Question

In regard to RNA modification, explain how RNA editing can be both advantages and disadvantages. You can focus on pre-mRNA to mature mRNA...

Note: The answer should fulfil a 3rd-year biochemistry major.

Answer 1

• For translation, mRNA moves to the cytoplasm, where it is subjected to constant degradation. Thus, Post-transcriptional processing of mRNA is essential.
• The half-life depends on the stability of the mRNA, that in turn is determined by capping (methylation) at 5’ end and poly A tail (polyadenylation) at 3’ end.
• Many eukaryotic gene contain, regions of “expressed sequence” or exons, which code for proteins and non-coding or” intervening sequence” or introns.
• The pre-RNA transcript requires splicing process, by which introns are removed, exons are joined to form mature RNA. The process of splicing is mediated by small nuclear ribonuclear proteins (snRNPs or SNURPS).
• Thus, the mRNA obtained by transcription is the pre-mRNA, which is converted to mature RNA by RNA processing.
• The proteins interact specifically with snRNA or small nuclear RNA for pre-mRNA processing.
• snRNA has specific sequences, based on which they are designated as U1 to U6.
• The U1 snRNA help in initiation of RNA processing.
• They can base paor with the pre-mRNA and protect it from adenylation or cleavage.
• They also form recognition point for 5’ capping point.
• The sequence specificity also determines the binding point of SNURPS for splicing initiation.

Three RNA processing eukaryotes, includes:

a. Capping- modification of pre-mRNA at 5’ end, by methylation.

Capping function:

• Aid in splicing of pre m-RNA.
• Help in transfer of mRNA from nucleus to cytoplasm.
• Protect mRNA from enzymatic degradation (like nucleases, phosphatases), when mRNA enters the cytoplasm.
• Help in initiation of translation by signaling ribosome attachment.

B. Poly A tail- Ploy Adenylation (A-A-A-A) at 3’ end of mRNA.

Poly A tail functions:

• Maintains stability of mRNA.
• Helps in splicing.
• Helps in crossing of mRNA from nucleus to cytoplasm.
• Helps in translation.

C. Splicing- removal of introns and joining of exons.

• The gene comprises of the exons, which are the translated regions and the introns, which are the untranslated regions. However, the introns determine the different products of the gene. The introns generally are removed during splicing. But by variation in the splicing process, different exon sequences might be obtained.
• The process of alternative splicing, the gene products may be modified. In alternate splicing alternative introns may be removed to get a different sequence producing an alternative protein.
• Thus, the introns and exons that are spliced, or the retained sequence after the splicing will express differently specifying different cell types.