PROKARYOTES: GENES Nearly all prokaryotes have a single circular chromosome contained within a conglomeration of ribosomes and other proteins related to a transcription and translation region called the nucleoid, as opposed to the well defined, double membrane bound eukaryotic nucleus. Certain exceptions do apply, however. For example, Borrelia burgdorferi and the genus Streptomyces contain linear chromosomes, like the eukaryotes. Vibrio cholerae, the causative agent of cholera, has two circular chromosomes, the smaller of which contains most of the genes responsible for virulence. Most notable, however, are the plasmids, which are small (about 1 to 10 thousand base pairs), circular pieces of DNA that are replicated by the host's DNA replication machinery, but whose genes are not absolutely critical for general survival. In nature, they usually contain special genes that confer some type of selective advantage such as antibiotic resistance, virulence, or gene transfer mechanisms. In genetic engineering artificially introduced plasmids carry genes to be expressed and studied. Prokaryotes also differ from eukaryotes in the structure, packing, density, and arrangement of their genes on the chromosome. Prokaryotes have incredibly compact genomes compared to eukaryotes, mostly because prokaryote genes lack introns and large non-coding regions between each gene. Whereas nearly 95% of the human genome does not code for proteins or RNAs, Nor includes a gene promoter, nearly all of the prokaryote genome codes or controls something. Prokaryote genes are also expressed in groups, known as operons, instead of individually, as in eukaryotes. In a prokaryote cell, all genes in an operon (three in the case of the famous lac operon) are transcribed on the same piece of RNA and then made into separate proteins, whereas if these genes were native to eukaryotes, they each would have their own promoter and be transcribed on their own strand of mRNA. This lesser degree of control over gene expression contributes to the simplicity of the prokaryotes as compared to the eukaryotes. It is worth noting that one of the most convincing pieces of evidence for the endosymbiotic theory of the origin of mitochondria is that mitochondrial genomes look like prokaryotic genomes, replete with circular genomes, operons, and plasmids, while that of the host follows the eukaryotic model. Reproduction is most often asexual, through binary fission, where the chromosome is duplicated and attaches to the cell membrane, and then the cell divides in two. However, they show a variety of parasexual processes where DNA is transferred between cells, such as transformation and transduction. * * * * * * * * *