Race for the Double Helix

Race for the Double Helix

The Watson-Crick model of DNA was a Nobel Prize winning structure. But what differed from this one to all of the other DNA structures that already existed? Well, Watson and Crick discovered the DNA was double-helical. Previously it was believed that DNA had a triple helix. Watson and Crick knew that the phosphates could not be located on the outside while the bases were on the inside.

The novel feature of their structure was that the two chains were held together by a purine and pyrimidine base. They are joined in pairs, a single base from one chain being hydrogen-bonded to a single base from the other chain, so that the two lie side by side with identical z-co-ordinates. Adenine (purine) is paired with thymine  (pyrimidine), and guanine (purine) is paired with cytosine (pyrimidine). Many times it will be written like: A-T, G-C.

As we already know, DNA consists of two strands, and each of those strands consist of a phosphate-sugar-phosphate backbone with bases (A, T, C or G) on the sugar molecules. The two strands run opposite of each other, with the bases pointing inwards and (as mentioned above) adenine is always paired with thymine, and guanine is always paired with cytosine. Therefore, if you know the sequence of one strand, then you know the sequence of the other strand. They are mirror images in reverse.