Page 32 - Genetics_From_Genes_to_Genomes_6th_FULL_Part2

6.2 The Watson and Crick Double Helix Model of DNA 191

FEATURE FIGURE 6.11

The Double Helix Structure of DNA

(a) Watson and Crick took the known facts about DNA’s chemi-
cal composition and its physical arrangement in space and
constructed a wire-frame model that could explain the mol-
ecule’s function.

(b) In the model, two DNA chains spiral around an axis with
the sugar-phosphate backbones on the outside and flat
pairs of bases meeting in the middle. One chain runs 5′
to 3′ upward, while the other runs in the opposite direc-
tion of 5′ to 3′ downward. In short, the two chains are
antiparallel. The two chains wrap around each other
once every 10 base pairs, or once every 34 Å. The result
is a double helix that looks like a twisted ladder with the
two spiraling structural members composed of sugar-
phosphate backbones and the perpendicular rungs con-
sisting of base pairs.
(a)
(c) In a space-filling representation of © A. Barrington Brown/Science Source
the model, the overall shape is that (b) 3' 5' (c)
of a grooved cylinder with a diame-
ter of 20 Å. The backbones spiral
around the axis of the double helix
like threads on a screw. Because
two backbones exist, there are two
threads, and these two threads are
vertically displaced from each other. Major groove
This displacement of the backbones
generates two grooves, one (the
major groove) much wider than the
other (the minor groove). Axis of helix
The two chains of the double helix Sugar-phosphate
are held together by hydrogen backbone
bonds between complementary
base pairs, A–T and G–C. The spa-
tial requirements of the double helix
require that each base pair must
consist of one small pyrimidine and Minor groove Base pairs
one large purine, and even then, 34 Å
only for the particular pairings of A–T
and G–C. In contrast, A–C and G–T
pairs do not fit well and cannot easily
form hydrogen bonds. Although any
one nucleotide pair forms only two 3.4 Å
or three hydrogen bonds, the sum of
these connections between succes-
sive base pairs in a long DNA mole-
cule composed of thousands of
nucleotides is a key to the molecule’s Major groove
great chemical stability. Base pair

5' 3' Base pairs
20 Å Sugar-phosphate backbones