FIGURE 10-2 Overview of mRNA processing in eukaryotes. Shortly after RNA polymerase II initiates transcription at the first nucleotide of the first exon of a gene, the 5′ end of the nascent RNA is capped with 7-methylguanylate (step 1 ). Transcription by RNA polymerase II terminates at any one of multiple termination sites downstream from the poly(A) site, which is located at the 3′ end of the final exon. After the primary transcript is cleaved at the poly(A) site (step 2 ), a string of adenosine (A) residues is added (step 3 ). The poly(A) tail contains ~250 A residues in mammals, ~150 in insects, and ~100 in yeasts. For short primary transcripts with few introns, splicing (step 4 ) usually follows cleavage and polyadenylation, as shown. For large genes with multiple introns, introns are often spliced out of the nascent RNA during its transcription, before transcription of the gene is complete. Note that the 5′ cap and the sequence adjacent to the poly(A) tail are retained in mature mRNAs. The diagram shown represents processing of human β-globin RNA.