Bacterial Genome Sequences

Genetic maps serve as the foundation for more detailed information provided by DNA sequencing, such as gene content and organization (see Chapter 19 for a discussion of gene sequencing).

Geneticists have now determined the complete nucleotide sequence of a number of bacterial genomes. The genome of E. coli, one of the most widely studied of all bacteria, is a single circular DNA molecule approximately 1 mm in length. It consists of 4,638,858 nucleotides and an estimated 4300 genes, more than half of which have no known function. These "orphan genes" may play important roles in adapting to unusual environments, coordinating metabolic pathways, organizing the chromosome, or communicating with other bacterial cells.

A number of other bacterial genomes have been completely sequenced (see Table 19.2), and many additional microbial sequencing projects are underway. A substantial proportion of genes in all bacteria have no known function. Certain genes, particularly those with related functions, tend to reside next to one another, but these clusters are in very different locations in different species, suggesting that bacterial genomes are constantly being reshuffled. Comparisons of the gene sequences of pathogenic and nonpathogenic bacteria are helping to identify genes implicated in disease and may suggest new targets for antibiotics and other antimicrobial agents. For a current list of completed and partial microbial genome projects and a list of microbial genome projects funded by the U.S. Department of Energy

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