The availibility of complete genomes has oppened up a whole research discipline named genomics. Genomics refers to scientific studies dealing with whole sets of genes. When we look at complete genomes, many questions spring to mind. Which genes are present? How did they get there? Are the genes present in more than one copy? Which genes are not present that we would expect to be present? What order are the genes in, and does this have any significance? How similar is the genome of one organism to that of another?
The GOLD (the Genome Online Database) database lists 315 bacterial and 22 archeal and 11 yeast complete genomes. Bacterial genomes range in size from around half a million to over seven million bases. Genome size and content can vary quite rapidly between related bacterial species. The smallest genomes are in bacteria that are parasites or symbionts inside other cells. In cases where more than one strain of a bacterial species has been completely sequenced, there can be a suprising degree of variability in gene content between the genomes. In pathogenic strains, it is often possible to identify specific regions of the genome, called pathogenity islands, that contain genes responsible for their pathogenic lifestyle, and that appear to have been inserted into the genome from elsewhere.
There is a good evidence that both mitochondria and chloroplasts are descended from bacteria that were taken up by ancient eukaryotic cells, became endosymbionts, and eventually became integral parts of the eukaryote. Analysis of genes from organelle genomes shows that mitochondria are related to -Proteobacteria, such as the present-day Rickettsia, and chloroplasts are related to cyanobacteria. Organelle genomes encode greatly reduced numbers of genes by comparison with even the smallest bacterial genomes. Many of the genes lost from the organelles have been transfered to the nuclear genome.
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