Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

GENETIC INFORMATION IN EUKARYOTES
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Genes that were once identical have diverged; many of the gene copies have been
lost through disruptive mutations; some have undergone further rounds of local
duplication; and the genome, in each branch of the vertebrate family tree, has
suffered repeated rearrangements, breaking up most of the original gene orderings.
Comparison of the gene order in two related organisms, such as the human
and the mouse, reveals that—on the time scale of vertebrate evolution—chromosomes
frequently fuse and fragment to move large blocks of DNA sequence
around. Indeed, it is possible, as discussed in Chapter 4, that the present state
of affairs is the result of many separate duplications of fragments of the genome,
rather than duplications of the genome as a whole.
There is, however, no doubt that such whole-genome duplications do occur
from time to time in evolution, for we can see recent instances in which duplicated
chromosome sets are still clearly identifiable as such. The frog genus Xenopus,
for example, comprises a set of closely similar species related to one another
by repeated duplications or triplications of the whole genome. Among these frogs
are X. tropicalis, with an ordinary diploid genome; the common laboratory species
X. laevis, with a duplicated genome and twice as much DNA per cell; and
X. ruwenzoriensis, with a sixfold reduplication of the original genome and six
times as much DNA per cell (108 chromosomes, compared with 36 in X. laevis, for
example). These species are estimated to have diverged from one another within
the past 120 million years (Figure 1–42).
The Frog and the Zebrafish Provide Accessible Models for
Vertebrate Development
Frogs have long been used to study the early steps of embryonic development
in vertebrates, because their eggs are big, easy to manipulate, and fertilized outside
of the animal, so that the subsequent development of the early embryo is
easily followed (Figure 1–43). Xenopus laevis, in particular, continues to be an
important model organism, even though it is poorly suited for genetic analysis
(Movie 1.6 and see Movie 21.1).
The zebrafish Danio rerio has similar advantages, but without this drawback.
Its genome is compact—only half as big as that of a mouse or a human—and it
has a generation time of only about three months. Many mutants are known, and
genetic engineering is relatively easy. The zebrafish has the added virtue that it is
transparent for the first two weeks of its life, so that one can watch the behavior
of individual cells in the living organism (see Movie 21.2). All this has made it an
increasingly important model vertebrate (Figure 1–44).
The Mouse Is the Predominant Mammalian Model Organism
Mammals have typically two times as many genes as Drosophila, a genome that
is 16 times larger, and millions or billions of times as many cells in their adult
bodies. In terms of genome size and function, cell biology, and molecular mechanisms,
mammals are nevertheless a highly uniform group of organisms. Even
anatomically, the differences among mammals are chiefly a matter of size and
proportions; it is hard to think of a human body part that does not have a counterpart
in elephants and mice, and vice versa. Evolution plays freely with quantitative
features, but it does not readily change the logic of the structure.
Figure 1–42 Two species of the frog genus Xenopus. X. tropicalis, above,
has an ordinary diploid genome; X. laevis, below, has twice as much DNA per
cell. From the banding patterns of their chromosomes and the arrangement
of genes along them, as well as from comparisons of gene sequences, it is
clear that the large-genome species have evolved through duplications of
the whole genome. These duplications are thought to have occurred in the
aftermath of matings between frogs of slightly divergent Xenopus species.
(Courtesy of E. Amaya, M. Offield, and R. Grainger, Trends Genet. 14:253–
255, 1998. With permission from Elsevier.)