RNA Processing: Definition, Steps, Types and Examples

What is RNA processing?

RNA processing is an important post-transcriptional modification, which converts a primary transcript of RNA into a mature RNA molecule that is expected to code for protein. It is a very crucial process in the appropriate expression of material genomics. There are introduced several sets of biochemical alterations important to the stability, exportation, and functionality of the nucleic acid.

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Viewed from this perspective, RNA processing turns out to be viably important in gene expression modulation. Cellular alteration of RNA transcripts allows representing a suitable mechanism for it to regulate the expression of a given protein in terms of its time, and quantity. This is rather essential in supporting cellular activity and responding to new environmental factors.

The key steps in RNA processing include capping, tailing, splicing, and at times, editing. Each of those steps has specific enzymatic actions that are effected to habituate the RNA molecule to proper processing and good functioning in translation into proteins.

Transcription: The First Step

The first process of gene expression is transcription, where the DNA is copied into a pre-mRNA by the RNA polymerase enzyme. It can be broadly divided into three stages:

Initiation

Binding of RNA polymerase with the promoter region of DNA and initiation to unwind the strands of DNA. Elongation: As the RNA polymerase moves along the template strand, it synthesizes a complementary RNA strand by the addition of ribonucleosides. Termination: After the transcription of the whole gene, this newly synthesized pre-mRNA would be released along with the release of RNA polymerase from DNA.

This pre-mRNA contains both introns and exons and is thus more or less a bit similar to the final mRNA. More processing is needed for it to be used in the body.

Primary Transcript Processing

This primary transcript is a copy of the gene's DNA sequence and thus contains all information that is contained within a gene. Additionally, it contains exons and introns. The features of this make extensive processing necessary before it is translated into a protein. This is to ensure that the mature mRNA will encode only for the coding regions, the exons and splice out the non-coding ones known as introns.

Capping

5' capping The process in which a modified guanine nucleotide, known as 7-methylguanosine, is added to the 5' end of the preRNA. Initiation has just occurred and this capping event happens very rapidly. The series capping takes the following sequence:-

• Hydrolysis: Getting rid of one phosphate group of the 5′ end of the nascent RNA.

• Guanylation: Attaching a guanosine monophosphate to this 5′ diphosphate end.

• Methylation: Contains a guanine nucleotide The nucleotide is methylated at the N-7 position, to have a cap structure.

The 5' cap has several important functions:

• Methylated nucleotide prevents the mRNA from being broken down by exonucleases that act on the 5' end.

• The cap-binding complex recognises the cap, which is recognised by the eukaryotic initiation factors as a part of translation initiation that aids in the binding of ribosomes to mRNA.

• The 5′ cap functions in exporting mRNA from the nucleus to the cytoplasm by interacting with nuclear export receptors.

Enzymes Involved in Capping: The capping process is catalysed by a capping enzyme complex that contains RNA triphosphatase, guanylyltransferase, and methyl transferase activities.

Tailing

The Addition of Poly-A tail at the 3' End: In the process, a chain of adenine nucleotides will have to bind with the 3' end of pre-mRNA. This method of binding is referred to as polyadenylation, and it often accounts for about 200 to 250 adenine residues.

Role of PolThis poly(A) tail protects mRNA from exonucleases whose function involves the degrading of RNA through a pathway from the 3'end.

The poly(A) tail can serve as a nuclear export signal, as there are poly(A) binding proteins recognized by nuclear export machinery, which mediate mRNA export from the nucleoplasm into the cytoplasm.

RNA Splicing

RNA splicing can be described below:

Introns And Exons:

While exons are the genes corresponding to coding sequences joined in the mature mRNA, introns are discarded sequences in genes. That simply means introns vary to a great extent in their length and sequence, while exons are relatively conserved and shorter.

Mechanism Of Splicing:

The spliceosome is a single large RNA-protein complex that carries out both of those reactions: intron removal and exon joining. It includes not only the small nuclear RNAs, the snRNAs, but also associated proteins that form small nuclear ribonucleoproteins, or snRNPs.

Splice Site Recognition: The spliceosome recognizes the boundaries of the intron. These are the 5′ splice site, also called the donor site and the 3′ splice site called the acceptor site.

Formation of the Branch Point: A branch point is identified within the intron that usually consists of an adenine nucleotide.

Formation of Lariat: In the 5' splice site cleavage, the 5' end of the intron may become attached covalently to the adenine of the branch point, forming a lariat structure.

Exon Ligation: At the 3' splice site, cutting occurs and the intron lariat is released, and thereby the exons are ligated in the mature mRNA.

Alternative Splicing

By including or excluding different sets of exons in one gene, one may get different variants of proteins. This will in all probability give a fillip to an increase in the degree of protein diversity from one gene.

Examples Of Alternative Splicing:

Immunoglobulins: single pre-mRNA gives rise to several antibodies because the exons encoding different regions of the antibody were differentially included or excluded.

Tropomyosin: this protein participates in the muscle contraction process. Once again, the alternative splicing gives rise to different isoforms of this protein. The isoforms show tissue specificity.

RNA Editing

RNA editing is a post-transcriptional modification in which the nucleotide sequence of an RNA molecule is altered. It changes codons in mRNA and, therefore, may lead to the expression of a distinctly different protein.

Rna Editing Mechanisms:

ADAR (Adenosine Deaminase Acting on RNA): It catalyses the hydrolytic deamination of adenosine to inosine, which during translation is recognised as guanine by the translating ribosome.

Cytidine Deaminases: These enzymes mediate the deamination of cytidine to uridine (C-to-U editing).

RNA Transport

The processed mature mRNA is then allowed to pass into the cytoplasm for translation.

The nuclear pore complex allows the export of mRNA by selectively and rapidly transporting millions of mRNA-protein complexes through the nuclear envelope.

This large, multimeric protein complex of NPC regulates and coordinates the transport process for molecules between the nucleus and the cytoplasm and furnishes a filter through which mRNAs can pass only after they are done with processing.

Conclusion

RNA processing is relevant in the correct expression of genetic information since, in RNA molecules, it confers stability, the right modifications, and the activity required for protein synthesis. It forms an important part of gene expression regulation and has significant ramifications in this way for cellular function and organismal development. An understanding of RNA processing offers great insight into a large number of diseases and possible therapeutic interventions.