section 23.2
Physical and Chemical Structure of DNA
(a) Schematic diagram indicating the dimensions and helical structure of double-stranded DNA. (b) The structure of
B-DNA showing the orientation of the atoms and the major and minor grooves in DNA. [From R. E. Dickerson,
Sci. Am.
December 1983, pp. 100-104; Copyright ©1983 by Scientific American, Inc. All rights reserved. Illustration
C. Irving Geis.]
1. The sugar-phosphate backbones of the double helix
follow helical paths at the outer edge of the molecule.
The sugar-phosphate strands are oriented in
antiparallel directions such that the 5'-phosphate end
of one strand is opposite the 3'-OH end of its partner.
As a result of the antiparallel orientation, if one reads
a sequence of bases in a 5' to
direction on one
strand, one is reading the complementary bases on the
other strand in a 3' to 5' direction (Figure 23-6a). The
overall width of the double helix is 2.0 nm.
2. The two strands in the double helix are
because of the base pairing rules that
dictate A = T and G = C. Because of the base
pairing rules, the information in DNA is redundant;
knowing the sequence of bases in one strand dictates
the sequence of bases in the opposite strand.
3. Each base pair in the double helix lies in a plane
that is perpendicular to the axis of the helix
(Figure 23-6b). Adjacent pairs of bases in DNA are
separated by 0.34 nm and rotated with respect to one
another so that 10 base pairs occupy each turn of the
helix, which repeats every 3.4 nm.
4. Space filling models of DNA reveal two grooves that
run the length of the molecule (Figure 23-6b). The
major groove
is wide and deep and the
minor groove
is narrow and shallow. These grooves in DNA provide
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