Evolution__3rd_Edition

28 PART 1 / Introduction
Original DNA sequence
…C C G C T C G T C A A C T A G…
…Gly . Glu . Gin . Leu . Ile…
Figure 2.4
Different sorts of mutation. (a) Synonymous mutations a
the base changes but the amino acid encoded does not.
(b) Transition a a change between purine types or between
pyrimidine types. (c) Transversion a a change from purine to
pyrimidine or vice versa. (d) Frameshift mutation a a base is
inserted. (e) Stop mutation a an amino acid-encoding triplet
mutates to a stop codon. The terms transition and transversion
can apply to synonymous or amino acid-changing mutations,
Different kinds of mutation can be
distinguished, such as ...
. . . point mutations ...
C → T
…C C G C T T G T C A A C T A G…
…Gly . Glu . Gin . Leu . Ile…
A → G
…C C G C T C G T A G C T A G…
…Gly . Glu . Gin . Ser . Ile…
A → C
…C C G C T C G T C C A C T A G…
…Gly . Glu . Gin . Val . Ile…
Insert C
→
…C C G C C T C G T C A A C T A G…
…Gly . Gly . Ala . Val . Asp…
G → A
…C C G C T C A T C A A C T A G…
…Gly . Gly . STOP
(a) Synonymous
(b) Transition,
changes
amnio acid
(c) Transversion,
changes
amino acid
(d) Frameshift
(e) Stop
but it has only been illustrated here for mutations that alter
amino acids. The base sequence here is for the DNA. The
genetic code is conventionally written for the mRNA sequence;
thus G has to be transcribed to C, etc. when comparing the
figure with Table 2.1 (the genetic code). (The figure is
stereochemically unconventional because the 3′ end has
been put at the left and 5′ at the right; but this detail is
unimportant here.)
DNA that results from a mutation may code for a form of protein with different properties
from the original. Mutations can happen in any cell, but the most important
mutations, for the theory of evolution, are those occuring in the production of the
gametes. These mutations are passed on to the offspring, who may differ from their
parents because of the mutation.
Various kinds of mutation can occur. One is point mutation, in which a base in the
DNA sequence changes to another base. The effect of a point mutation depends on the
kind of base change (Figure 2.4a–c). Synonymous, or silent, mutations (Figure 2.4a)
are mutations between two triplets that code for the same amino acid, and have no
effect on the protein sequence. Non-synonymous, or meaningful, point mutations do
change the amino acid. Because of the structure of the genetic code (Table 2.1), most
synonymous mutations are in the third base position of the codon. About 70% of
changes in the third position are synonymous, whereas all changes in the second and
most (96%) at the first position are meaningful. Another distinction for point mutations
is between transitions and transversions. Transitions are changes from one
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