Question

Give the different steps involved in formation of m-RNA from hn-RNA.

Hint:

Remember the processing with the mnemonic \textbf{C-S-T}: \textbf{C}apping at the 5' head, \textbf{S}plicing the introns out from the middle, and \textbf{T}ailing at the 3' end.

Answer 1

The formation of a mature messenger RNA (m-RNA) from the primary transcript, heterogeneous nuclear RNA (hn-RNA), in eukaryotes is called post-transcriptional processing. It involves three main steps:

Capping: A modified nucleotide, methyl guanosine triphosphate, is added to the 5' end of the hn-RNA. This "cap" is essential for protecting the m-RNA from degradation by ribonucleases, and it also helps in the binding of the ribosome to the m-RNA during translation.
Splicing: The hn-RNA contains both coding sequences, called exons, and non-coding sequences, called introns. In splicing, the introns are precisely removed, and the exons are joined together in the correct sequence. This process is carried out by a complex of proteins and RNA called the spliceosome.
Tailing (Polyadenylation): A tail of about 200-300 adenylate residues (poly-A tail) is added to the 3' end of the hn-RNA. This tail also protects the m-RNA from degradation, helps in its transport from the nucleus to the cytoplasm, and plays a role in initiating translation.
After these modifications, the hn-RNA is now considered a mature and functional m-RNA, ready for translation.