Evolution__3rd_Edition

58 PART 1 / Introduction
5' pG
Anticodon
ADH 3' (Amino acid-
C binding site)
C
A
C
Figure 3.7
Transfer RNA molecule. The amino acid is held at the other end
of the molecule from the anticodon loop where the triplet code of
the mRNA molecule is read.
(a) Wild type (b) Amber mutant (c) Amber mutant with suppressor
AUG UUG UUA AUG
AAC
Leu
Release
factor
Figure 3.8
Mutations that suppress amber mutations suggest that the
genetic code is chemically alterable. For example, (a) the
normal codon is UUG and encodes leucine. (b) The UUG
UAG UUA AUG UAG UUA
Tyr
codon
UAC
AUG
Suppressor
mutation
Tyr Tyr
If the genetic code is not chemically determined, why is it the same in all species? The
most popular theory is as follows. The code is arbitrary, in the same sense that human
language is arbitrary. In English the word for a horse is “horse,” in Spanish it is
“caballo,” in French it is “cheval,” in Ancient Rome it was “equus.” There is no reason
why one particular sequence of letters rather than another should signify that familiar
perissodactylic mammal. Therefore, if we find more than one people using the same
word, it implies they have both learned it from a common source. It implies common
ancestry. When the starship Enterprise boldly descends on one of those extragalactic
Tyr
AUC
mutates to the stop codon UAG (this is called an amber
mutation). (c) A tRNA for tyrosine mutates from AUG to AUC
(which recognizes UAG) and suppresses the amber mutation
by inserting a tyrosine.
..