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Source: Number of genes and protein-domain families from International Human Genome Sequencing Consortium, Initial sequencing and analysis of the human genome, Nature 409 (2001), Table 23.

Source: Number of genes and protein-domain families from International Human Genome Sequencing Consortium, Initial sequencing and analysis of the human genome, Nature 409 (2001), Table 23.

Eukaryotic Genomes

The genomes of only a few eukaryotic organisms have been completely sequenced, but some tentative statements can be made about the content and organization of eukaryotic genetic information from these organisms.

The genomes of eukaryotic organisms (Table 19.3) are larger than those of prokaryotes, and, in general, multicellu-lar eukaryotes have more DNA than do simple, single-celled eukaryotes such as yeast (see p. 000 in Chapter 11). There is no close relation, however, between genome size and complexity among the multicellular eukaryotes. For example, the roundworm Caenorhabditis elegans is structurally more complex than the plant Arabidopsis but has considerably less DNA. Eukaryotic genomes also contain more genes than do prokaryotes, and the genomes of multicellular eukaryotes have more genes than do the genomes of single-celled eu-karyotes. The number of genes among multicellular eukary-otes also is not obviously related to phenotypic complexity: humans have more genes than do invertebrates but only twice as many as fruit flies and only slightly more than the plant Arabidopsis. Eukaryotic genomes contain multiple copies of many genes, indicating that gene duplication has been an important process in genome evolution.

A substantial part of the genomes of multicellular organisms consists of moderately and highly repetitive sequences (see Chapter 11), and the percentage of repetitive sequences is usually higher in those species with larger genomes (Table 19.4). Most of these repetitive sequences appear to have arisen through transposition. This is particularly evident in the human genome, where 45% of the DNA is derived from transposable elements, many of which are defective and no longer able to move. The majority of DNA in multicellular organisms is noncoding, and many genes are interrupted by introns. In the more complex eukaryotes, both the number and the length of the introns are greater.

In spite of only a modest increase in gene number, vertebrates have considerably more protein diversity than do invertebrates. The human genome does not encode many new

[Table 19.4 Percentage of genome consisting of 1

interspersed repeats derived from

transposable elements

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