The Structure of DNA

The discovery of DNA structure by Watson and Crick would not have happened without data from Franklin and Chargaff
DNA Molecule. Getty/Pasieka

It was not always known that deoxyribonucleic acid (DNA) was the genetic material for all living things. In fact, this is a fairly recent discovery in the history of science. It was not until the mid-1900s that enough about DNA was known to build its structure.

DNA is the Genetic Material

Before it was proven that DNA was the material responsible for genetic inheritance, it was thought that proteins may have controlled the genetics of living things since there were so many different expressed traits.

DNA is a very simple molecule in its structure, so how could something so simple be responsible for so many differences in life on Earth? It was only logical to conclude that it were the numerous proteins that controlled genetics.

In 1952, Alfred Hershey and Martha Chase conducted a series of experiments that finally proved that it was DNA and not proteins that controlled inheritance of traits. The experiment used DNA with radioactively labeled phosphorus and proteins with radioactively labeled sulfur. A bacteriophage was used to infect regular bacteria and through the process of transformation incorporate its genetic material into the bacteria. The phosphorus tracer was found in the transformed bacteria, but there was no sulfur tracer found. Therefore, it was concluded that DNA had to be the genetic material, and not proteins.

Finding the Structure of DNA

Once DNA was proven to be the genetic material, the race was on to be the first to figure out its structure.

Even though DNA had been known to exist since the mid-1800s, it was not extensively researched because of its relatively simplicity. However, once Hershey and Chase published the results of their experiments, scientists across the globe turned their attention to figuring out more about the mysterious simple molecule that could create so much diversity in life on Earth.

A scientist named Erwin Chargaff was the first to figure out the rules of nitrogen base pairing that helped lead to the discovery of the double helix structure. DNA has a total of four, and only four, nitrogen bases. These four bases are named Adenine (A), Cytosine (C), Guanine (G), and Thymine (T). The structure of Adenine and Guanine are similar in that they both have two rings of carbons. These were designated as purines. Likewise, Thymine and Cytosine are alike in that they have only one ring of carbon. These bases were called pyrimidines.

Through some biochemical analyses of the DNA molecule, Chargaff determined that Adenine was present at nearly the same percentages as Thymine. He also noticed that the percentage of Guanine was extremely similar to the percentage of Cytosine. From this data, he created what would become known as the base pairing rules. A = T and C = G. He also noted that the percentages of each of the nitrogen bases differed from species to species. This explained how there could be so much diversity in life on Earth even though the DNA molecule was so simple.

James Watson and Francis Crick took particular interest in Chargaff's findings. Watson and Crick were pursuing the structure of DNA and this helped unlock the mystery of how the molecule was shaped.

Up until this point, it was thought that DNA was a single strand of the nitrogen bases. Watson and Crick interpreted Chargaff's rules to mean that instead of DNA being single stranded, it was actually double stranded.

Watson and Crick first proposed that DNA was shaped much like a ladder. The backbone of the DNA was the five carbon sugar called deoxyribose. This would make up the sides of the ladder. They also knew there were phosphate groups coming off of the backbone. The "rungs" of the ladder, they hypothesized, were made up of pairs of the nitrogen bases, either a rung composed of Adenine and Thymine or Cytosine and Guanine. It was very important that the DNA matched up one pyrimidine with one purine to keep the distance between the deoxyribose backbones constant at a total of three carbon rings between them.

However, the discovery of the structure of DNA did not end there. Watson and Crick collaborated with Rosalind Franklin who had been taking X-ray photographs of DNA molecules. Her pictures showed a sort of spiral shape to the molecule. Watson and Crick amended their model to be a sort of "twisted" ladder or even spiral staircase. This came to be known as the DNA double helix.