Question

How did Alfred Hershey and Martha Chase arrive at the conclusion that DNA is the genetic material?

Answer 1

Hint: Remember the results of Griffith’s experiment? where he used two strains of Pneumococcus (a non-virulent “rough” strain and a virulent “smooth” strain) and injected them into mice. The rough strain on its own and the heat “killed” smooth strain didn’t infect the mice on injection while the untreated smooth strain did. However, when the mouse was injected with heat-killed “smooth” strain and rough strain the mice still died. This implied there was something in the heat-killed smooth strain that transformed the rough strain into the smooth strain.

Complete answer:

In the early 1900s, most scientists still believed proteins to be the carrier of genetic information rather than DNA since,
DNA was concentrated inside the nucleus (hence generally inaccessible) and under most conditions chemically inert. It was considered to be the storehouse for the cellular phosphorus.
DNA was also a relatively simple structure compared to the massively complex structure that is common in tertiary and quaternary proteins.
Proteins on the other hand were much more ubiquitous and had a seemingly much more significant role in the biochemistry of the organisms.
This was the case when Hershey and Chase undertook the series of experiments that would go to confirm and establish DNA as the carrier of genetic information once and for all.
In an elegantly defined experiment, the scientists studied bacterial cells infected by bacteriophages (bacteria-killing virus). A typical virus consists of a proteinaceous coat and DNA. To identify the potential genetic carrier the virus was used to infect bacteria in two distinct cultures-
A medium rich in radioactive Sulphur (S), Sulphur is an important component of many proteins and it was assumed that the virus growing in this culture would incorporate the radioactive sulfur into their protein coats.
A medium rich in radioactive phosphorus (P), since DNA is the only concentrated store of Phosphorus inside the Virus, any phosphorus would be only found in the virus DNA.
Bacteriophages from each of these media were isolated and purified and used to infect a fresh batch of bacterial cells. The infected cells were then centrifuged and lysed to release the viral copies.
Since the virus infects their hosts by using the host for making copies of their own, the researchers hypothesized that whichever radioactive isotope is detected in the contents of the lysed cells has to be the carrier of genetic information of the virus. While the cells infected with radioactive sulfur phages didn’t show the presence of any radioactive sulfur and the cells infected with radioactive phosphorus phages were found to contain traces of radioactive phosphorus in their viral copies.
Thus determining that it's the DNA and not the proteins that carry the genetic information.

Note: Since 1869, many scientists believed that proteins carry the information related to inheritance as DNA appeared to be an inert molecule since it is located inside the nucleus, it was assumed to be a storage house for phosphorus. Hershey and Chase proved DNA as the genetic material and got a Nobel Prize in 1969 for such an astounding discovery.