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

36 Chapter 1: Cells and Genomes
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1 mm
fertilized egg 16 cells blastula gastrula
neurula
Figure 1–43 Stages in the normal development of a frog. These drawings
show the development of a Rana pipiens tadpole from a fertilized egg. The
entire process takes place outside of the mother, making the mechanisms
involved readily accessible for experimental studies. (From W. Shumway,
Anat. Rec. 78:139–147, 1940.)
tail bud
For a more exact measure of how closely mammalian species resemble one
another genetically, we can compare the nucleotide sequences of corresponding
(orthologous) genes, or the amino acid sequences of the proteins that these genes
encode. The results for individual genes and proteins vary widely. But typically, if
we line up the amino acid sequence of a human protein with that of the orthologous
protein from, say, an elephant, about 85% of the amino acids are identical.
A similar comparison between human and bird shows an amino acid identity of
about 70%—twice as many differences, because the bird and the mammalian lineages
have had twice as long to diverge as those of the elephant and the human
(Figure 1–45).
The mouse, being small, hardy, and a rapid breeder, has become the foremost
model organism for experimental studies of vertebrate molecular genetics. Many
naturally occurring mutations are known, often mimicking the effects of corresponding
mutations in humans (Figure 1–46). Methods have been developed,
moreover, to test the function of any chosen mouse gene, or of any noncoding
portion of the mouse genome, by artificially creating mutations in it, as we explain
MBoC6 n1.201/1.43
later in the book.
Just one made-to-order mutant mouse can provide a wealth of information for
the cell biologist. It reveals the effects of the chosen mutation in a host of different
contexts, simultaneously testing the action of the gene in all the different kinds of
cells in the body that could in principle be affected.
tadpole
Humans Report on Their Own Peculiarities
As humans, we have a special interest in the human genome. We want to know the
full set of parts from which we are made, and to discover how they work. But even
(A)
(B)
1 cm 150 µm
Figure 1–44 Zebrafish as a model for
studies of vertebrate development. These
small, hardy tropical fish are convenient
for genetic studies. Additionally, they have
transparent embryos that develop outside
of the mother, so that one can clearly
observe cells moving and changing their
character in the living organism throughout
its development. (A) Adult fish. (B) An
embryo 24 hours after fertilization. (A, with
permission from Steve Baskauf; B, from
M. Rhinn et al., Neural Dev. 4:12, 2009.)