placed the bacterial cells in the pellet and detached coats largely in the supernatant.
Most phosphorus-labelled material was associated with infected bacteria, whereas most sulfur-labelled protein remained outside. Thus DNA was the major phage component delivered into cells and responsible for directing production of progeny phages. The experiments were not a claim that every protein molecule absolutely remained outside or that every type of virus uses DNA; they supported DNA as the genetic material in the phage system being tested.
How experimental evidence becomes an explanation
The three experiments form an evidence chain. Griffith demonstrated transferable hereditary change; Avery's team associated that transfer with purified DNA and DNA digestion; Hershey–Chase independently tracked DNA and protein during viral infection. Different methods constrained the possible explanations. Scientific inference becomes strongest when independent techniques converge on the same mechanism while ruling out alternative hypotheses.
What the conclusion does and does not mean
DNA is the hereditary material of cells and most viruses, but some viruses carry RNA genomes. Proteins remain indispensable for packaging, replication and expression of hereditary information. Identifying the information-bearing molecule does not imply that it operates alone; a complete biological mechanism requires coordinated actions of nucleic acids, proteins and cellular structures.
Experimental controls and alternative explanations
A strong transforming-factor experiment asks not only whether material produces a changed bacterial phenotype, but whether that activity persists when a candidate molecule is selectively removed. If RNase-treated material still transforms cells, RNA is not required for the activity under those conditions. If protease-treated material still transforms cells, an intact protein is not required. If DNase-treated material loses activity, DNA becomes the leading candidate. Each conclusion assumes the treatments worked as intended and that the cells were otherwise able to survive and respond. The comparison is stronger than a claim based merely on the material's abundance.
Griffith's result alone could not distinguish DNA, RNA, protein or another component as the hereditary carrier. Avery, MacLeod and McCarty's biochemical approach addressed this uncertainty. Hershey and Chase used a distinct physical tracing strategy: radioactive phosphorus enriched DNA, while radioactive sulfur enriched protein. Because the labelled substances were tracked after infection, the experiment asked what actually entered the bacterial cells rather than what a purified extract could do. The independent designs provided complementary types of evidence.
Interpreting the phage fractions correctly
After radioactive phages had attached to bacteria and delivered their genetic material, the mixture was agitated to remove many external phage coats. Centrifugation separated the relatively large bacteria, which formed the pellet, from much of the detached material in the supernatant. A phosphorus-labelled pellet supported the inference that phage DNA entered cells. A predominantly sulfur-labelled supernatant supported the inference that most phage coat protein remained external. These fractions are enriched, not perfectly pure; real experimental data can show residual label in either fraction.
Scientific conclusions must be constrained to the experiment's system. The phage result established the hereditary role of DNA in the DNA phage tested. It did not show that proteins are irrelevant to infection, because protein coats are essential for particle integrity and recognition. Nor did it exclude RNA genomes in other viruses. The distinction between a molecule that carries hereditary sequence and a molecule required for the process prevents an invalid either-or conclusion.
Quick Quiz
Quiz
What did Griffith's observations establish directly?
Why was DNase treatment decisive in the transformation studies?
What was the purpose of radioactive sulfur in phage experiments?
After Hershey–Chase centrifugation, where was much of the radioactive DNA detected?
Which result alone would not establish the chemical identity of the transforming factor?
Practice Problems
- Describe the complete logic of Griffith's four experimental conditions.
- Explain why comparing protease, RNase and DNase treatments is more informative than using only one enzyme.
- Predict the fraction in which sulfur-35 and phosphorus-32 would predominantly appear after blending infected phages with bacteria.
- A student claims that DNA must be the genome of every virus because of Hershey–Chase. Evaluate the claim.
Live S cells were virulent, establishing an S phenotype; live R cells were nonvirulent, providing a contrasting recipient; heat-killed S cells were nonvirulent, excluding survival of the original S cells; mixing heat-killed S with live R produced virulent living S cells. Therefore a transferable, heritable property of the dead S preparation changed some R cells.
Key Takeaways
• Griffith discovered a transferable hereditary transformation. • Avery and colleagues linked bacterial transformation specifically to DNA. • Radioactive phosphorus and sulfur distinguished phage DNA from protein. • Hershey–Chase demonstrated that the entering phage genetic material was DNA. • Independent experiments strengthen a causal explanation. • RNA genomes in some viruses do not contradict DNA heredity in cells.