HUMAN BIOLOGY

404 Chapter 21

n DNA is built of nucleotides—the subunits of the biological

molecules called nucleic acids. DNA nucleotides are

arranged to form a double helix.

n Links to Nucleic acids 2.13, Genes 19.1

Dna is built of four kinds of nucleotides

As you know from Chapter 18, a chromosome consists of a
DNA molecule and proteins. A DNA molecule, in turn, is
built from four kinds of nucleotides (Figure 21.1), the build-
ing blocks of nucleic acids introduced in Section 2.13.
A DNA nucleotide is built of a five-carbon sugar (deoxy-
ribose), a phosphate group, and one of these four nitrogen-
containing bases:

Many researchers were in the race to discover DNA’s
structure, but James Watson and Francis Crick were the first
to realize that DNA consists of two strands of nucleotides

twisted into a double helix. Nucleotides in a strand are

linked together, like boxcars in a train, by strong covalent

bonds. Weaker hydrogen bonds link the bases of one strand

with bases of the other. The two strands run in opposite

directions, as shown in Figure 21.2.

Chemical “rules” determine which

nucleotide bases in Dna can pair up

The bases in the four DNA nucleotides have different

shapes, and different sites where hydrogen bonds can

form. These factors determine which bases can pair up.

Adenine pairs with thymine, and guanine pairs with cyto-

sine. Therefore, two kinds of base pairs occur in DNA:

A—T and G—C. In a double-stranded DNA molecule, the

amount of adenine equals the amount of thymine, and the

amount of guanine equals the amount of cytosine.

While base pairs must form as we’ve just described—

A with T and G with C—the nucleotides can line up in any

order. For example, these are just three possibilities of the

pattern one might find in DNA:

Dna: a Double helix

F i g u r e 21.1 There are four kinds of nucleo tides in DNA. Here the five-carbon sugars are

orange. Each one has a phosphate group attached to its ring structure (on the left). Small

numerals on the structural formulas identify the carbon atoms where various parts of

the molecule are attached. The photograph shows James Watson (left) and Francis Crick,

who figured out the structure of DNA. (© Cengage Learning)

CAAT

GTTA

GCAC

CGTG

A

T

or

AAAA

TTTT

AAAA

TTTT

A

T

or

CCTT

GGAA

GGCC

CCGG

C

G

one

base

pair

A. Barrington Brown/Science Source

21.1

cytosine

C

base with a

single-ring

structure

thymine

T

base with a

single-ring

structure

O

OH

HC

NH 2

P

H

C

N

HC CO

N

HO O CH 2

O

O–

O

OH

C

O

P

H

C

NH

HC CO

N

HO O CH 2

O

O–

CH 3

4 '

3 ' 2 '

5 ' 1 '

4 '

3 ' 2 '

1 '

5 '

sugar

(deoxyribose)

adenine

A

base with a

double-ring

structure

guanine

G

base with a

double-ring

structure

O

OH

C

NH 2

P

H

C

N

C CH

N

HO O CH 2

O

O–

4 '

3 ' 2 '

1 '

5 '

HC

N

N

O

OH

C

O

P

H

C

NH

CC

N

HO O CH 2

O

O–

4 '

3 ' 2 '

5 ' 1 '

HC

N

N NH^2

adenine guanine thymine cytosine

A G T C

© Cengage Learning

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