Hershey–Chase experiment

Hershey–Chase experiment. Conducted in 1952 by Alfred Hershey and his assistant Martha Chase at the Cold Spring Harbor Laboratory, this landmark study provided definitive evidence that DNA, not protein, is the genetic material of life. Using the bacteriophage T2 and a series of elegant radioactive labeling techniques, the experiment demonstrated that viral DNA enters a bacterial cell to direct the production of new viruses. The findings were a pivotal confirmation of earlier work by Oswald Avery and his colleagues, solidifying the central role of DNA in heredity and paving the way for the molecular biology revolution.

Background and context

By the early 20th century, scientists understood that chromosomes, composed of DNA and protein, carried hereditary information. However, the specific identity of the genetic material was hotly debated, with many favoring protein due to its greater chemical complexity. A crucial precursor was the 1944 Avery–MacLeod–McCarty experiment conducted at the Rockefeller Institute for Medical Research, which suggested DNA was the transforming principle in pneumococcus bacteria, but its conclusions were not universally accepted. Alfred Hershey, a member of the influential Phage Group centered around Max Delbrück and Salvador Luria, sought more direct proof using the simpler model system of bacteriophage viruses that infect E. coli. The intellectual environment of the Cold Spring Harbor Laboratory, a hub for genetics research, was instrumental in fostering this line of inquiry.

Experimental design

Hershey and Chase exploited the simple structure of the T2 bacteriophage, which consists of a protein coat surrounding a DNA core. They devised a method to differentially label these two components using radioactive isotopes. The phage's protein was labeled with radioactive sulfur-35 (³⁵S), which is incorporated into the amino acids cysteine and methionine but not into DNA. Conversely, the phage's DNA was labeled with radioactive phosphorus-32 (³²P), as phosphorus is a key component of the DNA backbone but absent from protein. They then allowed these labeled phages to infect cultures of E. coli bacteria. After infection, the mixture was subjected to vigorous blending in a Waring blender, a step designed to shear away any phage parts attached to the outside of the bacterial cells. Finally, the samples were centrifuged to separate the heavier bacterial cells from the lighter viral debris.

Results and interpretation

The centrifugation results were clear and decisive. In the batches with ³⁵S-labeled protein, most of the radioactivity was found in the supernatant, indicating the phage protein coats did not enter the bacterial cell. In contrast, in the batches with ³²P-labeled DNA, most of the radioactivity was found in the bacterial pellet. This demonstrated that the viral DNA entered the host cell during infection. Furthermore, the infected bacteria from the ³²P experiment went on to produce new, fully functional bacteriophage progeny. The conclusion was unambiguous: the genetic instructions for replicating the virus were carried by the injected DNA, not by the protein shell left outside. This provided direct physical evidence that DNA is the molecule responsible for heredity in this system.

Significance and legacy

The Hershey–Chase experiment is widely regarded as one of the most convincing and elegant proofs in the history of biology. It decisively resolved the debate over the chemical nature of genes, lending critical support to the earlier findings of Oswald Avery. This validation set the stage for the subsequent elucidation of the double helix structure of DNA by James Watson and Francis Crick in 1953, which relied on data from Rosalind Franklin and Maurice Wilkins. The work cemented the central dogma of molecular biology and directly fueled the rapid growth of molecular genetics. For their contributions, Alfred Hershey shared the 1969 Nobel Prize in Physiology or Medicine with Max Delbrück and Salvador Luria. The experiment remains a classic case study in experimental design within scientific education.

Criticisms and limitations

While overwhelmingly persuasive, the experiment had minor technical limitations noted by contemporaries. A small amount of ³⁵S-labeled protein did enter the bacterial cell, and a small amount of ³²P-labeled DNA was found in the supernatant, indicating the separation was not perfectly clean. Some scientists, such as Alfred Mirsky of the Carnegie Institution for Science, initially argued this left room for doubt, though these traces were later attributed to experimental noise or minor contamination. The experiment also did not itself explain the mechanism of DNA replication or gene expression; it solely identified DNA as the carrier of genetic information. Furthermore, its focus on bacteriophage left open the question of whether the same was universally true for all organisms, though subsequent work on tobacco mosaic virus and eukaryotic cells quickly confirmed DNA's primary role. Category:Molecular biology Category:Genetics experiments Category:1952 in science