one of the principal discoveries of modern molecular biology has been that genes come in pieces. More precisely, in higher organisms, the sequence of dna that encodes a protein message corresponds to the protein called exons, interspersed with portions called introns. The introns are cut and removed between the production of an initial rna transcript and its packaging to give rise to a final message.

It is not clear why the information required to make a protein is spread over different segments of dna because many cases are known of genes occurring as single contiguous stretches in bacteria and other prokaryotes. One school of thought holds that introns are just