question_answer 78)

  Give an account of post transcriptional modifications of a eukaryoticm RNA.

Answer:

                  Post-transcriptional Modifications The primary transcripts are non-functional, containing both the coding region, exon and non-coding region, intron in RNA and are called heterogenous RNA or /mRNA. In eukaryotes, three types of RNA polymerases are found in the nucleus (i) RNA polymerase I transcribes rRNAs (28 S and 5.8 S). (ii) RNA polymerase II transcribes the precursor of mRNA (called heterogeneous nuclear RNA or/mRNA). (iii) RNA polymerase III transcribes t RNA, 5 SrRNA and snRNAs (small nuclear RNAs). The /mRNA undergoes two additional processes called capping and tailing. In capping, an unusual nucleotide, methylguanosine triphosphate is added to the 5'-end of/mRNA.In tailing, adenylate residues (about 200-300) are added at 3'-end in a template independent manner. Now the mRNA undergoes a process where the introns are removed and exons are joined to form mRNA by the process called splicing.     Note In prokaryotes, mRNA does not require any processing.