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classes of genes—those that code for rRNA, tRNA, and proteins—may contain introns. The number and size 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 nucle-otides. Finally, most introns do not encode proteins (an intron of one gene is not usually an exon for another), although there are exceptions.

There are four major types of introns (Table 14.1). Group I introns, found in some rRNA genes, are self-splic-ing—they can catalyze their own removal. Group II in-trons are present in some protein-encoding genes of mitochondria, chloroplasts, and a few eubacteria; they also are self-splicing, but their mechanism of splicing differs from that of the group I introns. Nuclear pre-mRNA introns are the best studied; they include introns located in the protein-encoding genes of the nucleus. The splicing mechanism by

Table 14.1

1 Major types of introns

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