Читать книгу Cell Biology - Stephen R. Bolsover - Страница 61

In 1953 Rosalind Franklin used X‐ray diffraction to show that DNA was a helical (i.e. twisted) polymer. James Watson and Francis Crick demonstrated, by building three‐dimensional models, that the molecule is a double helix (Figure 3.4). Two hydrophilic sugar‐phosphate backbones lie on the outside of the molecule and the purine and pyrimidine bases lie on the inside of the molecule. There is just enough space for one purine and one pyrimidine in the center of the double helix. The Watson–Crick model showed that the purine guanine (G) would fit nicely with the pyrimidine cytosine (C). The purine adenine (A) would fit nicely with the pyrimidine thymine (T). Thus A always base pairs with T and G always base pairs with C.