Structural Biochemistry/Nucleic Acid/RNA/RNA modification/Exons
Exons are protein-coding segments and one of the two major divisions of DNA that is transcribed into RNA. All genes start with exons which are often interrupted by introns (non-protein coding segments). Exons received their name because they exit the nucleus and allow the DNA sequences to be expressed (prefix ex-comes from "expressed")1. Exons are the actual part that contains codes for particular protein parts. The number of exons in DNA can vary from one species to another. Before the functional mRNA is formed, a splicing complex called spliceosome cleaves exons to bring them together, then removes all introns and connects exons to each other. Exons join together and travel out of the nucleus where they eventually code for proteins.
Exons were first discovered in 1977 by American molecular biologists and Nobel Prize winners (1993) Richard Roberts and Phillip Sharp. They used electron microscopy to study mRNA and DNA hybrids. In the absence of introns, the entire region that is hybridized to the mRNA would be displaced. In their experiment, they observed regions that were not displaced, creating a loop that is indicative of an intron. Initially it was assumed that the sequence of both DNA and mRNA were identical or continuous. The result of their experiments revealed that DNA had stretches of bases not present in mRNA. Based on that they explained the nature of exons.
Roles of exonsEdit
With the help of introns, exons can undergo recombination or exon shuffling. Crossovers occur in random, but homologous, positions at a frequency that depends on DNA length. Exon shuffling is a natural process that allows the formation of new functional proteins by creating new arrangements and thus new interactions with minimal risk to the sequence encoding of the functional parts. In the absence of introns, crossovers are likely to disrupt the exon sequence and often create a loss of function. Moreover, exon shuffling can produce new and useful proteins which lead to evolution. It encode different domains of the protein products with the processes of transcription, RNA processing, and translation.
1 Jerry Bergman, The Functions of Introns: From Junk DNA to Designed DNA