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In bacterial cells most proteins are encoded by a single uninterrupted stretch of DNA sequence that is copied without alteration to produce an mRNA molecule. In 1977 molecular biologists were astonished by the discovery that most eucaryotic genes have their coding sequences (called exons) interrupted by noncoding sequences (called introns). To produce a protein, the entire length of the gene, including both its introns and its exons, is first transcribed into a very large RNA molecule - the primary transcript. Before this RNA molecule leaves the nucleus, a complex of RNA-processing enzymes removes all of the intron sequences, thereby producing a much shorter RNA molecule. After this RNA-processing step, called RNA splicing, has been completed, the RNA molecule moves to the cytoplasm as an mRNA molecule that directs the synthesis of a particular protein (see Figure 3-15).
This seemingly wasteful mode of information transfer in eucaryotes is presumed to have evolved because it makes protein synthesis much more versatile. The primary RNA transcripts of some genes, for example, can be spliced in various ways to produce different mRNAs, depending on the cell type or stage of development. This allows different proteins to be produced from the same gene. Moreover, because the presence of numerous introns facilitates genetic recombination events between exons, this type of gene arrangement is likely to have been profoundly important in the early evolutionary history of genes, speeding up the process whereby organisms evolve new proteins from parts of preexisting ones instead of evolving totally new amino acid sequences.