Figure 2–38. Portions of the polynucleotide chains of DNA (A) and RNA (B).
The Watson and Crick Model
The authors of the model — James Watson and Francis Crick (1953 a)
have established that the DNA molecule is a double helix consisting of
two complementary chains built from four bases — adenine (A), guanine
(G), thymine (T) and cytosine (C). The bases are situated inside the helix
linked by hydrogen bonds, and the sugar-phosphate skeletons are
outside forming the frame. At that, adenine is always bound to thymine
(A—T) and guanine to cytosine (G—C). The bound bases lie on planes
perpendicular to a helix axis and are located one under the other at a
distance of 0.34 nm. A full turn is accomplished by 10 pairs of bases
forming a step of 3.4 nm (Fig. 2–39).
This model has fully satisfied the requirements posed by the then
existing data from the X-ray diffraction studies. Besides it revealed the
possibility of self-replication, since the DNA molecule comprises two
complementary chains which can be uncoiled and each of the initial chains
may serve as a template for the synthesis of a new complementary chain
(Watson, Crick, 1953 b).
The biological essence born by this discovery was so great that it
has brought its authors immediate recognition and gave a strong impetus
to the development of biology, molecular biology and genetics in
particular. Proteins regarded as eventual bearers of heredity until then