Evolution__3rd_Edition

24 PART 1 / Introduction
Most genes code for proteins ...
. . . but there are complications,
such as alternative splicing
pigment proteins such as rhodopsin (actually, rhodopsin is made up of a protein called
opsin combined with a derivative of vitamin A); and metabolic processes are catalyzed
by a whole battery of proteins called enzymes (cytochrome c, for instance, is a respiratory
enzyme and alcohol dehydrogenase is a digestive enzyme). Other proteins, such
as the immunoglobulins, defend the body from parasites. The expression of genes is
regulated by yet other proteins, such as the transcription factors coded for by the Hox
genes that we shall discuss in Chapter 20.
Proteins are made up of particular sequences of amino acids. Twenty different
amino acids are found in most kinds of living things. Each amino acid behaves chemically
in distinct ways, such that different sequences of amino acids result in proteins
with very different properties. A protein’s exact sequence of amino acids determines its
nature. Hemoglobin, for example, is made up of two α-globin molecules, which are
141 amino acids long in humans, and two β-globins, which are 144 amino acids long;
insulin has another sequence of 51 amino acids. Hemoglobin binds oxygen in the
blood, whereas insulin stimulates cells (particularly muscle cells, and others) to absorb
glucose from the blood. The different behavior of hemoglobin and insulin is caused by
the chemical properties of their different amino acids, arranged in their characteristic
sequence. (As Chapters 4 and 7 discuss in more detail, the particular sequence of any
one protein can vary within and between species. Thus turkey hemoglobin differs
on average from human hemoglobin, though it binds oxygen in both species. There
are also different variants of hemoglobin within a species, a condition called protein
polymorphism. However, the sequences of all variants of hemoglobin from the same or
different species are similar enough for them to be recognizably hemoglobins.)
The idea that one gene encodes for one protein is a simplification. On the one hand,
some proteins are assembled from more than one gene. For example, hemoglobin is
assembled from four genes, in two main positions in the DNA. On the other hand, one
gene can be used to produce more than one protein. For example, the process of alternative
splicing generates a number of proteins from one gene. Alternative splicing can
be illustrated by the gene slo, which works in the development of our acoustic sensory
system. We are sensitive to a range of frequencies because we have a series of tiny hairs
in our inner ears; some of the hairs are bent by high frequency sounds, others by low
frequency sounds. The frequency that a hair is sensitive to depends on the chemical
properties of the proteins it is made of. Slo is one of the key genes that code for a protein
in the hair cells. It might be thought that we should contain a series of genes, coding for
a series of proteins, that produce the series of hair cells with a range of sensitivities. In
fact slo is read in many ways. The slo gene is made up of several units, which can be
combined in a large number of ways. Exactly how many ways slo can be read is uncertain,
but the alternative splicing of slo contributes part of the molecular diversity
underlying our acoustic sense. Thus, it is not strictly speaking correct to say that one
gene codes for one protein. Nevertheless, for many purposes it is not grievously wrong
to describe DNA as made up of genes (and non-coding regions), and genes as coding
for proteins.
How exactly do genes in the DNA code for proteins? The answer is that the sequence
of nucleotides in a gene specifies the sequence of amino acids in the protein. There are
four types of nucleotide in the DNA. They differ only in the base part of the nucleotide
unit; the sugar and phosphate group are the same in all four. The four are called
..