Introns Are There Because...

It is well-known that many eukaryotic pre-mRNA molecules have both introns and exons. The introns are removed from the mRNA, the exons are connected, and then the gene is expressed. A common question has passed through the minds of many. What exactly is the point of introns? What is their function? This article helps to answer these questions.
Introns have been called "junk DNA" but the fact that they are so prevalent and and have been preserved during evolution leads researchers to believe that introns serve some function.

Ashok Bidwai, an assistant professor in the department of biology at West Virginia University, explained how it is widely believed that introns are remnants of genetic sequences that once served as spacers between the stretches of DNA that coded for specific, simpler proteins.  As complex proteins were being evolved, regions of the genetic code (domains) may have been shuffled and brought together to generate new sequences that code for protein structures that took on new functions. This hypothesis is based on the observation that the relative positions of introns in genes remain largely the same in organisms from Drosophila melanogaster (the fruit fly) to Caenorhabditis elegans(a widely studied nematode) to mice and humans.
Some researchers have proposed that introns serve as a mechanism that selects for genes that will be expressed early during the development of an organism. There has not been much experimentation to support this hypothesis so its plausibility is uncertain.
Sandro J. de Souza, who works in Walter Gilbert's laboratory at Harvard University, expanded on the prevailing intron hypothesis. There are at least five different types of introns. Some are ribozymes. Some of these ribozymes can splice themselves out of the original transcript. The most common type of intron is called a spliceosomal or nuclear intron.This type of intron is the one found in the nuclear genes of humans.
Generally, nuclear introns are ubiquitous in complex eukaryotes. However, simple prokaryotes and eukaryotes lack introns. In complex multicellular organisms, introns are about ten times longer than the exons. The sequence and length of introns vary rapidly over evolutionary time.
Introns do sometimes have identifiable functions. Scientists have found 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 common feature of introns, though. There are also cases in which introns contain genes for small nuclear RNA. Nuclear introns can also be important in alternative splicing, which produces multiple types of messenger RNA from a single gene. Although these examples demonstrate roles for introns, they do not explain why introns are so ubiquitous in our genes.