This Week's Citation Classic

Hershey AD & Chase M. 1952. Independent functions of viral protein and nucleic acid in growth of bacteriophage. J. Gen. Physiol. 36: 39-56.

This week's citation classic is cited by practically every undergraduate genetics textbook, with good cause. The experiments they describe, the so-called Blender Experiments, are a marvel of elegance, simplicity and fundamental importance.

Until Alfred Hershey and Martha Chase, there was still debate regarding the nature of the genetic material. Was it DNA or protein? While Avery, MacLeod and McCarty's pneumococcal work ostensibly clinched the case for DNA, but this work was largely ignored and there remained some holdouts (isn't that always the case). Some, including double Nobel laureate Linus Pauling, considered DNA too simple a molecule to play such an important role as the genetic organizing principle for the entire organism. Hershey and Chase provided the final nail for the protein coffin.

Knowing that DNA, but not protein, possesses phosphorus, and protein, but not DNA, possesses sulfur, Hershey and Chase added either radioactive phosphorus or sulfur to cultures of T2 phages and E. coli, and later separated out the phages. Hershey and Chase now had two types of bacteriophages: one with a radioactive external protein coat, the other with highly radioactive DNA. These phages were added to fresh cultures, incubated for a short time, then whirled in a kitchen blender causing the phages to fall off the bacteria. The cultures were then separated using a centrifuge; the heavier bacterial cells fell to the bottom and formed a pellet, the lighter bacteriophages and loose phage parts remained in the liquid (called supernatant).

Hershey and Chase examined the pellet and the supernatant for radioactivity. In the cultures infected by bacteriophages with radioactive sulfur (with labeled protein), most of the radioactivity was in the liquid with the phages. In the cultures infected by bacteriophages with radioactive phosphorus (with most of the label in their DNA), most of the radioactivity was in the pellet of infected bacteria. Thus the radioactive DNA, but not the protein, entered the bacterial cells, clinching the case for DNA as the genetic material.

As further proof, Hershey and Chase found that offspring of the original viruses had radioactive DNA, but not radioactive protein, showing that the DNA was passed down from parent to offspring. It is fair to say that the scientific community, including Pauling, were now convinced of the fundamental nature of DNA.