SPLIT GENES

During 1970, in some mammalian viruses (e.g. adenoviruses) it was found that the DNA sequences coding for a polypeptide were not present continuously but were split into several pieces. Therefore, these genes were variously named as split genes or introns interrupted genes or interventing sequences inserts junk DNA. For the discovery of split genes in adenoviruses and higher organisms, Richards J.Roberts and Phillip sharp were awarded Nobel Prize in 1993.

We can see a DNA sequence codes for m RNA but the complete corresponding sequence of DNA is not found in m RNA. Certain sequences of DNA are missing in m RNA. The sequences present in DNA but missing in m RNA are called intervening sequences or introns, and the sequences of DNA found in RNA are known as exons. The exons code for m RNA.

Before the discovery of split genes in 1977, all the genes analysed in detail were the bacterial genes. Bacteria were considered to resemble the simpler cell from which eukaryotes must have been evolved. Now, it is supposed that split genes are the ancient condition and bacteria lost their introns only after evolution of most of their proteins. Evidence for the ancient origin of introns has been obtained by the examination of the gene that encodes the ubiquitous enzyme, triose phosphate isomerase (TPI). The TPI is coded by a gene that contains six introns (in vertebrates), five of these are present at the same position as in maize. The TPI play a key role in cell metabolism that catalyses the interconversion of glycolysis and glucogenesis. By comparing this enzyme in various organisms it appears that the TPI evolved before the divergence of prokaryotes and eukaryotes from a common ancestor cell-i.e. progenote (Nyberg and Cronhjort, 1992).