Classic experiments: DNA as the genetic material
Protein vs. DNA
Frederick Griffith: Bacterial transformation
- R strain. When grown in a petri dish, the R bacteria formed colonies, or clumps of related bacteria, that had well-defined edges and a rough appearance (hence the abbreviation "R"). The R bacteria were nonvirulent, meaning that they did not cause sickness when injected into a mouse.
- S strain. S bacteria formed colonies that were rounded and smooth (hence the abbreviation "S"). The smooth appearance was due to a polysaccharide, or sugar-based, coat produced by the bacteria. This coat protected the S bacteria from the mouse immune system, making them virulent (capable of causing disease). Mice injected with live S bacteria developed pneumonia and died.
- Rough strain (nonpathogenic). When this strain is injected into a mouse, the mouse lives.
- Smooth strain (pathogenic). When this strain is injected into a mouse, the mouse gets pneumonia and dies.
- Heat-killed smooth strain. When heat-killed smooth cells are injected into a mouse, the mouse lives.
- Rough strain & heat-killed smooth strain. When these two types of cells are injected into a mouse as a mixture, the mouse gets pneumonia and dies.
Avery, McCarty, and MacLeod: Identifying the transforming principle
- The purified substance gave a negative result in chemical tests known to detect proteins, but a strongly positive result in a chemical test known to detect DNA.
- The elemental composition of the purified transforming principle closely resembled DNA in its ratio of nitrogen and phosphorous.
- Protein- and RNA-degrading enzymes had little effect on the transforming principle, but enzymes able to degrade DNA eliminated the transforming activity.
The Hershey-Chase experiments
- One sample was produced in the presence of , a radioactive isotope of sulfur. Sulfur is found in many proteins and is absent from DNA, so only phage proteins were radioactively labeled by this treatment.
- The other sample was produced in the presence of , a radioactive isotope of phosphorous. Phosphorous is found in DNA and not in proteins, so only phage DNA (and not phage proteins) was radioactively labeled by this treatment.
- One batch of phage was labeled with 35S, which is incorporated into the protein coat. Another batch was labeled with 32P, which is incorporated into the DNA.
- Bacteria were infected with the phage.
- The cultures were blended and centrifuged to separate the phage from the bacteria.
- Radioactivity was measured in the pellet and liquid (supernatant) for each experiment. 32P was found in the pellet (inside the bacteria), while 35S was found in the supernatant (outside of the bacteria)