How and why are mRNA molecules processed or modified before leaving the eukaryotic nucleus?

Much of our DNA is known as "noncoding," simply meaning it does not code for proteins required by the cell. When RNA polymerase transcribes DNA, however, it has no mechanism with which to recognize coding vs. noncoding regions. Therefore, it transcribes the entire region, producing a "pre-mRNA" transcript.

This pre-mRNA transcript contains both exons (coding regions) and "introns" (noncoding regions). A protein complex known as the spliceosome removes particular classes of introns (there are multiple mechanisms to remove introns). After they've been cleaved, the enzyme DNA ligase joins the intron segments to be produce a cohesive mRNA molecule.

Prior to exiting the nucleus to be translated in the cytosol, the mRNA has two more additions - a 5'cap, composed of guanine nucleotides and poly-A tail, composed of adenine nucleotides. The 5' cap serves various functions, including exit from the nucleus, protecting the mRNA from being degraded by nucleases, and in the binding of the mRNA to the ribosome. The poly-A tail also protects against degradation by nucleases and termination of translation.