Principles of Biochemistry Volume 3 - A. Lehninger 1985

Molecular mechanisms of genetic information transfer
DNA replication and transcription
Small nuclear RNAs help remove introns from RNA

Current data allow us to answer these questions. They indicate that the removal of untranslated introns during pre-mRNA Processing occurs in such a way that consecutive exons—that is, the coding fragments of mRNA—are never physically separated. Exons are joined together with high precision by molecules of another Class of RNA present in The Nucleus, known as small nuclear RNAs (snRNAs). The function of these short nuclear RNAs, consisting of approximately one hundred NUCLEOTIDES, remained unclear for a long time. It was elucidated after it was discovered that their nucleotide sequence is complementary to the sequences at the ends of each intron. As a result of base pairing between the snRNA and the ends of the looped-out intron, the sequences of the two exons are brought into close proximity, enabling the removal of the intervening intron and the enzymatic joining (splicing) of the coding fragments (exons). Thus, snRNA molecules act as temporary templates that hold the ends of the two exons in close proximity to ensure that splicing occurs at the correct site (Fig. 28-22). Structure/19.html">The Importance of this process lies in the fact that an error of even a single nucleotide during splicing can alter the reading frame in the sequence downstream of that point, leading to the synthesis of a defective protein molecule from such an mRNA.

Once all introns have been removed from the RNA in this manner, thereby completing pre-mRNA processing, the mature mRNA leaves the nucleus. To do this, the mRNA first binds to two specialized Proteins that escort it into the Cytoplasm through Pores in the nuclear envelope (Section 2.7). These pores, surrounded by a complex assembly of protein molecules, apparently permit only fully 'matured' mRNAs to exit the nucleus. RNA fragments remaining after processing are degraded by Nucleases. The resulting nucleoside 5'-monophosphates are converted into nucleoside 5'-triphosphates via ATP and are reused for RNA Synthesis in the nucleus.