Читать книгу Cell Biology - Stephen R. Bolsover - Страница 104

Genes that code for proteins should be simple things: DNA makes RNA makes protein, and a gene codes for the amino acids of a protein by the three‐base genetic code. In prokaryotes, indeed, a gene is a continuous series of bases that, read in threes, code for the protein. This simple and apparently sensible system does not apply in eukaryotes. Instead, the protein‐coding regions of almost all eukaryotic genes are organized as a series of separate bits interspersed with noncoding regions. The protein‐coding regions of the split genes are exons. The regions between are called introns, short for intervening sequences. At the bottom of Figure 4.7 we show the structure of the β‐globin gene, which contains three exons and two introns. Introns are often very long compared to exons. As happens in prokaryotes, messenger RNA complementary to the DNA is synthesized, but then the introns are spliced out before the mRNA leaves the nucleus (page 22). This means that a gene is much longer than the mRNA that ultimately codes for the protein. The name exon derives from the fact that these are the regions of the gene that, when transcribed into mRNA, exit from the nucleus.