Name the three RNA polymerases found in eukaryotic cells and mention their functions.

OR

Explain the post-transcriptional modifications the hn-RNA undergoes in eukaryotic cells.

The three RNA polymerases found in eukaryotic cells with their functions are as shown below:

(i) RNA Polymerase I – it is like a workhorse, they produce nearly 50% RNA transcribed in the cell. It exclusively polymerizes ribosomal RNA, i.e., synthesizes all the rRNAs except for the 5S rRNA molecule.

(ii) RNA polymerase II – It synthesizes all protein-coding nuclear mRNA precursors. It also catalyzes the formation of small nuclear RNAs and micro RNAs.

(iii) RNA polymerase III – It transcribes a variety of structural RNAs, transfer pre-RNAs (tRNAs) and small nuclear pre-RNAs. It is important as its targets for normal functioning of the cell.

(iv) RNA polymerase IV and RNA polymerase V – are usually found in plants. They are necessary in the formation of small interfering RNA and heterochromatin in the nucleus.

OR

The post transcriptional modifications help in the following ways:

(i) These modifications help the RNA molecule to be recognized by the molecules which convert RNA translation to proteins.

(ii) Here the part of RNA which is not converted to protein are sliced out of the sequence. This helps in increasing the efficiency of protein synthesis.

Without post-transcriptional modification synthesis of protein will be slowly downed.

In eukaryotic cells, there are three types of RNA polymerase found in the nucleus. The precursor of mRNA is also known as hnRNA (heterogeneous nuclear RNA). They contain both exon (coding region) and intron (non-coding region). The hn-RNAs are modified before they leave cytoplasm to join protein synthesis.

The introns are removed and exons are joined by splicing process. After this the hn-RNA undergoes two additional processes called capping and tailing.

Capping: In this methylguanosine triphosphate, an unusual nucleotide is added to the 5’-end of hn-RNA.

Tailing: In this adenylate residues are added to 3’-end of hn-RNA in template independent manner.

After all these processes, the mRNA is transported to nucleus.