Hershey and Chase Experiment
In 1953, Alfred Hershey and Martha Chase confirmed DNA's preeminent role in
genetics by demonstrating that DNA is the genetic material of a virus called
phage T2.
The phage, which infects E. coli, consists of a head, sheath, tail, and base plate
made of different proteins. DNA is packaged within the head of the virus.
When T2 comes in contact with E. coli, the phage attaches to the bacterium by
its tail. Next, the phage injects genetic material into the cell. The genetic material
directs bacterial enzymes to produce viral offspring.
When the life cycle is complete, 100 to 200 progeny phages have been
assembled inside each bacterium. The bacterium breaks open, or lyses, and the
phages are released
Since T2 consists of only DNA and protein, Hershey and Chase reasoned that
the genetic material must be one of the two components. So they designed an
experiment to determine which it is.
For their experiment, the scientists prepared T2 phages that either had
radioactive DNA or radioactive proteins.
To make the components radioactive, Hershey and Chase infected E. coli with
T2, and grew the bacterial cells in two different culture media -- one containing
the radioactive isotope phosphorous-32 and the other containing the radioactive
isotope sulfur-35.
In the next phase of their experiment, Hershey and Chase infected E. coli with
the two types of labeled T2. They then used a blender and a centrifuge to
separate the protein parts of the phage -- called the phage ghosts -- from the rest
of the infected cell.
The scientists discovered that the labeled DNA appeared in the host cell but not
the phage ghosts, while the labeled proteins appeared in the phage ghosts but
not the host cell. In addition, the progeny of the T2 with labeled DNA were
radioactive, while the progeny of the T2 with labeled proteins were not
radioactive.
Because genetic material is passed on from parent to offspring, Hershey and
Chase concluded that DNA had to be the genetic material of T2. Their results
helped convince the scientific community that DNA was the hereditary material.