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are said to be homologous and the proteins they encode are homologs. If two homologous genes occur in the same species, they are said to be paralogous and their protein products are paralogs. Paralogous genes are presumed to have been derived by gene duplication followed by gradual changes in the sequences of both copies (Fig. 1-37). Typically, paralogous proteins are similar not only in sequence but also in three-dimensional structure, although they commonly have acquired different functions during their evolution.

Two homologous genes (or proteins) found in different species are said to be orthologous, and their protein products are orthologs. Orthologs are commonly found to have the same function in both organisms, and when a newly sequenced gene in one species is found to be strongly orthologous with a gene in another, this gene is presumed to encode a protein with the same function in both species. By this means, the function of gene products can be deduced from the genomic sequence, without any biochemical characterization of the gene product. An annotated genome includes, in addition to the DNA sequence itself, a description of the likely function of each gene product, deduced from comparisons with other genomic sequences and established protein functions. In principle, by identifying the pathways (sets of enzymes) encoded in a genome, we can deduce from the genomic sequence alone the organism's metabolic capabilities.

The sequence differences between homologous genes may be taken as a rough measure of the degree to which the two species have diverged during evo-lution—of how long ago their common evolutionary precursor gave rise to two lines with different evolutionary fates. The larger the number of sequence differences, the earlier the divergence in evolutionary history. One can construct a phylogeny (family tree) in which the evolutionary distance between any two species is represented by their proximity on the tree (Fig. 1-4 is an example).

As evolution advances, new structures, processes, or regulatory mechanisms are acquired, reflections of the changing genomes of the evolving organisms. The genome of a simple eukaryote such as yeast should have genes related to formation of the nuclear membrane, genes not present in prokaryotes. The genome of an insect should contain genes that encode proteins involved in specifying the characteristic insect segmented body plan, genes not present in yeast. The genomes of all vertebrate animals should share genes that specify the development of a spinal column, and those of mammals should have unique genes necessary for the development of the placenta, a characteristic of mammals—and so on. Comparisons of the whole genomes of species in each phylum may lead to the identification of genes critical to fundamental evolutionary changes in body plan and development.

Species A

Species B

Gene 1

) Mutations in many genes lead to evolution of a new species.

Gene 1*

Function 1

@ Gene duplication leads to a superfluous copy of gene 1

Gene 1

Gene 1 copy

Function 1 Function 1

Gene 1

Gene 2

Function 1 Function 2

Homologous genes 1 and 2 are paralogs, related in sequence but encoding proteins of different function in the same species.

Function 1

Mutations in gene 1 copy give rise to gene 2. Gene 2 encodes a protein with a new, different function.

Homologous genes 1 and 1* are orthologs, encoding proteins of the same function in different species.

FIGURE 1-37 Generation of genetic diversity by mutation and gene duplication. (T) A mistake during replication of the genome of species A results in duplication of a gene (gene 1). The second copy is superfluous; mutations in either copy will not be deleterious as long as one good version of gene 1 is maintained. (2) As random mutations occur in one copy, the gene product changes, and in rare cases the product of the "new" gene (now gene 2) acquires a new function. Genes 1 and 2 are paralogs. (3) If species A undergoes many mutations in many genes over the course of many generations, its genome may diverge so greatly from that of the original species that it becomes a new species (species B)—that is, species A and species B cannot interbreed. Gene 1 of species A is likely to have undergone some mutations during this evolutionary period (becoming gene 1*), but it may retain enough of the original gene 1 sequence to be recognized as homologous with it, and its product may have the same function as (or similar function to) the product of gene 1. Gene 1* is an ortholog of gene 1.

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