Page 132 - Genetics_From_Genes_to_Genomes_6th_FULL_Part2

8.3 Translation: From mRNA to Protein 291

Figure 8.20 Base pairing between an mRNA codon and in the codon.) A single tRNA charged with a particular
a tRNA anticodon determines which amino acid is amino acid can thus recognize several or even all of the
added to a growing polypeptide. A tRNA with an anticodon codons for that amino acid. This flexibility in base pairing
for cysteine, but carrying the amino acid alanine, adds alanine between the 3′ nucleotide in the codon and the 5′ nucleo­
whenever the mRNA codon for cysteine appears. tide in the anticodon is known as wobble (Fig. 8.21a). The
– SH combination of normal base pairing at the first two posi­
– CH 2 – CH 3
Cysteine H–C–NH 2 Treatment with H–C–NH 2 tions of a codon with wobble at the third position clarifies
–
–
– = C nickel hydride – = C Alanine why multiple codons for a single amino acid usually start
O O O O
changes amino acid
with the same two letters.
An important aspect of wobble is the chemical modifi­
cation of certain bases at the 5′ end of the anticodon (the
Treatment with
nickel hydride wobble position) (Fig. 8.21b and c). An A in the wobble
tRNA cysteine leaves anticodon position of a tRNA is almost always modified to inosine
anticodon
3' 5' unchanged 3' 5'
mRNA codon A C G A C G Codon
for cysteine 5' U G U 3' 5' U G U 3' for cysteine
Figure 8.21 Wobble: Some tRNAs recognize more than
one codon for the same amino acid. (a) The G at the 5′ end
codon triplets in the genetic code. E. coli, for example, of the anticodon shown here can pair with either U or C at the 3′
makes 79 different tRNAs containing 42 different antico­ end of the codon. (b) The table shows the pairing possibilities for
nucleotides at the 5′ end of an anticodon (the wobble position).
dons. Although several of the 79 tRNAs in this collection xo U only rarely pairs with C. k C occurs only in certain bacteria.
2
5
obviously have the same anticodon, 61 − 42 = 19 of 61 (c) Chemical structures of the modified bases in anticodons.
potential anticodons are not represented. Thus 19 mRNA (a)
codons will not find a complementary anticodon in the Phe
E. coli collection of tRNAs. How can an organism construct
proper polypeptides if some of the codons in its mRNAs Phe tRNA
cannot locate tRNAs with complementary anticodons?
The answer is that some tRNAs can recognize more 3' 5'
than one codon for the amino acid with which they are tRNA anticodon A A G Wobble position
charged. That is, the anticodons of these tRNAs can inter­ mRNA codon 5' U U U/C 3'
act with more than one codon for the same amino acid, in (b)
keeping with the degenerate nature of the genetic code.
Francis Crick spelled out a few of the rules that govern the Wobble Rules
promiscuous base pairing between codons and anticodons. 5' end of can pair 3' end of
Crick reasoned first that the 3′ nucleotide in many co­ anticodon with codon
dons adds nothing to the specificity of the codon. For ex­ G U or C
G
C
ample, 5′ GGU 3′, 5′ GGC 3′, 5′ GGA 3′, and 5′ GGG 3′ I U, C, or A
all encode glycine (review Fig. 8.2). It does not matter xm U G
5
whether the 3′ nucleotide in the codon is U, C, A, or G as xm s U A or G
5 2
long as the first two letters are GG. The same is true for xo U A, G, U, or (C)
5
other amino acids encoded by four different codons, such 2
as valine, where the first two bases must be GU, but the k C A
third base can be U, C, A, or G. (c) Modified bases in anticodon wobble position
For amino acids specified by two different codons, the Unmodified U
first two bases of the codon are, once again, always the X X X
same, while the third base must be either one of the two
purines (A or G) or one of the two pyrimidines (U or C). S
2-thio-Uridine
Thus, 5′ CAA 3′ and 5′ CAG 3′ are both codons for glu­ Uridine 5-methylene-Uridine derivatives (xm s U) derivatives (xo U)
5-oxy-Uridine
5
5 2
5
derivatives (xm U)
tamine; 5′ CAU 3′ and 5′ CAC 3′ are both codons for histi­ O O O O
dine. If Pu stands for either purine and Py stands for either – CH COCH 3 – CH CNH 2 – OCH COOH – OCH COCH 3
2
2
2
2
pyrimidine, then CAPu represents the codons for glu­ (eukaryotes) (prokaryotes) – OCH
tamine, while CAPy represents the codons for histidine. 3
In fact, the 5′ nucleotide of a tRNA’s anticodon can Unmodified A O Unmodified C
often pair with more than one kind of nucleotide in the 3′ COOH
position of an mRNA’s codon. (Recall that after base pair­ CH(CH 2 4 ) HN
ing, the bases in the anticodon run antiparallel to the bases Adenosine Inosine (I) Cytidine NH 2 Lysidine (k C)
2
(bacteria)