What is the role of introns? Why are they here in our genes? Almost 20 years later we still do not have definitive answers, even though some DNA databases now contain around megabases of sequences--that is, strings of genetic code that represent million chemical letters of our genome. There are at least five different types of introns. Some of them are ribozymes, RNA molecules that are catalytically active, meaning that they facilitate certain chemical reactions; some of these ribozymes are able to perform a reaction in which they splice themselves out of the original transcript.
The most common type of intron is called a spliceosomal or nuclear intron; the name comes from the cellular machinery, known as the spliceosome, which is responsible for splicing and making sure that the genetic sequences in introns are not translated into junk proteins. This type of intron is the one found in the nuclear genes of humans. Simple prokaryotes and eukaryotes such as fungi and protozoa lack them. In complex multicellular organisms such as plants and vertebrates , introns are about fold longer than the exons, the active, coding parts of the genome.
The sequence and length of introns vary rapidly over evolutionary time. Scientists have found clear examples of 'functional nuclear introns' that can accommodate sequences important for the expression of the gene on which the intron resides. This function is not a general feature of introns, however, because several genes that lack introns express themselves normally histones and olfactory receptor genes, for instance.
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Volume Article Contents Abstract. Materials and Methods. Supplementary Material. Xin Hong , Xin Hong. Department of Biology, Indiana University. Oxford Academic. Douglas G. Michael Lynch. Kenneth Wolfe, Associate Editor.
Select Format Select format. Permissions Icon Permissions. Abstract Most research concerning the evolution of introns has largely considered introns within coding sequences CDSs , without regard for introns located within untranslated regions UTRs of genes. Open in new tab. Open in new tab Download slide.
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