RNA splicing is the process by which the newly synthesized pre-mRNA, also known as hnRNA, (heterogeneous nuclear RNA) is processed and forms the mature mRNA. hnRNA is processed in the nucleus and converted to mRNA, which then comes to the cytoplasm and undergoes translation or protein synthesis. It is a post-transcriptional modification.
In prokaryotes such as bacteria, the newly transcribed RNA is ready for translation and both the processes can even occur simultaneously in the mRNA. Most of the eukaryotic genes are transcribed in the form of pre-mRNA and have to be processed before undergoing protein synthesis.
In the RNA splicing process, the non-coding intervening regions called ‘introns’ are removed and the coding regions known as ‘exons’ are joined together. Spliceosome catalyses the RNA splicing process. Ribozymes (catalytic RNA) catalyse their own splicing.
Additionally, 5’ capping with the modified Guanine nucleotide and tailing with Poly-A (Adenylate) residues at 3’ end is also done to protect the coding segments and to provide stability to the mature mRNA.
RNA Splicing Process
In this process, introns are spliced out. RNA splicing is catalysed by spliceosomes, which is a protein-RNA complex, i.e. a complex of small nuclear ribonucleoproteins (snRNPs or snurps). It recognises and removes introns. Exons, which are the coding parts, are joined together.
Introns are removed at the specific sequences present at 5’ and 3’ ends of the introns known as splice sites.
Sometimes RNAs can be spliced differently giving rise to different mRNA molecules that code for different proteins. It is called alternative splicing.
Some of the genes undergo self-splicing, e.g. phage genes, protozoan ribosomal RNA genes, etc. Some mitochondrial genes are also capable of self-splicing.
Importance of RNA Splicing
- RNA splicing facilitates the formation of multiple functional mRNAs from a single transcript, which codes for different proteins.
- It also helps in the regulation of gene expression and protein content of the cell.
- It assists in the evolution process by forming different combinations of exons and thereby making new and improved proteins.
- New exons can be inserted into the introns to create new proteins without disrupting the functionality of the original gene.
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