In 1977 Sharp and his team discovered that the messenger RNA (mRNA) of an adenovirus corresponded to four separate, discontinuous segments of DNA. They found that the segments of DNA that coded for proteins, now called exons, were separated by long stretches of DNA, now called introns, that did not contain genetic information. At the same time, a team working independently under Roberts came up with the same finding. Previously biologists had believed that genes were continuous stretches of DNA that served as direct templates for mRNA in the assembly of proteins; this model was based on studies of prokaryotic organisms such as bacteria. Following the discovery of Sharp and Roberts, it was demonstrated that the discontinuous gene structure is the most common one found in eukaryotes, among which are all higher organisms, including human beings.
Subsequent studies, many of them carried out in Sharp's laboratory,
showed that DNA transcription initially produces a precursor RNA molecule
containing copies of the introns; the final mRNA molecule is produced
by the removal of the introns and the splicing together of the exons.
A significant proportion of genetic diseases result from mutations that
arise during splicing. Biologists also believe that the discontinuous
structure of genes may be an evolutionary mechanism, permitting the
"reshuffling" of exons and thus the synthesis of new proteins.
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