Human Physiology, 14th edition (2016)

Chemical Composition of the Body 45

deoxyribose can be covalently bonded to one of four possi-

ble bases. These bases include the two purines ( guanine and

adenine ) and the two pyrimidines ( cytosine and thymine )

( fig. 2.32 ). There are thus four different types of nucleotides

that can be used to produce the long DNA chains. If you

remember that there are about 20 different amino acids used

to produce proteins, you can now understand why many scien-

tists were deceived into thinking that genes were composed of

proteins rather than nucleic acids.

When nucleotides combine to form a chain, the phos-

phate group of one condenses with the deoxyribose sugar

of another nucleotide. This forms a sugar-phosphate chain

as water is removed in dehydration synthesis. Because the

nitrogenous bases are attached to the sugar molecules, the

sugar-phosphate chain looks like a “backbone” from which

the bases project. Each of these bases can form hydrogen

bonds with other bases, which are in turn joined to a differ-

ent chain of nucleotides. Such hydrogen bonding between

bases thus produces a double-stranded DNA molecule; the

two strands are like a staircase, with the paired bases as steps

( fig. 2.33 ).

Actually, the two chains of DNA twist about each other

to form a double helix, so that the molecule resembles a spi-

ral staircase ( fig. 2.33 ). It has been shown that the number

of purine bases in DNA is equal to the number of pyrimi-

dine bases. The reason for this is explained by the law of

complementary base pairing: adenine can pair only with

thymine (through two hydrogen bonds), whereas guanine can

pair only with cytosine (through three hydrogen bonds). With

Figure 2.32 The four nitrogenous bases in
deoxyribonucleic acid (DNA). Notice that hydrogen
bonds can form between guanine and cytosine and
between thymine and adenine.

Phosphate

H

H

H H

H

H

H

N

N

N

N

N

H

H

H

H

H H

H H

H

H

N

NN

N

N

NN

N

N

N

O

O

O

O

O

O O

O

C C

C

C

C

C

C

C

CCC C C

CC

C C

C

C

CH 2

CH 2

H 2 C

H 2 C

Deoxyribose

Guanine Cytosine

Thymine Adenine

Figure 2.31 The structure of nucleic acids. The
components of a single nucleotide are shown above, and the
structure of a polynucleotide is shown below. The polynucleotide
was formed by dehydration reactions between nucleotides that join
the nucleotides together by sugar-phosphate bonds.

Phosphate

group

Five-carbon

sugar

Base

Guanine

Thymine

Cytosine

A Adenine

O

O

O

O

O

T

C

G

Bases

Nucleotide