Question

What is self- splicing?

Answer 1

RNA processing in which a newly made precursor mRNA is transformed into mature RNA by removal of non-coding sequences is called RNA splicing. The process includes removal of introns and joining of exons. The process takes place during or immediately after transcription within the nucleus in the case of nucleus encoded genes. Introns are non-coding DNA sequences that are removed by the process of splicing. They have a donor site(5’ end), an acceptor site(3’end), and a branch site( near 3’end) that are required for splicing. Exons are coding DNA sequences that contain necessary genetic information essential for protein synthesis.

Complete answer:
RNA splicing process is regulated by spliceosome which is a large and complex molecule formed of RNAs and proteins. Spliceosome consists of five small nuclear RNAs and about 80 protein molecules, the combination resulting in the formation of a complex as small nuclear ribonucleoprotein.
Spliceosomes are confined within the nucleus and remain associated with immature pre- RNA transcript. Spliceosome functions as an editor that cuts out unnecessary materials to produce a functional final cut. All spliceosomes are involved in removal of introns and ligation of exons.
Minor spliceosomes are found in eukaryotic cells which have less abundant RNAs and are involved in the splicing of a rare class of pre-mRNA introns.
A type of RNA splicing which occurs in some rare introns that are capable of phosphodiester cleavage and formation without the help of other proteins or spliceosomes. These introns are unique in the context that they can mediate their excision from precursor RNA and the subsequent ligation of the flanking exons. Tertiary structure of introns, which recognize the splice site of precursor RNA and perform the cutting and ligation reaction in a very precise manner.
Three types of self-splicing introns are grouped as Group I , Group II and Group III introns. The mechanism of Group I and Group II introns are similar to that of spliceosomes. During the process of self-splicing, the 5’ splice site is recognized by a short sequence element in the intron called the internal guide sequence. Other strongly conserved introns sequences P, Q, R and S are needed to catalyze the cutting and ligation reactions.
Self splicing involves two trans-esterification reactions resulting in the removal of introns and ligation of exons.

Note:
RNA splicing ensures that an immature RNA molecule is converted into a mature molecule that can be trans-located into protein. That’s why RNA splicing is crucial in eukaryotic cells. For prokaryotic cells, post transcriptional modification is not necessary. RNA splicing is a controlled process being regulated by a various ribonucleoprotein.