68
UNIT ONE
DNA molecules have a unique ability to make copies of,
or replicate, themselves. They replicate prior to cell division,
and each newly formed cell receives an exact copy of the
original cell’s DNA molecules. Chapter 4 (p. 130) discusses
the storage of information in nucleic acid molecules, use of
the information to manufacture protein molecules, and how
these proteins control metabolic reactions.
Table 2.8
summarizes the four groups of organic com-
pounds.
Figure 2.23
shows three-dimensional (space-fill-
ing) models of some important molecules, illustrating their
shapes. From Science to Technology 2.3 describes two tech-
niques used to view human anatomy and physiology.
PRACTICE
20
Compare the chemical composition of carbohydrates, lipids,
proteins, and nucleic acids.
21
How does an enzyme aF
ect a chemical reaction?
22
What is likely to happen to a protein molecule
exposed to intense
heat or radiation?
23
What are the functions of DNA and RNA?
Nucleic Acids
Nucleic acids
(nu-kle
ik as
idz) carry the instructions that con-
trol a cell’s activities by encoding the amino acid sequences
of proteins. The very large and complex nucleic acids include
atoms of carbon, hydrogen, oxygen, nitrogen, and phospho-
rus, which form building blocks called
nucleotides
(nu
kle-o-
tıˉdz). Each nucleotide consists of a 5-carbon sugar (ribose or
deoxyribose), a phosphate group, and one of several nitrogen-
containing organic bases, called nitrogenous bases
(f g. 2.20)
.
Such nucleotides, in a chain, form a polynucleotide
(f g. 2.21)
.
There are two major types of nucleic acids.
RNA
(ribo-
nucleic acid) is composed of nucleotides that have ribose
sugar. RNA is a single polynucleotide chain. The second type
of nucleic acid,
DNA
(deoxyribonucleic acid), has deoxyri-
bose sugar. DNA is a double polynucleotide chain wound
into a double helix.
Figure 2.22
compares the structures of
ribose and deoxyribose, which differ by one oxygen atom.
DNA and RNA also differ in that DNA molecules store the
information for protein synthesis and RNA molecules use
this information to construct speci±
c protein molecules.
S
P
B
S
P
S
P
S
P
S
P
S
P
S
P
B
B
B
B
B
B
S
P
S
P
S
P
S
P
S
P
S
P
B
B
B
B
B
B
S
S
S
S
S
S
P
P
P
P
P
P
B
B
B
B
B
B
(a)
(b)
FIGURE 2.20
A nucleotide consists
of a 5-carbon sugar (S = sugar), a
phosphate group (P = phosphate), and a
nitrogenous base (B = base).
FIGURE 2.21
A schematic
representation of nucleic acid structure. A
nucleic acid molecule consists of (
a
) one
(RNA) or (
b
) two (DNA) polynucleotide
chains. DNA chains are held together by
hydrogen bonds (dotted lines) and they
twist, forming a double helix. That the
sugars of each chain point in opposite
directions aF
ects the way that the
information in genes is “read.” Chapter
4 discusses gene structure and function,
and chapter 24 covers heredity.
Ribose
Deoxyribose
OH
H
H
H
CC
OH
O
H
C
C
H
H
H
OH
H
CC
OH
O
H
C
OH
C
FIGURE 2.22
The molecules of ribose and deoxyribose diF
er by a
single oxygen atom.
Recall that water molecules are polar. Many larger molecules have
polar regions where nitrogen or oxygen bond with hydrogen. Such
molecules, including carbohydrates, proteins, and nucleic acids, dis-
solve easily in water. They are water soluble, or hydrophilic (“liking”
water). Molecules that do not have polar regions, such as triglycer-
ides and steroids, do not dissolve in water (“oil and water don’t mix”).
Such molecules do dissolve in lipid and are said to be lipophilic (“lik-
ing” lipid). Water solubility and lipid solubility are important factors in
drug delivery and in movements of substances throughout the body.
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