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Viral infection experiments confirmed that DNA is the genetic material

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Even with the bacterial transformation experiments, many biologists were still not convinced that DNA is the genetic material. One problem was that DNA, being made up of only four nucleotides (see Key Concept 4.1), seemed too uniform a substance to be able to confer instructions for all the functions and variety of life. The possibility remained that proteins, with all their chemical and structural diversity (20 amino acids), fulfilled that role. Experiments with a virus were designed to distinguish between these alternatives.

Alfred Hershey and Martha Chase of the Cold Spring Harbor Laboratory in New York studied bacteriophage T2 (phage T2), which infects the bacterium Escherichia coli. T2 phage consists of a DNA core packed inside a protein coat (Figure 13.3). When it attacks a bacterium, part (but not all) of the virus enters the bacterial cell. About 20 minutes later, the cell bursts, releasing dozens of particles that are virtually identical to the infecting virus particle. Clearly the virus is somehow able to hijack the host cell’s molecular machinery and convert it into a viral replication machine. Hershey and Chase set out to determine what part of the virus—DNA or protein—enters the host cell to bring about this genetic change. To trace the two components of the virus over its life cycle, the scientists labeled each component with a specific radioisotope:

Proteins were labeled with radioactive sulfur. Proteins contain some sulfur (in the amino acids cysteine and methionine), but DNA does not. Sulfur has a radioactive isotope, ³⁵S. Hershey and Chase grew bacteriophage T2 in a bacterial culture in the presence of ³⁵S, so the proteins of the resulting viruses were labeled with (contained) the radioisotope.
DNA was labeled with radioactive phosphorus. DNA contains a lot of phosphorus (in the deoxyribose–phosphate backbone—see Figure 4.4), whereas proteins contain little or none. Phosphorus also has a radioisotope, ³²P. The researchers grew another batch of T2 in a bacterial culture in the presence of ³²P, thus labeling the viral DNA with ³²P.

Figure 13.3 Bacteriophage T2: Reproduction Cycle Bacteriophage T2 is parasitic on E. coli, dependent on the bacterium to produce new viruses. The external structures of bacteriophage T2 consist entirely of protein, and the DNA is contained within the protein coat. When the virus infects an E. coli cell, its genetic material is injected into the host bacterium.

Hershey and Chase used these radioactively labeled viruses in their experiments (Figure 13.4). In one experiment, they allowed ³²P-labeled bacteriophage to infect bacteria; in the other, the bacteria were infected with ³⁵S-labeled bacteriophage. After a few minutes they agitated each mixture vigorously in a kitchen blender, stripping away the parts of the viruses that had not penetrated the bacteria, without bursting the bacteria. Then they separated the bacteria from the rest of the material (the remains of the viruses) from each experiment individually in a centrifuge. The result was that the bacterial cells in the centrifuge pellet contained most of the ³²P (and thus the viral DNA), and the supernatant fluid with the viral remains contained most of the ³⁵S (and thus the viral protein). These results indicated that it was the DNA that had been transferred into the bacteria, and that DNA was the molecule responsible for redirecting the genetic program of the bacterial cell.

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experiment

Figure 13.4 The Hershey–Chase Experiment

Original Paper: Hershey, A. D. and M. Chase. 1952. Independent functions of viral protein and nucleic acid in growth of bacteriophage. The Journal of General Physiology 36: 39–56.

When Hershey and Chase infected bacterial cells with radioactively labeled T2 bacteriophage, only labeled DNA was found in the bacteria. The infected cells were agitated to remove the viral coats from the bacteria and were then centrifuged to pellet the bacteria. The labeled protein remained in the supernatant. This showed that DNA, not protein, is the genetic material.

A work with the data exercise that accompanies this figure may be assigned in LaunchPad.

Animation 13.1 The Hershey–Chase Experiment

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