FIGURE 3-10. Structure and translation of messenger RNA (mRNA). In most cases, the mature mRNA represents the fusion of multiple exons. These sequences encode two major regions: translated and untranslated. The translated or coding region is delimited by the translation initiator codon, AUG, at its 5′ end and the termination codon, UGA, UAA, UAG, at its 3′ end. This coding region represents a series of codons in an open-reading frame that determines the amino-acid sequence of its encoded polypeptide. The 5′ and 3′ untranslated regions are shown. The mRNA enters the cytoplasm to interact with the ribosome. There, protein synthesis is initiated, and by way of a series of several cotranslational events, secretory polypeptide hormone precursors are processed. The steps involve cleavage of the signal or leader peptide, followed by addition of asparagine-linked carbohydrate moieties in glycoprotein subunits or hormones, and intramolecular folding with the formation of disulfide linkages. These events occur within the lumen of the rough endoplasmic reticulum. These partially processed polypeptide hormones are then shuttled to the Golgi stack, where these molecules are transported, sorted, and further processed posttranslationally to yield the bioactive hormone located in secretory granules or vesicles.