Double Helix

The double helix refers to the "spiral staircase" shape or structure of the deoxyribonucleic acid (DNA) molecule. DNA is the genetic material of all living organisms. Also described as a twisting ladder, this double helix model enabled scientists to finally account for both the similarities as well as the immense variety of life.

The discovery of the double helix molecular structure of DNA in 1953 by American biochemist James Dewey Watson and English biochemist Francis Harry Compton Crick was one of the major scientific events of the twentieth century and some would say in the history of the life sciences. Prior to this discovery, it was not understood how such a relatively simple nucleic acid as DNA could contain such a vast complex of hereditary material, and few, if any, believed that it did.

As a nucleic acid, DNA was known to be composed of only four different submolecules called nucleotides—adenine (A), guanine (G), thymine (T), and cytosine (C)—and Watson and Crick believed that if they could determine the structure of DNA, they could explain how DNA actually works. The major way of learning about the structure of a chemical is to crystallize it and x-ray the crystals. When x rays pass through the crystals, they bend or diffract and create a pattern that can be studied. Watson and Crick worked with English physicist Maurice H. F. Wilkins and his associate Rosalind Franklin, whose excellent x rays in

1951 provided important evidence that DNA had a spiral shape. Franklin's x rays showed that the DNA molecule was a double strand of twisted material which came to be called a double helix. (Helix is taken from the Greek word for spiral.)

Knowing this did not allow Watson and Crick to correctly describe the actual structure of a DNA molecule, and for some time they tried to build a model of what it might look like. Although, until they discovered that the four nucleotides always formed themselves into a definite pattern of pairs (A always pairs with T, and G always pairs with C), they were unable to make further progress. Once they knew that these "base pairs" were complementary (in other words they always paired up the same way), Watson and Crick designed and built a model in which the correctly paired bases were the "rungs" of a ladder that connected the two sides or "rails" of the ladder. These sides were then twisted in the shape of a compact spiral or coil. Watson and Crick also explained that the rungs on the ladder (called bases) were the coded instructions, and that the order of these four nucleotides (A,T,G,C) spelled out the instructions for all of the different characteristics of an organism.

In March 1953, the two scientists announced their discovery of the double helix structure of the DNA molecule, offering to science what was basically the explanation of the chemical basis of life itself. Unlike many discoveries in the life sciences, knowledge of the structure or shape (the double helix) of the DNA molecule was essential to explaining how it could carry all the information needed to make a living creature, as well as how it could make exact duplicates of itself. In 1962 Wilkins shared the Nobel Prize in Physiology or Medicine with Watson and Crick for their discovery. Rosalind Franklin would surely have been included as well, but she had died in 1958 and the prize is only given to living scientists.

[See alsoDNA ]