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ONE GENE, MANY PROTEINS The classical view of gene expression was simple and consistent: a gene’s DNA sequence is first transcribed by cellular machinery into an RNA version, then that primary transcript is edited to remove “junk” stretches called introns, and only the information-bearing exons are included in the final messenger-RNA that is translated into a protein. As it turns out, these rules do not always apply. The genes of complex organisms can have their exons and introns mixed and matched, discarded and retained, in alternative combinations during editing, permitting one gene to encode a variety of different proteins, depending on how its information is edited. DNA gene CLASSIC GENE EXPRESSION A DNA sequence is transcribed into a single-strand copy made of RNA. Cellular splicing machinery then edits this primary transcript to remove introns, each of which is defined by distinctive nucleotide sequences at its beginning and end, known, respectively, as the 5 (“fiveprime”) and 3 (“three-prime”) splice sites. The exons are then spliced into a messenger-RNA version of the original gene that will be translated into a protein by the cell. Exon Intron Transcription 5’ 3’ Primary RNA transcript Splicing Intronic RNA Messenger RNA Translation Discarded Protein ALTERNATIVE SPLICING A gene’s primary transcript can be alternatively edited in several different ways, shown at right, where splicing activity is indicated by dashed lines. An exon may be left out (a). Splicing machinery may recognize alternative 5 ’ splice sites for an intron (b), or alternative 3 ’ splice sites (c). An intron may be retained in the final mRNA transcript (d). Exons may be retained on a mutually exclusive basis (e). a) EXON SKIPPING RESULTING mRNA b) ALTERNATIVE 5 ’ SPLICE SITES c) ALTERNATIVE 3’ SPLICE SITES d) INTRON RETENTION Exon always spliced in Exon alternatively spliced e) MUTUALLY EXCLUSIVE CREDIT Intron w w w. s c ia m . c o m SCIENTIFIC A MERIC A N 35