Evolution, 4th Edition

358 CHAPTER 14

important to bacteria and archaea, in which they are widespread in the genome,

but they are also found in viruses and a small number of eukaryotes.

Gene structure

A shocking discovery was made in 1977, in the early days of DNA sequencing.

The genes that code for many proteins are broken into pieces. These pieces—the

exons—are separated by stretches of noncoding DNA—the introns (see Chapter

4). Further research revealed that most genes in prokaryotes lack introns, while

most genes in eukaryotes have them. In humans, about one-fourth of the entire

genome is made up of introns (FIGURE 14.14).

When a gene with introns is transcribed into messenger RNA, the message

is processed by splicing to produce the final message that is then translated into

a protein. The splicing removes all the segments of the message corresponding

to introns. Splicing can also remove one or more segments that correspond to

exons. Alternative splicing brings together different combinations of exons from

the same locus. As a result, a single gene can produce more than one protein. The

current record holder is the gene Dscam in Drosophila. Alternative splicing of its

95 exons could potentially produce more than 38,000 kinds of proteins from this

single gene [69]. Alternative splicing is a major mechanism used by eukaryotes to

increase organismal complexity without increasing the size of their genomes, and

it may be as important as amino acid changes in the functional diversification of

proteins. Changes in alternative splicing can evolve quite quickly. About one-third

of alternative splicing events are different between the genes of humans and mice

[49], while differences in their coding sequences have evolved only half as fast.

Alternative splicing also contributes to phenotypic plasticity (see Chapters 6 and

15). The plant Arabidopsis splices some of its genes in different ways depending on

the environment in which it grows [68]. In sum, introns are essential to the proper

function of the eukaryotic genome. You will see shortly, however, that introns may

have originated for nonadaptive reasons.

Another surprising feature of how genes are put together is that they sometimes

overlap, so that one stretch of chromosome encodes two different proteins [30].

(Imagine a string of letters that form two different sentences depending on where

Futuyma Kirkpatrick Evolution, 4e

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26%

Introns

12%

Miscellaneous

unique sequences 2%

Coding

sequences

(exons)

Miscellaneous 8%

heterochromatin

Duplications

Simple

sequence repeats

5%

3%

3%

8% 13%

SINEs

LINEs

Transposons

LT R

retrotransposons

DNA transposons

20%

FIGURE 14.14 The ingredients that make up the human

genome. Less than 2 percent is devoted to protein-coding

sequences. Different kinds of transposable elements (SINEs,

LINEs, LTR retrotransposons, and DNA transposons) together

make up almost half the genome, while introns make up more

than one-fourth. (After [23].)

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