Further Proof "Junk DNA" Has Value
February 12, 2008
The prevalence of noncoding regions of DNA in the genome of humans and many other eukaryotic organisms has long been a subject of controversy. DNA is composed of alternating areas called exons and introns. When DNA is transcribed to mRNA (the first step of protein synthesis), the introns (from "intragenic regions") are spliced out before the final mRNA sequence is formed. The exons become part of the mRNA and can code for amino acids involved in protein formation. The function of introns has been a mystery, and their ostensible superfluousness has caused some to refer to them as “junk DNA”.
Numerous hypotheses have been developed to explain the existence of introns. Some have suggested they are merely relics of old genes that no longer have any use to us, and thus have become non-functional. Others have postulated that junk DNA provides a buffer around integral genes to save them from being cut when portions of chromosomes “cross over” in meiosis. But many other scientists have not been satisfied with the suggestion that such a large portion of a genome (by some estimates up to 97%) would have a passive, or even meaningless, role.
Included in that group is a team of researchers at the University of Pennsylvania School of Medicine, who have discovered an important role in cellular function that is played by an intron. In 2005, they found that dendrites, the branch-like arms of a neuron that receive input from other neural cells, have the ability to splice mRNA. This was previously thought to occur only in the nucleus of cells. More recently, the group discovered an mRNA outside the nucleus that contains an intron. The mRNA encodes for a protein important to the functioning of the dendrite.
When the group removed the intron from the mRNA and left a spliced RNA molecule in the cell, the electrical properties of the cell became irregular. They believe the intron plays an integral role in guiding the mRNA to the dendrite, and may be involved in determining how many mRNAs are brought there to form electrically conducting channels. To serve this function, the intron may be spliced out of the mRNA by the dendrite and then incorporated into the dendrite itself. The details are not yet certain, but what is clear is this particular intron has an essential role in the cell, thus bringing the moniker “junk DNA” further into question, and inviting more research into the greater part of our genome.