NEL Beyond Mendel 647
Proteins contain sulfur but no phosphorus, whereas DNA contains phosphorus but
no sulfur. Therefore, to track the location of DNA and proteins, Hershey and Chase
tagged the viral proteins with an isotopeof sulfur,^35 S, and the viral DNA with an iso-
tope of phosphorus,^32 P.^35 S and^32 P are radioisotopesof sulfur and phosphorus, respec-
tively. They are easy to track in an experiment because radioisotopes are unstable and the
radiation that they emit as they decay can be measured.
Each type of tagged bacteriophage was allowed to infect a separate batch of bacterial
host cells and to multiply. The bacterial cells were put into a blender to remove the pro-
tein coats of the viruses from the surfaces of the bacteria. The mixtures were then sub-
jected to centrifugation to isolate the individual components (bacteria as a pellet and viral
particles in the liquid). The bacterial cells that were exposed to viruses containing radioac-
tively labelled DNA contained^32 P. The bacterial cells that were exposed to viruses whose
protein coats were radioactively tagged with^35 S did not contain any radioactivity; instead,
the radioactive^35 S was found in the culture medium (Figure 8). These experiments
illustrate that phosphorus-rich DNA was injected into the bacterial cells. In addition,
Hershey and Chase found that the bacteriophages in both experiments reproduced and
destroyed the bacterial cells that they had infected. This observation further supported
the claim that DNA entering the host bacterial cell carries all the genetic information.
Hershey and Chase’s experiments ended the debate. DNA was accepted as the hereditary
material.
Section19.3bacterial cell infection^32 P inside cell radioactive phosphorus
found within cellsvirus particle
DNA tagged with radioactive^32 PExperiment 1bacterial cell infection^35 S outside cell radioactive sulfur
found outside cellvirus particle
protein tagged with radioactive^35 SExperiment 2Figure 8
Hershey and Chase’s experiment conclusively showed that DNA was the hereditary material.
isotopeone of two or more atoms
of the same element containing the
same number of protons but a
different number of neutronsradioisotopean unstable isotope
that decays spontaneously by
emitting radiation