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662 Chapter 20 NEL

20.120.1 DNA Structure and Replication

According to the model proposed by Watson and Crick, DNA consists of two strands of

nucleotides. Each nucleotide contains a deoxyribose sugar, a phosphate group, and a

nitrogenous base, all covalently bonded to each other. Each strand of DNA has a back-

bone of sugar and phosphate groups (Figure 1). The nitrogenous bases stick out from the

backbone of each DNA strand.

Watson and Crick’s model also indicates that the two strands of DNA form a structure

that resembles a twisted ladder. The base pairs are the rungs of the ladder and the sugar–

phosphate backbones are the struts. This structure is called a double helix (see Figure 1).

Each DNA strand in the double helix twists in a clockwise direction.

complementary base pairing
pairing of the nitrogenous base of
one strand of DNA with the
nitrogenous base of another strand

H

H

N

CH 3

O

T

O

HH

H

N

N

N

N

A

NN

H

sugar

sugar

H

N

N

H

H

H

O

C

N

H

H

H

N

N

N

O

G

NN

H

sugar

sugar

Figure 2
Adenine forms two hydrogen bonds
with thymine, while guanine forms
three hydrogen bonds with
cytosine.

5’ end P

3’ end

P

P

P

P

P

P

P

S TA

GC

CG

AT

S

S

S

S

S

S

S

nitrogenous bases

one nucleotide sugar–phosphate backbone

nitrogenous bases

deoxyribose sugar molecule

a simplified

version of the

DNA molecule

showing the

double coil

or helix

phosphate molecule

Figure 1
A simplified diagram of the
structure of DNA

In the DNA molecule, the bases of one DNA strand are paired with bases in the other

strand. A purine is always paired with a pyrimidine. Adenine (a purine) is always paired

with thymine (a pyrimidine), and guanine (a purine) is always paired with cytosine (a

pyrimidine). This type of pairing is termed complementary base pairing.Hydrogen

bonds, between the complementary bases (A-T and G-C) on opposite strands, hold the

double helix together (Figure 2). Although a single hydrogen bond is very weak, large

numbers of hydrogen bonds are collectively strong, so the DNA molecule is very stable.

CAREER CONNECTION

Biological Technician

Biological technicians may work in

the field, in the laboratory, or both.

They perform routine analysis and

technical duties to support the

work of scientists and engineers

working in fields that include

molecular biology. What

educational background is

required to enter this field?

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