The Hershey-Chase Experiment: A woman, a man, and a blender Chasing down the makeup of genes.
Nowadays, many of us take for granted that DNA is the source of hereditary information, but this is actually a very recent discovery, that was quite controversial when announced in the 1950s. But how was the protein/DNA debate settled? With a blender and a woman named Martha Chase (you can learn more about her in this companion CSHL WiSE WiSE Wednesday piece).
Most scientists before the mid-1950s believed that genetic information was stored and transmitted through proteins, not nucleic acids (DNA and RNA). This was a sensible hypothesis because, unlike the 4-letter code of nucleic acids, proteins are made up of a larger (20-letter) alphabet of amino acids. And, while the 4 bases of DNA are all fairly similar, the 20 amino acids have very different chemical properties. Many scientists believed that this diversity made proteins the better candidate for genetic storage.
Not all scientists were convinced, however...
Early Evidence for "genes are DNA" camp: In 1928, Frederick Griffith showed that a “transforming principle” could be transferred from virulent to non-virulent bacteria, “transforming” the once harmless bacteria into killing machines. A group of scientists at the Rockefeller Institute (Oswald Avery, Colin MacLeod, & Maclyn McCarty), expanded upon the work and, in 1944, announced the results of an experiment that supported the “DNA camp.” They showed that enzymes that destroyed DNA inactivated the “transforming principle,” whereas chemicals that destroyed proteins but not DNA had no effect. Still, there was wide skepticism.
To settle the debate, Martha Chase and her colleague Alfred Hershey at Cold Spring Harbor Laboratory devised an elegant yet powerful experiment that has gone down as one of the greatest molecular biology experiments of all times, often referred to as the “Hershey-Chase experiment” or the “Waring blender experiment.” In addition to a blender (yep, one just like you might have in your kitchen), the experiment used a special type of virus, called a bacteriophage or “phage” for short, that infects bacteria.
It was known at the time that, when a phage infects a bacterium, it docks on the bacterium’s surface, then injects “stuff” into the bacterium, while leaving its “shell” outside (Fig. 1). This injected “stuff” contains genetic information that tells the bacterium to make more virus. The bacterium follows these instructions, makes more virus, then burst open (lyses), releasing these viruses to infect more bacteria. But what’s in this “stuff”? Proteins, nucleic acids, or both? Chase and Hershey knew that answering this question would help answer the larger question of what genetic information is made up of, so they devised a plan.
Key to the experiment was figuring out how to distinguish between nucleic acids and proteins. Biochemists can radioactively label the atoms of specific elements in biologically-produced molecules by including a radioactive isotope of that element in an organism’s growth media. When the organism makes new proteins and nucleic acids, it will incorporate the radioactive isotope into them, “labeling” them. Nucleic acids and amino acids both contain carbon, nitrogen, and oxygen, but nucleic acids also contain phosphorus while amino acids don’t and some amino acids contain sulfur, while no nucleic acids do. Therefore, radioactive phosphorus can be used to selectively label nucleic acids, while radioactive sulfur can be used to selectively label proteins.
Chase & Hershey grew bacteria in growth media containing either radioactive phosphorus (P-32) or radioactive sulfur (S-35) and infected these bacteria with a phage called T2. The bacteria in both media made more of the T2 phage, but the T2 produced by the bacteria in P-32 media had radiolabeled nucleic acids while the T2 produced by the bacteria in S-35 had radiolabeled proteins.
Chase & Hershey isolated these labeled viruses and used them to infect bacteria that were grown in normal media. They allowed the T2 to dock on the bacteria and inject the mysterious “stuff,” then, before the bacteria lysed, they used a blender to shear the “shells” of the T2 off of the bacterial cells and centrifuged the resultant mixture to separate the heavier bacteria (now containing the T2’s genetic “stuff”) from the lighter T2 “shells.” They then measured how much radioactivity was in each portion.
When planning an experiment, it’s important to think about what results you would expect in different cases, so let’s think about this for a minute. We have 2 experimental conditions, labeled protein and labeled DNA, and for each of these we’re comparing radioactivity in 2 populations (T2 “shells” and bacteria). We have 2 main hypotheses (genetic information is made up of protein versus nucleic acid). (Of course, there is also the possibility that the “stuff” contains both, but we’re going to ignore that here for simplicity).
|protein labeled||nucleic acid labeled|
|genetic info = protein||radioactivity in bacteria||radioactivity in T2
no radioactivity in bacteria
|genetic info = nucleic acid||radioactivity in T2
no radioactivity in bacteria
|radioactivity in bacteria|
So, what did they find? When they labeled the nucleic acid, almost all of the radioactivity was in the bacterial portion (Fig. 2a); whereas, when they labeled the protein, almost all of the radioactivity was in the T2 portion (Fig. 2b). This told them that the “stuff” being injected into the bacteria contained nucleic acids, but NOT proteins! And, since this “stuff” held the T2’s genetic information, this information is made up of nucleic acids, NOT proteins.
As we know now, DNA stores information in “words” of three consecutive bases (termed codons) that code for one amino acid, thus providing the diversity required for storing complex information. Furthermore, each nucleic acid base is complementary to another base, so the information can be easily copied and transmitted. These properties make nucleic acid ideal for the job (in fact, with the “big data” revolution generating enormous quantities of data, scientists are currently working on using synthetic DNA to store some of it!).
The experimental results weren’t exactly this cut-and-dry, but they played a large role in establishing DNA as the source of genetic information. The experiment won Hershey the Nobel Prize in 1969. Martha Chase was not included, and Hershey didn’t even acknowledge her contributions in his acceptance speech. I hope that this article helped you better understand and appreciate the elegance of this groundbreaking experience and I hope that when you think of the Hershey-Chase experiment you will think about Martha Chase!