Exons and Introns

Exons and Introns

Exons and introns refer to specific nucleotide base sequences in the genetic code that are involved in producing proteins. Exons are the DNA bases that are transcribed into mRNA and eventually code for amino acids in the proteins. Introns are DNA bases, which are found between exons, but are not transcribed. Genes which contain introns are known as interrupted genes.

The discovery of introns and exons occurred independently in 1977 by American molecular biologists Richard Roberts and Phillip Sharp. Prior to this time, it was thought that eukaryote genes were strange because they contained more DNA than was transcribed into RNA. The two scientists ran experiments which attempted to identify DNA from the resulting mRNA. It was assumed that the mRNA would have the same base sequence as the DNA from which it was transcribed. This, however, was not the case. Roberts and Sharp found stretches of DNA sequences that were not part of the mRNA. Further, these sequences were interspaced between coding sequences thereby interrupting the code. These data led to the description of exons, the coding DNA, and introns, the interrupting DNA. For their work, Roberts and Sharp shared in the 1993 Nobel Prize in physiology.

In most living systems, genes are made up of nucleic acid sequences which are translated into mRNA and then into proteins. In eukaryotic cells, the interrupted genes have a sequence with four different regions including a regulatory region, exons, introns and a stop sequence. The proportion of interrupted genes varies with each organism. Simple organisms such as yeasts have very few interrupted genes. In more complex organisms like mammals, almost every gene has an intron.

While each section is important, the exons are the sequences that actually code for proteins. The number of exons that code for a protein vary. Some proteins may have three or four exons but others can have 30 or 40. These differences are found in most species.

The introns are interspaced between the exons in an alternating fashion. Their nucleic acid sequence is highly variable and can be as short as a few dozens bases or as long as a few hundred. One thing that is constant, however, is the sequences at the beginning and end of the intron. These capping sequences are of importance when the gene is transcribed into mRNA.

When a gene is transcribed into RNA, the entire sequence, including the introns, is copied. This primary transcript of RNA is further processed to produce the protein coding mRNA. In this process, the exons are spliced together by a series of enzymes. First, the ends of exons are brought together. Then the introns are removed and the exons are chemically bonded.

Since introns are not translated into proteins, scientists have tried to determine how and why they evolved. While theories about their evolution are complex, it is generally believed that on primeval Earth, both introns and exons existed and were formed by the random combination of nucleotides. Exons helped code for proteins so they were incorporated into living organisms. Introns were also incorporated, perhaps randomly, and are thought to play a regulatory role in cell activity.