Organic Chemistry

046 CHAPTER 27 Nucleosides, Nucleotides, and Nucleic Acids

a.

b.

c.

Figure 27.7

(a) The DNA double helix. (b) View looking down the long axis of the helix. (c) The bases

are planar and parallel on the inside of the helix.

CG

O

O

O

O

O

O

AT

O

−OOP

O

OOP −

5 ′

3 ′

3 ′

5 ′

O O

O O

Figure 27.6

The sugar–phosphate backbone of DNA is on the outside, and the bases are on the inside,

with A’s pairing with T’s and G’s pairing with C’s. The two strands are antiparallel—they

run in opposite directions.

bonds that hold the bases together are all about the same length

Å).

The two DNA strands are antiparallel—they run in opposite directions, with the

sugar–phosphate backbone on the outside and the bases on the inside (Figures 27.4

and 27.6). By convention, the sequence of bases in a polynucleotide is written in the

direction (the -end is on the left).

The DNA strands are not linear but are twisted into a helix around a common axis

(see Figure 27.7a). The base pairs are planar and parallel to each other on the inside

of the helix (Figures 27.7b and c). The secondary structure is therefore known as a

5 ′-end 3 ′-end

ATGAGCCATGTAGCCTAATCGGC

5 ¿ ¡ 3 ¿ 5 ¿

(2.9;0.1

N¬H  O