any sulfur atoms. Conversely, DNA contains phosphorus atoms and
proteins do not. Thus, phages grown with radioactive sulfur would
have radioactive protein, and phages grown with radioactive phos-
phorus would have radioactive DNA.
Figure 4.3. Drawing of a bacteriophage virus (left) and electron micrograph of
bacteriophages attached to a bacterial cell during the infection process (right).
The size of a bacteriophage virus is approximately 0.1 micrometer (100
nanometers), too small to be seen with light microscopy. The outside shell of
the bacteriophage is made of protein, and its “head” portion has a strand of
DNA packed inside.
Hershey and Chase then allowed the phages to infect E. coli bacte-
ria, a process that can be accomplished in a liquid nutrient solution.
In such an infection, a phage T2 attaches to the surface of a bacterium
(Fig. 4.3, right). Less than thirty minutes later the bacterial cell
bursts open, and several hundred newly formed phage particles are
released. The phage that originally attached to the bacterial cell has
presumably injected its genes into the bacterium, commandeered the