38
Chapter 2
RNA
RNA molecules differ in only a few respects from DNA
(
Table 2–7
): (1) RNA consists of a single (rather than a
double) chain of nucleotides; (2) in RNA, the sugar in each
nucleotide is
ribose
rather than deoxyribose; and (3) the
pyrimidine base thymine in DNA is replaced in RNA by the
pyrimidine base
uracil
(U) (see Figure 2–23), which can base-
pair with the purine adenine (A–U pairing). The other three
bases, adenine, guanine, and cytosine, are the same in both
DNA and RNA. Because RNA contains only a single chain of
nucleotides, portions of this chain can bend back upon them-
selves and undergo base pairing with nucleotides in the same
chain or in other molecules of DNA or RNA.
ATP
The purine bases are important not only in DNA and RNA
synthesis, but also in a molecule that serves as the molecular
energy source for all cells.
The functioning of a cell depends upon its ability to
extract and use the chemical energy in the organic mole-
cules discussed in this chapter. For example, when, in the
presence of oxygen, a cell breaks down glucose to carbon
dioxide and water, energy is released. Some of this energy
is in the form of heat, but a cell cannot use heat energy
to perform its functions. The remainder of the energy is
transferred to another important molecule that can in turn
transfer it to yet another molecule or to energy-requiring
processes. In all cells, from bacterial to human,
adenosine
triphosphate (ATP)
(
Figure 2–26
) is the primary molecule
that receives the transfer of energy from the breakdown of fuel
molecules—carbohydrates, fats, and proteins.
Energy released from organic molecules is used to add
phosphate groups to molecules of adenosine. This stored
energy can then be released upon hydrolysis:
ATP + H
2
O
⎯→
ADP + P
i
+ H
+
+ energy
TA
TA
TA
AT
AT
CG
CG
GC
GC
Figure 2–24
Base pairings between a purine and pyrimidine base link the two
polynucleotide strands of the DNA double helix.
N
N
N
N
C
C
C
C
C
N
H
H
O
N
N
H
N
N
H
H
H
H
H
H
N
N
C
C
C
C
N
N
O
H
H
CC
C
C
CH
3
N
N
O
H
H
CC
C
C
O
H
C
Adenine
Guanine
Cytosine
Thymine
phosphate–sugar sequence
Figure 2–25
Hydrogen bonds between the nucleotide bases in DNA determine
the specifi city of base pairings: adenine with thymine and guanine
with cytosine.
Table 2–7
Comparison of DNA and RNA
Composition
DNA
RNA
Nucleotide sugar
Deoxyribose
Ribose
Nucleotide bases
Purines
Adenine
Adenine
Guanine
Guanine
Pyrimidines
Cytosine
Cytosine
Thymine
Uracil
Number of chains
Two
One
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