▲ FIGURE 4-13 Structure of the 5' methylated cap of eukaryotic mRNA. The distinguishing chemical features are the 5'n5' linkage of 7-methylguanylate to the initial nucleotide of the mRNA molecule and the methyl group on the 2' hydroxyl of the ribose of the first nucleotide (base 1). Both these features occur in all animal cells and in cells of higher plants; yeasts lack the methyl group on nucleotide 1. The ribose of the second nucleotide (base 2) also is methylated in vertebrates. [See A. J. Shatkin, 1976, Cell 9:645.]

p-Globin genomic DNA

Primary 5' RNA

transcript p-Globin genomic DNA

3' cleavage and addition of poly(A) tail

Primary 5' RNA


3' cleavage and addition of poly(A) tail on

Intron excision, exon ligation p-Globin mRNA

▲ FIGURE 4-14 Overview of RNA processing to produce functional mRNA in eukaryotes. The p-globin gene contains three protein-coding exons (coding region, red) and two intervening noncoding introns (blue). 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 encoding the last amino acid, resulting in noncoding regions (gray) 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 p-globin.

untranslated regions (UTRs), at each end. In mammalian mRNAs, the 5' UTR may be a hundred or more nucleotides long, and the 3' UTR may be several kilobases in length. Prokaryotic mRNAs also usually have 5' and 3' UTRs, but these are much shorter than those in eukaryotic mRNAs, generally containing fewer than 10 nucleotides.

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