Human Physiology, 14th edition (2016)

46 Chapter 2

knowledge of this rule, we could predict the base sequence
of one DNA strand if we knew the sequence of bases in the
complementary strand.
Although we can be certain which base is opposite a given
base in DNA, we cannot predict which bases will be above or
below that particular pair within a single polynucleotide chain.
Although there are only four bases, the number of possible
base sequences along a stretch of several thousand nucleotides
(the length of most genes) is almost infinite. To gain perspec-
tive, it is useful to realize that the total human genome (all of
the genes in a cell) consists of over 3 billion base pairs that
would extend over a meter if the DNA molecules were unrav-
eled and stretched out.
Yet even with this amazing variety of possible base se-
quences, almost all of the billions of copies of a particular gene
in a person are identical. The mechanisms by which identical
DNA copies are made and distributed to the daughter cells
when a cell divides will be described in chapter 3.

Ribonucleic Acid

DNA can direct the activities of the cell only by means of
another type of nucleic acid— RNA (ribonucleic acid). Like
DNA, RNA consists of long chains of nucleotides joined
together by sugar-phosphate bonds. Nucleotides in RNA, how-
ever, differ from those in DNA ( fig. 2.34 ) in three ways: (1) a
ribonucleotide contains the sugar ribose (instead of deoxy-
ribose), (2) the base uracil is found in place of thymine, and
(3) RNA is composed of a single polynucleotide strand (it is
not double-stranded like DNA).

There are three major types of RNA molecules that function

in the cytoplasm of cells: messenger RNA (mRNA), transfer RNA

(tRNA), and ribosomal RNA (rRNA). All three types are made

within the cell nucleus by using information contained in DNA

as a guide. The functions of RNA are described in chapter 3.

In addition to their participation in genetic regulation as

part of RNA, purine-containing nucleotides are used for other

Figure 2.34 Differences between the nucleotides

and sugars in DNA and RNA. DNA has deoxyribose and

thymine; RNA has ribose and uracil. The other three bases are

the same in DNA and RNA.

DNA nucleotides contain RNA nucleotides contain

Deoxyribose Ribose

Thymine Uracil

HOCH 2

CH 3

H

H H

H

H

H

H

N

N

OH

OH

O

O HOCH^2

instead

of

instead

of

H

H H

H

OH

OH

OH

O

O

H H

H

H

N

N

O

O

H

Figure 2.33 The double-helix structure of DNA. The two strands are held together by hydrogen bonds between
complementary bases in each strand.

C

AT

GC

G

AT

TA

TA

GC

AT

CG

A

CG

AT

G

A

C

T

G

G

C

C

Sugar-phosphate

backbone

Sugar-phosphate

backbone

Complementary

base pairing

Hydrogen

bond