FoundationalConceptsNeuroscience

cell’s metabolic machinery, and produced copies of itself. However,
in their experiment, Hershey and Chase did not allow the infection
process to go to completion. They allowed phage particles to attach
to the bacteria and let a few minutes elapse so that the infection
process could commence—time for the phage genes to be introduced
into the bacteria. Then, they agitated the cells just enough to dislodge
the virus particles from the surface of the bacteria but not enough to
break open the bacterial cells. The resulting solution would contain E.
coli bacterial cells (with viral genes inside) and whatever remained of
the T2 bacteriophage particles after their genes had been transferred
to the bacteria. Because bacterial cells and virus particles are very
different in their sizes and masses, and because they are all suspended
in a liquid solution, it is possible to separate them by rapid spinning in
a device called a centrifuge. Spinning in a centrifuge causes the larger,
heavier bacteria to settle into a pellet at the bottom of the tube, while
the smaller, lighter virus particles remain in the supernatant (the liq-
uid at the top of the tube).
Here’s what they found: Infection with phages containing ra-
dioactive sulfur led to the finding that the radioactivity remained in
the supernatant and thus stayed with the original phages after the
infection. Infection with phages containing radioactive phosphorus
led to the radioactivity being in the pellet, and thus it must have been
transferred to the bacteria during the infection. The interpretation is
simple and straightforward: it is viral DNA (containing phosphorus)
and not viral protein (containing sulfur) that is transferred from virus
to bacteria during infection. Whatever genes are, they are made of
DNA! Eight years after Avery’s seminal publication, there was now
another independent demonstration that everyone accepted and
believed.