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RNA Processing:- Both Introns and Exons Are Transcribed from DNA into RNA

المؤلف:  David L. Nelson، Michael M. Cox

المصدر:  Lehninger Principles of Biochemistry

الجزء والصفحة:  p1008-1009

2026-07-23

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RNA Processing:- Both Introns and Exons Are Transcribed from DNA into RNA

In bacteria, a polypeptide chain is generally encoded by a DNA sequence that is colinear with the amino acid sequence, continuing along the DNA template without interruption until the information needed to specify the polypeptide is complete. However, the notion that all genes are continuous was disproved in 1977 when Phillip Sharp and Richard Roberts independently dis covered that many genes for polypeptides in eukaryotes are interrupted by noncoding sequences (introns). The vast majority of genes in vertebrates contain introns; among the few exceptions are those that encode histones. The occurrence of introns in other eukaryotes varies. Many genes in the yeast Saccharomyces cerevisiae lack introns, although in some other yeast species introns are more common. Introns are also found in a few eubacterial and archaebacterial genes. Introns in DNA are transcribed along with the rest of the gene by RNA polymerases. The introns in the primary RNA transcript are then spliced, and the exons are joined to form a mature, functional RNA. In eukaryotic mRNAs, most exons are less than 1,000 nucleotides long, with many in the 100 to 200 nucleotide size range, encoding stretches of 30 to 60 amino acids within a longer polypeptide. Introns vary in size from 50 to 20,000 nucleotides. Genes of higher eukaryotes, including humans, typically have much more DNA devoted to introns than to exons. Many genes have introns; some genes have dozens of them.