Essential Cell Biology 5th edition

Model Organisms
35
TABLE 1–2 SOME MODEL ORGANISMS AND THEIR GENOMES
Organism
Genome Size*
(Nucleotide
Pairs)
Approximate Number
of Protein-coding
Genes
Homo sapiens (human) 3200 × 10 6 19,000
Mus musculus (mouse) 2800 × 10 6 22,000
Drosophila melanogaster (fruit fly) 180 × 10 6 14,000
Arabidopsis thaliana (plant) 103 × 10 6 28,000
Caenorhabditis elegans (roundworm) 100 × 10 6 22,000
Saccharomyces cerevisiae (yeast) 12.5 × 10 6 6600
Escherichia coli (bacterium) 4.6 × 10 6 4300
*Genome size includes an estimate for the amount of highly repeated, noncoding
DNA sequence, which does not appear in genome databases.
model organisms we have described have genomes that fall somewhere
between E. coli and human in terms of size. S. cerevisiae contains about
2.5 times as much DNA as E. coli; D. melanogaster has about 10 times
more DNA than S. cerevisiae; and M. musculus has about 20 times more
DNA than D. melanogaster (Table 1–2).
In terms of gene numbers, however, the differences are not so great. We
have only about five times as many protein-coding genes as E. coli, for
example. Moreover, many of our genes—and the proteins they encode—
fall into closely related family groups, such as the family of hemoglobins,
which has nine closely related members in humans. Thus the number of
fundamentally different proteins in a human is not very many times more
than in the bacterium, and the number of human genes that have identifiable
counterparts in the bacterium is a significant fraction of the total.
This high degree of “family resemblance” is striking when we compare
the genome sequences of different organisms. When genes from different
organisms have very similar nucleotide sequences, it is highly probable
that they descended from a common ancestral gene. Such genes (and
their protein products) are said to be homologous. Now that we have the
complete genome sequences of many different organisms from all three
domains of life—archaea, bacteria, and eukaryotes—we can search systematically
for homologies that span this enormous evolutionary divide.
By taking stock of the common inheritance of all living things, scientists
are attempting to trace life’s origins back to the earliest ancestral cells.
We return to this topic in Chapter 9.
Genomes Contain More Than Just Genes
Although our view of genome sequences tends to be “gene-centric,” our
genomes contain much more than just genes. The vast bulk of our DNA
does not code for proteins or for functional RNA molecules. Instead, it
includes a mixture of sequences that help regulate gene activity, plus
sequences that seem to be dispensable. The large quantity of regulatory
DNA contained in the genomes of eukaryotic multicellular organisms
allows for enormous complexity and sophistication in the way different
genes are brought into action at different times and places. Yet, in the
end, the basic list of parts—the set of proteins that the cells can make, as
specified by the DNA—is not much longer than the parts list of an automobile,
and many of those parts are common not only to all animals, but
also to the entire living world.