The big idea: A virus injects its genetic material into a bacterium. Hershey and Chase labelled the virus's protein and its DNA with different radioactive atoms, then looked to see which one went in.
The points to remember
- In the 1940s many scientists thought protein was the genetic material. Hershey and Chase tested protein or DNA.
- They used a bacteriophage (a virus of bacteria): just a protein coat with DNA inside.
- Protein contains sulfur but DNA does not; DNA contains phosphorus but protein has almost none.
- So they labelled the protein with radioactive sulfur-35 (³⁵S), and in a second batch the DNA with phosphorus-32 (³²P).
- After infection, a blender knocked the empty phage coats off the bacteria.
- A centrifuge spun the bacteria into a pellet; the light coats stayed in the liquid (the supernatant).
Remember it as: Sulfur for protein, phosphorus for DNA.
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The big idea: The protein label stayed outside the cells and the DNA label went in, so DNA carries the genes.
The points to remember
- ³⁵S stayed mostly in the supernatant: the protein coats stayed outside the bacteria.
- ³²P was mostly in the pellet: the DNA went inside the bacteria.
- The new phages made inside the bacteria contained some of the ³²P, and almost none of the ³⁵S.
- Whatever enters the cell must carry the instructions for new phages, so DNA is the genetic material.
Nature of science: a new tool, a new experiment: After the Second World War, radioactive isotopes made in nuclear reactors became available to biologists. Being able to label sulfur and phosphorus separately is what made this experiment possible.
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How it is asked: Paper 1A: the conclusion from a pellet and supernatant table. Paper 1B: a graph or table of results, then explain how it shows DNA is the genetic material.
IB-style questionExplain[3 marks]
The graph shows the percentage of ³⁵S and ³²P left outside bacteria after blending. Explain how these data provide evidence that DNA is the genetic material.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.