FIGURE 5-15 Overview of RNA processing. RNA processing produces functional mRNA in eukaryotes. The β-globin gene contains three protein-coding exons (constituting the coding region) and two intervening noncoding introns. The introns interrupt the protein-coding sequence between the codons for amino acids 31 and 32 and 105 and 106. Transcription of eukaryotic protein-coding genes starts before the sequence that encodes the first amino acid and extends beyond the sequence that encodes the last amino acid, resulting in noncoding regions at the ends of the primary transcript. These untranslated regions (UTRs) are retained during processing. The 5′ cap (m⁷Gppp) is added during formation of the primary RNA transcript, which extends beyond the poly(A) site. After cleavage at the poly(A) site and addition of multiple A residues to the 3′ end, splicing removes the introns and joins the exons. The small numbers refer to positions in the 147–amino acid sequence of β-globin.