Splicing

In eukaryotes, pre-mRNA or primary transcripts need to undergo some modifications before the mRNA sequence is functional for translation. One of the modifications is RNA Splicing. The splicing process is done by a complex of small proteins and small nuclear RNAs (snRNAs) called 'spliceosomes'. 

Splicing involves the removal of noncoding sequences, 'introns', that interrupt the coding sequence of the gene. Length of these 'introns' vary between a single nucleotide to 10,000 nucleotide bases. Once the 'introns' are removed, the remaining cluster of sequences are attached together, forming a complete mRNA that is now a functional gene sequence. 

Self - Splicing

RNA's have the ability to self - splyce. Thomas Cech and his coworkers studied a Tetrahymena (a ciliated protozoan), in which a 414 - nucleotide intron is removed form a 6.4 - kb precursor. This experiment established, that RNA under certain conditions can be a ribozyme and is catalytic. Since this experiment, more than 1500 similar introns havee been found in bacteria and eukaryotes. These intron are now reffered to as group I introns. The self - splicing reaction requires an additional guanosine nucleotide. At first, nucleotides were added in the reaction mixture because it was thought that certain energy molecules (ATP or GTP) could be needed as an energy source. However, it was established that the nucleotides were necessary as cofactors. Specifically, the main cofactor required was a guanosine unit in the form of GMP. GDP or GTP. Despite being in the form of an energy molecule, guanosine is not needed as an energy source, but instead as an attacking group that becomes transiently incorporated into the RNA.

Alternative Splicing

Splicing can be done differently. For example, some 'introns' may be removed and the 'exons' rearranged before joining together. This gives rise to different translation products for one fragment of pre-mRNA.