Question

Where do the removed introns go?

Answer 1

Most of the spliced out introns are immediately degraded. During the spliceosomal splicing introns are released as lariat intermediates. The first step in intron degradation is the clevage of the 2'-5' phosphoester linkage that connects the branch site Adenine residue with the 5'-phosphate of the intron. A debranching enzyme cleaves this branch. Then the intron is degraded by the activity of other nucleases.

In some cases the removed introns perform some funtions. The following are some examples.

Numerous spliced out introns function as miRNAs that are involved in various gene regulation mechanisms. Over 100 such intronic miRNAs have been identified using the bioinformatic tools.

A large majority of snoRNAs i(small nucleolar RNAs) involved in the processong of pre-rRNA are spliced out introns of genes encoding functional mRNAs. These genes invariably encode proteins involved in ribosome synthesis or translation. Some snoRNAs are introns spliced out non-functional mRNAs. The genes encoding these mRNAs seem to exist only to express snoRNAs from excised introns.