Essential Cell Biology 5th edition

58 CHAPTER 2 Chemical Components of Cells
_ O
5′ end
5′ CH 2 O
4′
3′
O
_ O
P
O
_ O
P
O
O
CH 2
_ O
O
P
O
2′
O
O
CH 2
N
N
O
H 3 C
O
P
O
O
1′
O
N
N
5′ CH 2 O
4′
3′
O
3′ end
NH
Figure 2–28 A short length of one
chain of a deoxyribonucleic acid
(DNA) molecule shows the covalent
phosphodiester bonds linking four
consecutive nucleotides. Because the
bonds link specific carbon atoms in the
sugar ECB5 ring—known e2.26/2.28 as the 5ʹ and 3ʹ carbon
atoms—one end of a polynucleotide chain,
the 5ʹ end, has a free phosphate group and
the other, the 3ʹ end, has a free hydroxyl
group. One of the nucleotides, T, is shaded
in gray, and one phosphodiester bond is
highlighted in yellow. The linear sequence
of nucleotides in a polynucleotide chain is
commonly abbreviated using a one-letter
code, and the sequence is always read from
the 5ʹ end. In the example illustrated, the
sequence is GATC.
O
N
2′
O
N
NH
NH 2
N
N
NH 2
O
NH 2
1′
N
N
O
G
A
T
C
Nucleotides also have a fundamental role in the storage and retrieval of
biological information. They serve as building blocks for the construction
of nucleic acids—long polymers in which nucleotide subunits are
linked by the formation of covalent phosphodiester bonds between the
phosphate group attached to the sugar of one nucleotide and a hydroxyl
group on the sugar of the next nucleotide (Figure 2–28). Nucleic acid
chains are synthesized from energy-rich nucleoside triphosphates by
a condensation reaction that releases inorganic pyrophosphate during
phosphodiester bond formation (see Panel 2–7, pp. 78–79).
There are two main types of nucleic acids, which differ in the type of sugar
contained in their sugar–phosphate backbone. Those based on the sugar
ribose are known as ribonucleic acids, or RNA, and contain the bases
A, G, C, and U. Those based on deoxyribose (in which the hydroxyl group
at the 2ʹ position of the ribose carbon ring is replaced by a hydrogen) are
known as deoxyribonucleic acids, or DNA, and contain the bases A, G,
C, and T (T is chemically similar to the U in RNA; see Panel 2–7). RNA usually
occurs in cells in the form of a single-stranded polynucleotide chain,
but DNA is virtually always in the form of a double-stranded molecule:
the DNA double helix is composed of two polynucleotide chains that run
in opposite directions and are held together by hydrogen bonds between
the bases of the two chains (see Panel 2–3, pp. 70–71).
The linear sequence of nucleotides in a DNA or an RNA molecule encodes
genetic information. The two nucleic acids, however, have different roles
in the cell. DNA, with its more stable, hydrogen-bonded helix, acts as
a long-term repository for hereditary information, while single-stranded
RNA is usually a more transient carrier of molecular instructions. The
ability of the bases in different nucleic acid molecules to recognize and
pair with each other by hydrogen-bonding (called base-pairing)—G with
C, and A with either T or U—underlies all of heredity and evolution, as
explained in Chapter 5.
MACROMOLECULES IN CELLS
On the basis of mass, macromolecules are by far the most abundant of
the organic molecules in a living cell (Figure 2–29). They are the principal
building blocks from which a cell is constructed and also the components
that confer the most distinctive properties on living things. Intermediate
in size and complexity between small organic molecules and organelles,
macromolecules are constructed simply by covalently linking small
bacterial
cell
30%
chemicals
inorganic ions,
small molecules (4%)
phospholipid (2%)
DNA (1%)
RNA (6%)
MACROMOLECULES
Figure 2–29 Macromolecules are
abundant in cells. The approximate
composition (by mass) of a bacterial cell
is shown. The composition of an animal
cell is similar.
70%
H 2 O
protein (15%)
polysaccharide (2%)