Deoxyribonucleic acid (DNA) is the genetic material, but early on, most of science thought the genetic material was protein —that changed in 1952

Ok, it didn’t change overnight, because scientists are pretty stubborn.

It actually wasn’t accepted until the 60’s, but, in 1951, Alfred Hershey and Martha Chase began a series of experiments that conclusively showed that DNA was the genetic material.

There was already good evidence that the genetic material wasn’t protein, but early molecular biologists couldn’t wrap their heads around how 4 bases of DNA could code for all of the diversity we see, especially when proteins are made up of long chains of 20 amino acids.

Surely, since 20 is greater than 4, you get more diversity with 20!

So the hypothesis was that DNA must be the scaffold that holds everything together.

Of course, we know now that’s absolutely not true, and it’s basically the opposite - DNA is the genetic material and protein serves a support function.

The work that synthesized all of these ideas together was completed by Hershey and Chase in what are now referred to as the Hershey-Chase Experiments.

They worked with bacterial viruses called bacteriophages.

These are very simple viruses that consist of a protein shell surrounding the DNA of the virus.

But before the 1950’s, no one knew what phages looked like or how they infected bacteria.

In 1951, Thomas F. Anderson, a pioneer of electron microscopy, showed that phages are like little moon landers and attach themselves to the outside of the bacteria but do not enter them.

This created a bit of a conundrum, how can protein be the genetic material if none of the phage protein gets inside?

The answer to this question involved Hershey-Chase, a Waring blender and phages grown in the presence of radioactive food items.

Luckily, proteins do not contain Phosphorus (P) and DNA does not contain Sulfur (S), so radioactive P-32 and S-35 could be used to label each component of the bacteriophage individually.

Radioactive phages were then combined with bacteria and stuck in a blender.

Hershey and Chase discovered that this technique didn’t kill the bacteria, but it was perfect for shaking the empty phage husks from the bacteria.

This is documented in the figure below which shows 90% viability of the infected bacteria but that 80% of the protein (S35) remained external while only 30% of the DNA (P32) does after blending.

This result supported the electron microscopy observation that the phages are basically just little protein covered DNA injectors!

Hershey and Chase followed up by showing that 30% of the DNA that makes it into baby phages after the blender experiment was radiolabeled while <1% of the protein was, essentially proving that DNA, and not protein, is the heritable genetic material.