Question

. Describe how the primary RNA transcript is modified in the nucleus of eukaryotic cells (unlike mRNA in prokaryotes) before exported to the cytoplasm for translation by ribosomes.

Answer 1

The process of formation of RNA using DNA as a template is called as transcription. It takes place in nucleus of eukaryotes. It uses enzyme DNA dependent RNA polymerase. The RNA formed by this process is premature or primary RNA transcript. Before it is exported into cytoplasm, it is converted into mature RNA transcript by the process of RNA processing.

The first step of this process is the addition of a 7 methyl guanosine residue at 5' end. This is called as capping. The two sugars which are immediately present after the cap are phosphorylated. It occurs with the help of capping enzyme.

The second step of this process is the addition of multiple residues of adenine at 3' end. This is called as tailing. Cleavage and polyadenylation stimulating factor (CPSF) and cleavage stimulating factor (CStF) cleave the messenger RNA at polyA site. Than enzyme polyA polymerase adds many residues of adenine.

The third step of this process is the removal of non coding sequences called as introns and joining together of coding sequences called as exons. This is called as splicing. It can either occur in a constitutive way, constitutive splicing, in which all the introns are removed and all the exons are joined in one to one manner. But most of the time it occurs alternatively, which is called as alternative splicing. In Alternative splicing, exons and introns are joined in different combinations.

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