Introns are common in eukaryotic genes but are rare in bacterial genes. For a number of years after their discovery, introns were thought to be entirely absent from prokaryotic genomes, but they have now been observed in archaea, bacteriophages, and even some bacteria. Introns are present in mitochondrial and chloroplast genes as well as the nuclear genes of eukaryotes. Among eukaryotic genomes, the sizes and numbers of introns appear to be directly related to organismal complexity: yeast genes contain only a few short introns, Drosophila introns are longer and more numerous, and most vertebrate genes are interrupted by long introns. All classes of eukaryotic genes—those that encode rRNA, tRNA, and proteins—may contain introns. The numbers and sizes of introns vary widely: some eukaryotic genes have no introns, whereas others may have more than 60; intron length varies from fewer than 200 nucleotides to more than 50,000. Introns tend to be longer than exons, and most eukaryotic genes contain more noncoding nucleotides than coding nucleotides. Finally, most introns do not encode proteins: an intron of one gene is not usually an exon for a different gene.
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