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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time in millions of years
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50
100
150
200
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300
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400
450
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Tertiary
Cretaceous
Jurassic
Triassic
Permian
Carboniferous
Devonian
Silurian
Ordovician
Cambrian
Proterozoic
human/chimp
human/orangutan
mouse/rat
cat/dog
pig/whale
pig/sheep
human/rabbit
human/elephant
human/mouse
human/sloth
human/kangaroo
bird/crocodile
human/lizard
human/chicken
human/frog
human/tuna fish
human/shark
human/lamprey
if you were a mouse, preoccupied with the molecular biology of mice, humans
would be attractive as model genetic organisms, because of one special property:
through medical examinations and self-reporting, we catalog our own genetic
(and other) disorders. The human population is enormous, consisting today
of some 7 billion individuals, and this self-documenting property means that a
huge database of information exists
MBoC6
on
m1.52/1.45
human mutations. The human genome
sequence of more than 3 billion nucleotide pairs has been determined for thousands
of different people, making it easier than ever before to identify at a molecular
level the precise genetic change responsible for any given human mutant phenotype.
By drawing together the insights from humans, mice, fish, flies, worms, yeasts,
plants, and bacteria—using gene sequence similarities to map out the correspondences
between one model organism and another—we are enriching our understanding
of them all.
100
98
84
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77
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35
percent amino acids identical in hemoglobin α chain
Figure 1–45 Times of divergence of
different vertebrates. The scale on the left
shows the estimated date and geological
era of the last common ancestor of each
specified pair of animals. Each time
estimate is based on comparisons of the
amino acid sequences of orthologous
proteins; the longer the animals of a pair
have had to evolve independently, the
smaller the percentage of amino acids
that remain identical. The time scale
has been calibrated to match the fossil
evidence showing that the last common
ancestor of mammals and birds lived
310 million years ago.
The figures on the right give data on
sequence divergence for one particular
protein—the α chain of hemoglobin. Note
that although there is a clear general trend
of increasing divergence with increasing
time for this protein, there are irregularities
that are thought to reflect the action of
natural selection driving especially rapid
changes of hemoglobin sequence when
the organisms experienced special
physiological demands. Some proteins,
subject to stricter functional constraints,
evolve much more slowly than hemoglobin,
others as much as five times faster. All this
gives rise to substantial uncertainties in
estimates of divergence times, and some
experts believe that the major groups of
mammals diverged from one another as
much as 60 million years more recently
than shown here. (Adapted from S. Kumar
and S.B. Hedges, Nature 392:917–920,
1998. With permission from Macmillan
Publishers Ltd.)
Figure 1–46 Human and mouse: similar
genes and similar development. The
human baby and the mouse shown
here have similar white patches on their
foreheads because both have mutations in
the same gene (called Kit), required for the
development and maintenance of pigment
cells. (Courtesy of R.A. Fleischman.)