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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Table 1–2 Some Model Organisms and Their Genomes
Organism
Genome size*
(nucleotide pairs)
Approximate number
of genes
Escherichia coli (bacterium) 4.6 × 10 6 4300
Saccharomyces cerevisiae (yeast) 13 × 10 6 6600
Caenorhabditis elegans
(roundworm)
130 × 10 6 21,000
Arabidopsis thaliana (plant) 220 × 10 6 29,000
Drosophila melanogaster (fruit fly) 200 × 10 6 15,000
Danio rerio (zebrafish) 1400 × 10 6 32,000
Mus musculus (mouse) 2800 × 10 6 30,000
Homo sapiens (human) 3200 × 10 6 30,000
*Genome size includes an estimate for the amount of highly repeated DNA sequence not in
genome databases.
many genes, and a great deal more noncoding DNA (~98.5% of the genome for a
human does not code for proteins, as opposed to 11% of the genome for the bacterium
E. coli). The estimated genome sizes and gene numbers for some eukaryotes
are compiled for easy comparison with E. coli in Table 1–2; we shall discuss how
each of these eukaryotes serves as a model organism shortly.
Eukaryotic Genomes Are Rich in Regulatory DNA
Much of our noncoding DNA is almost certainly dispensable junk, retained like
a mass of old papers because, when there is little pressure to keep an archive
small, it is easier to retain everything than to sort out the valuable information
and discard the rest. Certain exceptional eukaryotic species, such as the puffer
fish, bear witness to the profligacy of their relatives; they have somehow managed
to rid themselves of large quantities of noncoding DNA. Yet they appear similar in
structure, behavior, and fitness to related species that have vastly more such DNA
(see Figure 4–71).
Even in compact eukaryotic genomes such as that of puffer fish, there is more
noncoding DNA than coding DNA, and at least some of the noncoding DNA certainly
has important functions. In particular, it regulates the expression of adjacent
genes. With this regulatory DNA, eukaryotes have evolved distinctive ways of
controlling when and where a gene is brought into play. This sophisticated gene
regulation is crucial for the formation of complex multicellular organisms.
The Genome Defines the Program of Multicellular Development
The cells in an individual animal or plant are extraordinarily varied. Fat cells, skin
cells, bone cells, nerve cells—they seem as dissimilar as any cells could be (Figure
1–33). Yet all these cell types are the descendants of a single fertilized egg cell, and
all (with minor exceptions) contain identical copies of the genome of the species.
The differences result from the way in which the cells make selective use of
their genetic instructions according to the cues they get from their surroundings
in the developing embryo. The DNA is not just a shopping list specifying the molecules
that every cell must have, and the cell is not an assembly of all the items
on the list. Rather, the cell behaves as a multipurpose machine, with sensors to
receive environmental signals and with highly developed abilities to call different
sets of genes into action according to the sequences of signals to which the cell
has been exposed. The genome in each cell is big enough to accommodate the
neuron
neutrophil
25 µm
Figure 1–33 Cell types can vary
enormously in size and shape. An
animal nerve cell is compared here with a
neutrophil, a type of white blood cell. Both
are drawn to scale.
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