Essential Cell Biology 5th edition

From DNA to RNA
229
(A)
SUGAR DIFFERENCES
HOCH 2
O OH
HOCH 2
O
OH
5′ end
O
sugar–phosphate
backbone
H
H H
H
OH OH
ribose
H H
H H
OH H
deoxyribose
P
O
H 2 C
O
O
C
used in RNA
used in DNA
bases
(B)
BASE DIFFERENCES
O
HC
HC
C
U
N
NH
C
O
H 3 C
C
HC
O
C
T
N
NH
C
O
O
P
O
H 2 C
O
OH
O
A
– O
C
– O P
O
H
uracil
H
thymine
O
OH
used in RNA
used in DNA
Figure 7–3 The chemical structure of RNA differs slightly from
that of DNA. (A) RNA contains the sugar ribose, which differs from
deoxyribose, the sugar used in DNA, by the presence of an additional
–OH group. (B) RNA contains the base uracil, which differs from
thymine, the equivalent base in DNA, by the absence of a –CH 3 group.
(C) A short length of RNA. The chemical linkage between nucleotides
in RNA—a phosphodiester bond—is the same as that in DNA.
of RNA. We then discuss how these RNAs are processed, the variety of
roles they play in the cell, and, ultimately, how they are removed from
circulation.
ribose
phosphodiester
bond
P
O
H 2 C
O
– O P
O
H 2 C
O
OH
O
U
G
– O P
O
Portions of DNA Sequence Are Transcribed into RNA
The first step a cell takes in expressing one of its many thousands of
(C)
genes is to copy the nucleotide sequence of that gene into RNA. The process
is called transcription because the information, though copied into
another chemical form, is still written in essentially the same language—
the language of nucleotides. Like DNA, RNA is a linear polymer made
of four different nucleotide subunits, linked together by phosphodiester
3′
bonds. It differs from DNA chemically in two respects: (1) the nucleotides
in RNA are ribonucleotides—that is, they contain the sugar ribose (hence
the name ribonucleic acid) rather than the deoxyribose found in DNA;
and (2) although, like DNA, RNA contains the bases adenine (A), guanine
(G), and cytosine (C), it contains uracil (U) instead of the thymine (T)
found in DNA (Figure 7–3). Because U, like T, can base-pair by hydrogenbonding
with A (Figure 7–4), the complementary base-pairing properties
ECB5 e7.03/7.03
described for DNA in Chapter 5 apply also to RNA.
Although their chemical differences are small, DNA and RNA differ quite
dramatically in overall structure. Whereas DNA always occurs in cells
as a double-stranded helix, RNA is largely single-stranded. This difference
has important functional consequences. Because an RNA chain is
single-stranded, it can fold up into a variety of shapes, just as a polypeptide
chain folds up to form the final shape of a protein (Figure 7–5);
Figure 7–4 Uracil forms a base pair with adenine. The hydrogen
bonds that hold the base pair together are shown in red. Uracil has the
same base-pairing properties as thymine. Thus U-A base pairs in RNA
closely resemble T-A base pairs in DNA (see Figure 5−4A).
5′
O
N
C
hydrogen
bond
H
C
N
O
3′ end
H
U
N
H
N
A
C
N
5′
C
C
C
C
OH
C
3′
O
H
N
N
H
H
sugar–phosphate backbone
uracil
H
adenine