Essential Cell Biology 5th edition

248 CHAPTER 7 From DNA to Protein: How Cells Read the Genome
structure that looks like a cloverleaf when drawn schematically (Figure
7–31A). As shown in the figure, for example, a 5ʹ-GCUC-3ʹ sequence
in one part of a polynucleotide chain can base-pair with a 5ʹ-GAGC-3ʹ
sequence in another region of the same molecule. The cloverleaf undergoes
further folding to form a compact, L-shaped structure that is held
together by additional hydrogen bonds between different regions of the
molecule (Figure 7–31B–D).
Two regions of unpaired nucleotides situated at either end of the L-shaped
tRNA molecule are crucial to the function of tRNAs in protein synthesis.
One of these regions forms the anticodon, a set of three consecutive
nucleotides that bind, through base-pairing, to the complementary codon
in an mRNA molecule (Figure 7–31E). The other is a short, single-stranded
region at the 3ʹ end of the molecule; this is the site where the amino acid
that matches the codon is covalently attached to the tRNA.
attached amino
acid (Phe)
5′ end
A 3′ end
C
C
A
C
G
CUUAAG
A CA C C
We saw in the previous section that the genetic code is redundant; that
is, several different codons can specify a single amino acid (see Figure
7–27). This redundancy implies either that there is more than one tRNA
for many of the amino acids or that some tRNA molecules can base-pair
with more than one codon. In fact, both situations occur. Some amino
acids have more than one tRNA, and some tRNAs require accurate basepairing
only at the first two positions of the codon and can tolerate a
mismatch (or wobble) at the third position. This wobble base-pairing
explains why so many of the alternative codons for an amino acid differ
only in their third nucleotide (see Figure 7–27). Wobble base-pairings
make it possible to fit the 20 amino acids to their 61 codons with as few
as 31 kinds of tRNA molecules. The exact number of different kinds
of tRNAs, however, differs from one species to the next. For example,
humans have approximately 500 different tRNA genes, but this collection
includes only 48 different anticodons.
(A)
D D G A UUUAGGCG
A CUC G
G
G G A GC
G A G
C
C
A GA
C
U
GA A
C U G U G
C UGG
A
G
G
U
C Ψ
Y
A
anticodon
U A
G
T C
Ψ
anticodon
loop
a cloverleaf
(B)
(C)
G A A
(D)
5′ GCGGAUUUAGCUCAGDDGGGAGAGCGCCAGACUGAAYAΨCUGGAGGUCCUGUGTΨCGAUCCACAGAAUUCGCACCA 3′
(E)
anticodon
Figure 7–31 tRNA molecules are molecular adaptors, linking amino acids to codons. In this series of diagrams, the same tRNA
molecule—in this case, a tRNA specific for the amino acid phenylalanine (Phe)—is depicted in various ways. (A) The conventional
“cloverleaf” structure shows the complementary base-pairing (red lines) that creates the double-helical regions of the molecule.
The anticodon loop (blue) contains the sequence of three nucleotides (red letters) that base-pairs with the Phe codon in mRNA. The
amino acid matching the anticodon is attached at the 3ʹ end of the tRNA. tRNAs contain some unusual bases, which are produced by
chemical modification after the tRNA has been synthesized. The bases denoted ψ (for pseudouridine) and D (for dihydrouridine) are
derived from uracil. (B and C) Views of the actual L-shaped molecule, based on x-ray diffraction analysis. These two images are rotated
90º with respect to each other. (D) The schematic ECB5 e7.29/7.31 representation of tRNA that will be used in subsequent figures emphasizes the
anticodon. (E) The linear nucleotide sequence of the tRNA molecule, color-coded to match (A), (B), and (C).