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(Left) Classical textbook picture in which all pre-mRNA processing reactions are depicted as posttranscriptional (cf. Alberts et al. 2002, Figs. 6–21). (Right) pre-mRNA processing is cotranscriptional. In the depicted pre-mRNA molecule, splicing of intron 1 has already occurred, introns 2 and 3 are being processed, and exon 4 has not been transcribed yet.
