Genetic Material Experiments
High-Yield Summary
- Three classic experiments, each closing a gap left by the one before: Griffith (1928) showed SOMETHING transfers heritable traits; Avery-MacLeod-McCarty (1944) identified that something as DNA; Hershey-Chase (1952) confirmed it with an independent system (phages).
- Griffith: heat-killed virulent S strain + live avirulent R strain → mouse DIES, live S strain recovered. Neither alone kills. Conclusion: a 'transforming principle' from dead S cells converted live R cells to virulent S cells.
- Avery-MacLeod-McCarty: purified S-strain extracts treated with protease (transformation still occurs), RNase (still occurs), DNase (transformation BLOCKED). Only destroying DNA stopped transformation → DNA is the transforming principle.
- Hershey-Chase: bacteriophages labeled with P-32 (DNA, no sulfur) or S-35 (protein, no phosphorus). After infection + blending + centrifugation: S-35 stayed in supernatant (protein never entered cell); P-32 ended up in pellet (DNA entered the cell) → DNA is the genetic material.
Avery-MacLeod-McCarty: Enzyme Treatments
| Enzyme (Destroys) | Transformation Still Occurs? |
|---|---|
| Protease (proteins) | Yes |
| RNase (RNA) | Yes |
| DNase (DNA) | No — the only one that blocks transformation |
Hershey-Chase: Labeling Results
| Labeled Molecule | Where Radioactivity Ends Up |
|---|---|
| S-35 → protein coat | Supernatant (lighter phage coats) — protein did NOT enter the cell |
| P-32 → DNA | Pellet (heavier bacterial cells) — DNA DID enter the cell |
Key Terms
- Transforming principle
- Griffith's name for the unidentified substance that transferred virulence between bacteria — later identified as DNA.
- Bacterial transformation
- Uptake of genetic material from the environment by a bacterial cell.
- Bacteriophage
- A virus that infects bacteria; consists of a protein coat and DNA.
The Experimental Timeline
- 1Griffith (1928) — proves some transferable substance exists, doesn't identify it chemically
- 2Avery, MacLeod, McCarty (1944) — enzyme digestion pinpoints DNA specifically as the transforming principle
- 3Hershey-Chase (1952) — radioactive isotope labeling in phages independently confirms DNA (not protein) enters cells and directs replication
Common MCAT Trap
- Griffith's experiment alone does NOT identify DNA — it only shows something is transferable. Don't credit Griffith with proving DNA is the genetic material; that's Avery-MacLeod-McCarty.
- In Hershey-Chase, phosphorus-32 labels DNA (phosphate backbone) and sulfur-35 labels protein (sulfur-containing amino acids) — mixing these up flips the entire logic of the result.
- The pellet = heavier bacterial cells after centrifugation; the supernatant = lighter phage coats. Radioactivity in the pellet means that molecule entered the bacteria.
Quick Recall
In Griffith's experiment, why did the mouse die from heat-killed S strain + live R strain when neither alone was lethal?
Which enzyme treatment in the Avery-MacLeod-McCarty experiment blocked transformation, and what does that prove?
Why did Hershey and Chase use two different radioactive isotopes?