Evolution__3rd_Edition

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Figure 19.5
(a) A chromosomal inversion
has a set of genes inverted.
The letters represent genes
along the chromosomes.
(b) Recombination in a
heterozygote for a
chromosomal inversion can
produce chromosomes that lack
some genes and have others in
double doses. These forms are
probably selected against.
Much of our DNA orginated from
transposable elements
CHAPTER 19 / Evolutionary Genomics 567
(a) Chromosomal inversion (b) Recombination in inversion heterozygote
Chromosome
form A
Inverted chromosome
form A'
…ABCDEFGH… …ABCGFEDH… …ABCDEFGH…
…ABCGFEDH…
…ABCDFEDH… …ABCGEFGH…
able to date the four events. Their hypothesis may or may not hold up as further
research is done, but is a good example of the kind of inferences that genomic data are
making possible for chromosomal evolution.
19.7 Genome sequences can be used to study the history of
non-coding DNA
The “coding” part of human DNA a the part coding for the genes that regulate, build,
and defend our bodies a makes up less than 5% of our genome. The rest is “noncoding”
DNA. Non-coding DNA may be useless “junk” DNA that has no function
in the body or it may have some structural or regulatory function. Here we can look
at how the human genome sequence has been used to infer the evolutionary history
of one large class of non-coding DNA, that derived from transposable elements
(Section 2.5, p. 29).
About 45% of the human genome is derived from transposable elements. These
stretches of DNA are of four main kinds: short interspersed elements (SINEs), long
interspersed elements (LINEs), long terminal repeat (LTR) retrotransposons, and
DNA transposons. The first three kinds are “jumping genes” that are copied via an
RNA intermediate. The most important SINE in our DNA is a sequence called Alu. The
Alu unit sequence is somewhat less than 300 nucleotides long, and our DNA contains
over a million copies of it: rather over 10% of human DNA consists of the Alu sequence.
A LINE called LINE1 makes up even more of our DNA a about 17%.
We can take any two copies of a sequence such as Alu in our DNA, count the number
of differences between them, and use a molecular clock to estimate how long ago the
duplicative “jumping” event took place to give rise to them. The International Human
Genome Sequencing Consortium (2001) performed an analysis of this kind for all the
identified transposon-derived DNA in the human genome. Three patterns can be
noticed. One is that the different kinds of transposable element have been more or less
active at different times in human history. Table 19.1 gives the percentage of the human
genome that consists of each of the three kinds of transposon, dating to particular times
of origin. We can see that the Alu element, for example, had a burst of proliferation