Question

How are the eukaryotic primary mRNA transcripts processed before they can be transported from the cell nucleus to the ribosomes in the cytosol?

Answer 1

Answer :-

The RNA polymerase Il synthesis the primary mRNA { it also called hnRNA ( heterogenous nuclear RNA)} during eukaryotic transcription.

The primary transcript mRNA undergoes many alterations - additions,terminal base, base modification, splicing, which are collectively called post transcriptional modification .

Process in which primary transcript mRNA convert into funcational mRNA :-

1. The 5' capping :- During 5' capping, the 5' end of primary transcript mRNA is capped with 7-methyl guanosine by an unusual 5'-5' triphosphate linkage. S-adenosyl methionine is the donor of methyl group.

2. Poly A tail :- In tailing, Adenylate residues ( 200 - 300) are added at 3'-end in template independent manner. The poly A tail protects the mRNA molecule in cytoplasm from enzymatic degradation

3. Removal of Introns :-

Primary transcript mRNA contain both exon and the Introns.

Exons of mRNA possess genetic code and responsible for protein synthesis. Introns do not code for proteins so it is non funcational.

The process in which removal of Introns and exons are joined in a defined order is called spicing.

The post transcriptional modification of mRNA occur in the nucleus. The mature RNA then enters the cytoplasm to perform the protein synthesis.