Many eukaryotic genes contain coding regions called exons and noncoding regions called intervening sequences, or introns. For example, the gene encoding the protein ovalbumin has eight exons and seven introns; the gene for cytochrome b has five exons and four introns (Figure 10.16). The average human gene contains from eight to nine introns. All the introns and exons are initially transcribed into RNA, but during or after transcription, the introns are removed and the exons are joined to yield the mature RNA.

Introns are common in eukaryotic genes but are rare in bacterial genes. 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. TRY PROBLEM 28

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