A bacteriophage virus infects a bacterium and harnesses its DNA (deoxyribonucleic acid) to make numerous replications of itself. These replicants go on to infect other bacteria.
Even in 1951, is spite of the work of Oswald Avery and his colleagues (see here), it was still widely thought that genetic information was carried by proteins, not by DNA. But then came the experiment that persuaded even the doubters that DNA is ‘the’ life molecule.
The experimental steps along the road that led to an understanding of DNA used a succession of smaller and more rapidly reproducing organisms. Gregor Mendel worked with peas; Thomas Hunt Morgan worked with fruit flies; and Avery’s team worked with bacteria. The final step was taken using the smallest entities that carry genetic material: viruses.
Viruses are little more than bags of protein, far smaller than a bacterium, filled with genetic material. When a virus attacks a cell, it injects this genetic material into the cell, where it hijacks the machinery of the cell and uses it to make copies of the virus out of the chemical material inside the cell. Then, the cell bursts open, releasing the copies of the virus to repeat the process. At the beginning of the 1950s, Alfred Hershey and Martha Chase, working at the Cold Spring Harbor Laboratory, developed a neat experiment which showed definitively that it was DNA that carried these instructions into the cell being attacked.
They worked with viruses known as bacteriophage (sometimes shortened to phage; from the Greek -phagos, to devour), because they ‘eat’ bacteria. The idea behind the experiment was based on growing phage in a medium that contained radioactive isotopes of either phosphorus (phosphorus-32) or sulphur (sulphur-35), then using the radioactive phage to attack a colony of non-radioactive bacteria. The sulphur and phosphorus isotopes have different and distinctive radioactive signatures, and they can be traced through the cycle of infection and reproduction from one generation of phage to the next. Bacteria were grown in colonies laced with one or other of the radioactive isotopes before phage was added. The phages in the next generation took up the radioactive material and were then used to infect non-radioactive bacteria. The point of all this is that phosphorus is present in DNA, but not in protein, while sulphur is present in protein but not in DNA. Everywhere the team detected phosphorus they would be tracing the path of DNA, while everywhere they detected sulphur they would be tracing the path of protein.
The snag was, after the radioactive phage had done their work in the culture of bacteria, what was left was a mass of cells filled to bursting point with new viruses but with discarded phage husks still attached to the hijacked bacterial cells. The culture still contained both kinds of radioactive isotope. Somehow, Hershey and Chase had to separate out the leftover debris from the original generation of phage from the new viruses manufactured inside the bacteria. The difficulty was solved when a colleague lent them an ordinary kitchen utensil known as a Waring Blender.
On a low setting, the blender provided just enough agitation to shake the empty phage bags loose from the cells they had infected, without smoothing everything out into an amorphous goo. After this agitation, the mixture was whirled in a centrifuge, where the bacterial cells, full of new virus, fell to the bottom and could be extracted, while the husks of the old phage were left behind. When the two components were analysed, the results were persuasive. Radioactively labelled DNA was found in the cells (in other words, in the new generation of viruses), while radioactively labelled protein was found in the leftover husks. It was DNA, not protein, that was passed from one generation to the next, although in the scientific paper announcing their results the team cautiously concluded only that ‘this protein probably has no function in the growth of intracellular phage. The DNA has some function’.
The success of this superficially simple experiment owed a great deal to the expertise of Martha Chase, although officially she was ‘only’ the assistant to Alfred Hershey. Waclaw Szybalski, another Cold Spring Harbor biologist, later recalled: ‘Experimentally, she contributed very much. The laboratory of Alfred Hershey was very unusual. At that time there were just the two of them, and when you entered the laboratory there was absolute silence and just Al directing the experiments by pointing with his finger to Martha, always with a minimum of words. She was perfectly fitted to work with Hershey.’45
It was now clear that the protein provided the structural material in the bacteriophage, while the DNA carried the genetic information. The results were published in 1952, and the work became known as ‘the Waring Blender experiment’. After this, hardly any biologists believed that the genetic material could be anything but DNA, and the stage was set for the investigations that would reveal the structure of DNA itself.