protein domains; there are 1262 domains in humans compared with 1035 in fruit flies (see Table 19.3). However, the existing domains in humans are assembled into more combinations, leading to many more types of proteins. For example, the human genome contains almost two times as many arrangements of protein domains as worms or flies contain and almost six times as many as yeast contains. Humans, worms, and flies have many of the same families of genes in common, but each family in the human genome has a greater number of different genes, suggesting that gene duplication has been an important process in vertebrate evolution.

Concepts j"

Comparative genomics compares the content and organization of whole genomic sequences from different organisms. Prokaryotic genomes are small, usually ranging from 1 million to 3 million base pairs of DNA, with several thousand genes. Among multicellular eukaryotic organisms, there is no clear relation between organismal complexity and amount of DNA or gene number. A substantial part of the genome in eukaryotic organisms consists of repetitive DNA, much of which is derived from transposable elements.

(a) Saccharomyces cerevisiae (yeast)

I6 pairs of linear chromosomes Genome size: I2.I million bp Number of genes: 6I00 G + C content: 38%

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