Evolution__3rd_Edition

30 PART 1 / Introduction
(a) Transposition (by reverse transcription)
Transposable element
(b) Unequal crossing-over
A B
RNA
Reverse transcription
Target
A B B
A B A
Figure 2.7
Chromosomes can mutate by:
(a) deletion; (b) duplication
of a part; (c) inversion; or
(d) translocation. Translocation
may be either “reciprocal” (in
which the two chromosomes
exchange equal lengths of
DNA) or “non-reciprocal”
(in which one chromosome
gains more than the other). In
addition, whole chromosomes
may fuse, and whole
chromosomes (or the whole
genome) may duplicate.
. . . and chromosomal mutations
(a) (b)
A B C D E F G H
Deletion
A B C D E F G H
A E F G H A B C D E F G H F G H
A B C D E F G H
Inversion
Figure 2.6
Transposition and unequal crossing-over are mutation
mechanisms that affect stretches of DNA longer than one or two
nucleotides. They duplicate DNA laterally through the genome.
(a) Transposition can occur by more than one mechanism. Here
transposition occurs via an RNA intermediate that is copied back
into the DNA by reverse transcription. Transposable elements of
this kind are called retroelements. (b) Unequal crossing-over
happens when the sequences of the two chromosomes are
misaligned at recombination (for recombination, see Figure 2.9
below). In the simple case illustrated here, chromosomes with
three and with one copy of a gene could be generated from two
chromosomes with two genes each. In practice, misalignment is
more likely if there are a long series of copies of similar sequences.
(c) (d)
Duplication
A B C D E N O P Q R
Reciprocal
translocation
A E D C B F G H A B C O N E D P Q R
chromosome, or to another place on the same chromosome, or be inverted. Whole
chromosomes may fuse, as has happened in human evolution; chimps and gorillas (our
closest living relatives) have 24 pairs of chromosomes whereas we have 23. Some or all
of the chromosomes may be duplicated. The phenotypic effects of these chromosomal
mutations are more difficult to generalize about. If the break-points of the mutation
divide a protein, that protein will be lost in the mutant organism. But if the break is
between two proteins, any effect will depend on whether the expression of a gene
depends on its position in the genome. In theory, it might not matter whether a protein
is transcribed from one chromosome or another; though in practice gene expression
is probably at least partly regulated by relations between neighboring genes and a
chromosomal mutation will then have phenotypic consequences.
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