Figure 1. Overview of the steps involved in human gene expression. In the nucleus, genes are transcribed to form pre-mRNA. Pre-mRNA, which is characterized by a ^7-methylGpppG cap structure that is added cotranscriptionally, is then processed to mRNA. Processing includes (i) splicing, that is the ligation of exons (boxes) and the concomitant removal of intervening sequences or introns (lines between boxes), and (ii) 3¢ end formation, that is endonucleolytic cleavage followed by the addition of poly(A) ((A)_n) to the upstream cleavage product. Depending on the particular transcript and type of editing, editing can occur either before or after splicing. Finally, processed mRNA is exported to the cytoplasm, where it is translated and, ultimately, degraded. The translation initiation codon usually resides within the first exon, and the translation termination codon usually resides within the last exon.

Figure 2. Examples of splicing defects that alter the coding sequence of a gene. AUG and Norm Stop specify the initiation codon and the normal termination codon, respectively, while STOP signs specify premature termination codons. Patterned boxes and thick lines represent exons and introns respectively. Thin and dashed lines connect normally used and abnormally used (cryptic) 5¢ and 3¢ splice sites (ss) respectively. Intron retention can result in an intron-derived premature termination codon, a shift in the reading frame that generates a downstream premature termination codon or an insertion in the reading frame. Exon skipping can result in a shift in the reading frame that generates a downstream premature termination codon or deletion of the reading frame.

Figure 3. Intronic mutation can create a new splice site within -globin pre-mRNA. Splicing of exon 1 (E1), exon 2 (E2) and exon 3 (E3) of a normal (a) or mutated (b) -globin pre-mRNA that harbours a U-to-G transversion (bold capital letters) within the second of two introns (thick bold lines). In the case of the mutated pre-mRNA, the transversion is recognized as a 5¢ splice site and is paired with a cryptic 3¢ splice site that resides upstream. As a consequence, an intronic region (dotted box) that harbours a premature termination codon is retained in the spliced product so that it encodes abnormal -globin protein.

Figure 4. Deletion within the polyadenylation sequence of the 2-globin gene can lead to decreased expression of both 2- and 1-globin genes. 3¢ end formation of a normal (a) or mutated 2-globin gene (b) that lacks the last two base pairs of the polyadenylation sequence (bold ). In the case of the mutated gene, 3¢ end formation is inefficient, causing transcription termination to be inefficient. As a consequence, transcription of the downstream 1-globin gene is compromised, and the 2-globin transcript is unstable.