Evolution__3rd_Edition

..
r
Either Or
r
r' r'
r r
r'
. . . frameshift mutations ...
. . . slippage ...
. . . transposition . . .
r
Repair
r
r'
r'
r r
r' r'
Old strand
Newly copied strand
CHAPTER 2 / Molecular and Mendelian Genetics 29
Figure 2.5
Slippage happens when a stretch of DNA is copied twice or not at
all. r in the figure is a certain sequence of DNA. It is repeated three
times in a region of the DNA molecule. This molecule is being
copied, and r′ refers to the same unit sequence in the new DNA
molecule. In this case the DNA polymerase has slipped past one
repeat and the new molecule has two rather than three repeats.
It is also possible for the polymerase to slip back and copy a unit
repeat twice; then the new molecule has four repeats. The old and
new copies will have different numbers of repeats. They may be
repaired to create a mutant DNA (with two or four repeats) or to
restore the original number of three repeats. Sections of DNA
with many repeats of a similar sequence may be particularly
vulnerable to slippage.
pyrimidine to the other, or from one purine to the other: between C and T, and
between A and G. Transversions replace a purine base by a pyrimidine, or vice versa:
from A or G to T or C (and from C or T to A or G). The distinction is interesting
because transitional changes are much commoner in evolution than transversions.
Successive amino acids are read from consecutive base triplets. If, therefore, a mutation
inserts a base pair into the DNA, it can alter the meaning of every base “downstream”
from the mutation (Figure 2.4d). These are called frameshift mutations, and
will usually produce a completely nonsensical, functionless protein. Another kind of
mutation is for a previously coding triplet to mutate to a “stop” codon (Figure 2.4e);
the resulting protein fragments will probably again be functionless.
Some stretches of non-coding DNA consist of repeats of a short unit sequences.
These sequences are particularly vulnerable to a kind of error called slippage (Figure
2.5). In slippage, the DNA strand that is being copied from slips relative to the new
strand that is being created. A short stretch of nucleotides is then missed out or copied
twice. Slippage contributes to the origin of non-coding DNA that consists of repeats of
short unit sequences. However, slippage can also occur in DNA other than repetitive
non-coding DNA. Slippage can cause frameshift mutations (Figure 2.4d), for instance.
The mutational mechanisms we have considered so far concern single nucleotides,
or short stretches of nucleotides. Other mutational mechanisms can influence larger
chunks of DNA. Transposition is an important example. Transposable elements a
informally known as “jumping genes” a can copy themselves from one site in the DNA
to another (Figure 2.6a). If a transposable element inserts itself into an existing gene, it
will corrupt that gene; if it inserts itself into a region of non-coding DNA, it may do less
or even no damage to the body. Transposable elements can pick up a stretch of DNA
and copy it as well as itself into the new insertion site. Transposition usually alters the
total length of the genome, because it creates a new duplicated stretch of DNA. This
contrasts with a simple miscopying of a nucleotide, in which the total length of the
genome is unchanged. Unequal crossing-over is another kind of mutation that can
duplicate (or, unlike transposition, delete) a long stretch of DNA (Figure 2.6b).
Finally, mutations may influence large chunks of chromosomes, or even whole
chromosomes (Figure 2.7). A length of chromosome may be translocated to another