Genetic Material Experiments
The three classic experiments — Griffith, Avery-MacLeod-McCarty, and Hershey-Chase — that together established DNA as the molecule of heredity.
It's easy to take for granted that DNA carries hereditary information — but scientists in the early twentieth century didn't know that. Proteins, with their far greater structural complexity, were actually the leading candidate for the genetic material for decades. Three classic experiments changed that, each closing a gap left open by the one before it.
Key Takeaways
Griffith's transformation experiment (1928) showed that a "transforming principle" from dead virulent bacteria could permanently convert live nonvirulent bacteria into virulent ones — without identifying what that substance was.
Avery, MacLeod, and McCarty (1944) identified DNA as the transforming principle by showing that only DNase treatment (not protease or RNase) blocked transformation.
Hershey and Chase (1952) used differential radioactive labeling (phosphorus-32 for DNA, sulfur-35 for protein) in bacteriophages to confirm that DNA, not protein, enters bacterial cells during infection and directs viral replication.
Together, these three experiments — each closing a gap left by the previous one — established DNA as the molecule of heredity.
How Do We Know DNA Is the Genetic Material?
MCAT Callout — Experiment Timeline: Griffith's transformation experiment (1928) showed that some substance could transfer heritable traits between bacteria, without identifying what that substance was. Avery, MacLeod, and McCarty (1944) identified that substance as DNA specifically. Hershey and Chase (1952) confirmed the conclusion using an entirely different experimental system — bacteriophages — closing out the case.
Griffith's Transformation Experiment (1928)
Frederick Griffith was studying Streptococcus pneumoniae, a bacterium that can cause pneumonia in mammals, using two strains: the S strain (S for smooth), which produces smooth colonies because of a protective polysaccharide capsule and is virulent — capable of causing disease and killing infected mice — and the R strain (R for rough), which lacks the capsule, produces rough colonies, and is nonvirulent.
Condition | Injected Into Mouse | Result |
|---|---|---|
Live R strain alone | Mouse | Lives (R strain is nonvirulent) |
Live S strain alone | Mouse | Dies (S strain is virulent) |
Heat-killed S strain alone | Mouse | Lives (heat destroyed virulence) |
Heat-killed S strain + live R strain (mixed) | Mouse | Dies — and live S strain bacteria are recovered from the body |
The fourth result was the surprising one: neither component alone should have caused death, since the live R strain was harmless and the heat-killed S strain had already lost its ability to cause disease. The only explanation was that some substance from the dead S cells had been taken up by the live R cells and converted them into virulent S cells.
The Transforming Principle
Griffith called this unidentified substance the transforming principle. He didn't know what it was chemically, but he'd demonstrated something fundamentally important: hereditary information could be transferred from one cell to another, permanently altering the recipient cell's characteristics. This introduced the concept of bacterial transformation — the uptake of genetic material from the environment — but it didn't yet prove that DNA specifically was the transforming principle. That question required a different experimental approach.
The Avery-MacLeod-McCarty Experiment (1944)
Oswald Avery, Colin MacLeod, and Maclyn McCarty set out to determine exactly which molecule was responsible for transformation. Rather than working with whole cells, they created purified extracts from virulent S strain bacteria and selectively destroyed specific types of macromolecules using targeted enzymes, testing whether transformation could still occur after each treatment.
Enzyme Treatment | What It Destroys | Transformation Still Occurs? |
|---|---|---|
Protease | Proteins | Yes |
Ribonuclease (RNase) | RNA | Yes |
Deoxyribonuclease (DNase) | DNA | No — R strain remains nonvirulent |
The result was decisive: the only condition that prevented transformation was the destruction of DNA. Avery, MacLeod, and McCarty concluded that DNA — not protein, not RNA — is the transforming principle: the molecule that carries hereditary information. Even so, many scientists at the time remained skeptical, since proteins' greater structural complexity made them seem like better candidates for genetic material. That skepticism set the stage for one more experiment.
The Hershey-Chase Experiment (1952)
Alfred Hershey and Martha Chase turned to bacteriophages — viruses that infect bacteria — to settle the question with an independent experimental system. A bacteriophage consists of two main components: a protein coat surrounding the outside, and DNA contained inside that coat. When a phage infects a bacterium, it attaches to the cell surface and injects its genetic material into the bacterial cell.
Labeling Strategy: Phosphorus-32 and Sulfur-35
The key idea was to label DNA and protein differently using radioactive isotopes. DNA was labeled with radioactive phosphorus (phosphorus-32), since DNA contains phosphorus in its phosphate backbone but no sulfur. Protein was labeled with radioactive sulfur (sulfur-35), since many proteins contain sulfur in some of their amino acids but no phosphorus in that same structural context. Whichever molecule actually entered the bacterial cell during infection would carry its radioactive label along with it.
Experiment | Labeled Molecule | After Infection, Blending, and Centrifugation | Conclusion |
|---|---|---|---|
Sulfur-35 experiment | Protein coat | Radioactivity found in the supernatant (lighter viral coats) | Protein did not enter the bacterial cell |
Phosphorus-32 experiment | DNA | Radioactivity found in the pellet (heavier bacterial cells) | DNA did enter the bacterial cell |
In each case, phages were grown with the radioactive isotope, allowed to infect bacteria, then blended to physically separate the empty phage protein coats from the bacterial cells, and finally centrifuged to separate the heavier bacterial cells (forming a pellet) from the lighter viral coats (remaining in the supernatant). The sulfur-35 experiment showed radioactivity stayed in the supernatant — protein never entered the cell. The phosphorus-32 experiment showed radioactivity ended up in the pellet — DNA did enter the cell. Since DNA, not protein, enters the bacterial cell and directs the production of new viruses, DNA must be the genetic material.
Common MCAT Mistakes
Confusing what Griffith's experiment actually proved. Griffith showed that a transforming principle exists and can convert bacteria — he did not identify DNA as that substance. Attributing the DNA identification to Griffith instead of Avery, MacLeod, and McCarty is a common mix-up.
Forgetting which enzyme targets which molecule in the Avery-MacLeod-McCarty experiment. Protease destroys protein, RNase destroys RNA, DNase destroys DNA — only the DNase condition blocked transformation, which is what pins the transforming principle down to DNA specifically.
Mixing up phosphorus-32 and sulfur-35 in the Hershey-Chase experiment. Phosphorus-32 labels DNA (via the phosphate backbone); sulfur-35 labels protein (via sulfur-containing amino acids). Swapping these flips the entire logic of the experiment.
Misremembering pellet vs. supernatant. The pellet contains the heavier bacterial cells; the supernatant contains the lighter, empty phage protein coats. Radioactivity in the pellet means that labeled molecule entered the cell — that's the phosphorus-32 (DNA) result, not the sulfur-35 (protein) result.
MCAT-Style Concept Check
Question: In the Hershey-Chase experiment, after blending and centrifugation, most of the radioactivity from phosphorus-32-labeled phages was found in the pellet, while most of the radioactivity from sulfur-35-labeled phages was found in the supernatant. What does this pattern indicate?
A) Protein enters the bacterial cell and directs viral replication, while DNA remains outside
B) DNA enters the bacterial cell during infection, while the protein coat remains outside and separates into the supernatant
C) Both DNA and protein enter the bacterial cell in equal amounts
D) The radioactive labeling failed to distinguish DNA from protein
Answer: B
Explanation: The pellet contains the heavier bacterial cells, and the supernatant contains the lighter, empty phage protein coats. Because phosphorus-32 (which labels DNA) ended up in the pellet, DNA must have entered the bacterial cells. Because sulfur-35 (which labels protein) stayed in the supernatant, the protein coat never entered the cells and was discarded along with the empty phage shells. Option A reverses the correct conclusion. Option C is incorrect because the two labels showed opposite, not equal, distributions. Option D is incorrect — the clean separation of the two labels is exactly what allowed the experiment to distinguish DNA from protein.
FAQ
What did Griffith's transformation experiment show?
Griffith's experiment showed that a "transforming principle" from heat-killed virulent S strain bacteria could permanently convert live nonvirulent R strain bacteria into virulent S strain bacteria — establishing that hereditary information could pass between cells, without identifying what chemical substance was responsible.
What did the Avery-MacLeod-McCarty experiment add that Griffith's didn't?
Avery, MacLeod, and McCarty identified the transforming principle chemically as DNA. By selectively destroying proteins, RNA, or DNA in purified bacterial extracts, they showed that only destroying DNA (with DNase) blocked transformation — pinning the effect on DNA specifically.
Why did Hershey and Chase use phosphorus-32 and sulfur-35 specifically?
DNA contains phosphorus in its phosphate backbone but no sulfur, while many proteins contain sulfur in certain amino acids but no phosphorus in that same structural context. Labeling DNA with phosphorus-32 and protein with sulfur-35 let Hershey and Chase track exactly which molecule entered bacterial cells during phage infection.
Why was the Hershey-Chase experiment needed if Avery-MacLeod-McCarty had already identified DNA?
Many scientists remained skeptical of the Avery-MacLeod-McCarty conclusion because proteins' greater structural complexity made them seem like more plausible carriers of genetic information. Hershey and Chase confirmed the same conclusion using an entirely independent experimental system — bacteriophages instead of bacterial transformation — which closed out the remaining doubt.
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