section 23.2

Physical and Chemical Structure of DNA

525

(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

3'

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

complementary

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