Both intron and exon sequences are transcribed into RNA. The intron sequences are removed from the newly synthesized RNA through the process of RNA splicing. The vast majority of RNA splicing that takes place in cells func- tions in the production of mRNA, and our discussion of splicing focuses on this so-called precursor-mRNA (or pre-mRNA) splicing. Only after 5ʹand 3ʹ-end pro- add methyl group to base cessing and splicing have taken place is such RNA termed mRNA.
Figure 6–23 The reactions that cap the 5ʹ end of each RNA molecule synthesized by RNA polymerase II. The final cap contains a novel 5ʹ-to-5ʹ add methyl group to ribose linkage between the positively charged 7-methyl G residue and the 5ʹ end of the RNA transcript (see Figure 6–21B). The letter N represents any one of the four ribonucleotides, although the nucleotide that starts an RNA chain is usually a purine (an A or a G). (After A.J. Shatkin, BioEssays 7:275–277, 1987. With permission from Wiley-Liss, Inc., a subsidiary of John Wiley & Sons, Inc.) CH3 exons 2000 200,000 nucleotide pairs
Each splicing event removes one intron, proceeding through two sequential phosphoryl-transfer reactions known as transesterifications; these join two exons together while removing the intron between them as a “lariat” (Figure 6–25). The machinery that catalyzes pre-mRNA splicing is complex, consisting of five addi-tional RNA molecules and several hundred proteins, and it hydrolyzes many ATP molecules per splicing event. This complexity ensures that splicing is accurate, while at the same time being flexible enough to deal with the enormous variety of introns found in a typical eukaryotic cell. It may seem wasteful to remove large numbers of introns by RNA splicing. In attempting to explain why it occurs, scientists have pointed out that the exon– intron arrangement would seem to facilitate the emergence of new and useful proteins over evolutionary time scales. Thus, the presence of numerous introns in DNA allows genetic recombination to readily combine the exons of different genes, enabling genes for new proteins to evolve more easily by the combination of parts of preexisting genes. The observation, described in Chapter 3, that many proteins in present-day cells resemble patchworks composed from a common set of protein domains, supports this idea (see pp. 121–122). RNA splicing also has a present-day advantage. The transcripts of many eukaryotic genes (estimated at 95% of genes in humans) are spliced in more than one way, thereby allowing the same gene to produce a corresponding set of dif-ferent proteins (Figure 6–26). Rather than being the wasteful process it may have seemed at first sight, RNA splicing enables eukaryotes to increase the coding potential of their genomes. We shall return to this idea again in this chapter and the next, but we first need to describe the cellular machinery that performs this remarkable task. O(A) (B) 5˜5˜AOH AHO 3˜5˜exon sequence intron sequence 3˜exon 2˜
Figure 6–24 Structure of two human genes showing the arrangement of exons and introns. (A) The relatively small β-globin gene, which encodes a subunit of the oxygen-carrying protein hemoglobin, contains 3 exons (see also Figure 4–7).
(B) The much larger Factor VIII gene contains 26 exons; it codes for a protein (Factor VIII) that functions in the blood-clotting pathway. The most prevalent form of hemophilia results from mutations in this gene.