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
Now the mRNA undergoes a process
where the introns are removed and exons are joined to form mRNA by the process
Note In prokaryotes, mRNA does not
require any processing.
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