Describe the molecular mechanism of RNA modification.

Explanation

The processing of pre-mRNA to mature mRNA/Molecular mechanism of RNA modification.
In eukaryotes three major types of RNA, mRNA, tRNA and rRNA are produced from a precursor RNA molecule termed as the primary transcript or preRNA. The RNA polymerase II transcribes the precursor of mRNA, which are also called the heterogenous nuclear RNA or hnRNA which are processed in the nucleus before they are transported into the cytoplasm.
(i) Capping Modification at the 5' end of the primary RNA transcript (hn RNA) with methylguanosine triphosphate is called capping.
Purpose of capping
(a) Protects RNA from degradation.
(b) Capping plays an important role in removal of first intron in pre mRNA.
(c) It regulates the mRNA export from the nucleus into the cytoplasm.
(d) It helps in binding of mRNA to the ribosome.
(ii) Tailing/Polyadenylation
The 3' end of hnRNA is cleaved by an endonuclease and a string of adenine nucleotides is added to the 3' end of hnRNA (pre mRNA) is known as Poly (A) tail : Polyadenylation. This process is called tailing or polyadenylation.
Purpose of tailing:
(a) Translation of RNA transcript is facilitated.
(b) Helps in the synthesis of polypeptides.
(c) It enhances the mRNA stability in the cytoplasm.
The protein coding regions are not continuous in eukaryotes. Exons are the coding sequences or expressed sequences contain biological informations in the matured processed mRNA. Introns are intervening sequences, which are non- coding sequences (non-amino acid- coding sequences) that should be removed from a gene before the mRNA product is made. These exons and introns are known as Split Genes.
(iii) RNA splicing in plants
(a) RNA splicing is a process which involves the cutting or removing out of introns and knitting of exons. This process takes place in spherical particles which is a multiprotein complex called SPLICISOMES. It is approximately 40 - 60 nm in diameter.
(b) A spliceosome removes the introns with an enzyme ribozyme. Now the mature mRNA comes away from the spliceosomes through the nuclear pore and is transported out from the nucleus into cytoplasm, and gets attached to ribosome to carry out translation.