Q.Describe the molecular mechanism of RNA modification.

A eukaryotic gene's protein-coding sequence is interrupted by non-coding introns (discovered by Roberts and Sharp, 1977 work, 1993 Nobel Prize), while the coding, retained segments are exons. RNA splicing removes the introns from the primary transcript and joins the exons into one continuous coding sequence, carried out inside a large multi-protein machine, the spliceosome (about 40-60 nm across), built from small nuclear ribonucleoprotein particles (snRNPs, combining snRNAs with proteins) that recognise intron boundaries and use a ribozyme to catalyse removal; the mature mRNA then exits the nucleus through a nuclear pore (an energy-dependent process) to reach the ribosomes. Alternative splicing lets one gene generate multiple distinct mature mRNAs, and so multiple protein isoforms, by removing introns (and sometimes even exons) in different combinations from one primary transcript; this expands protein diversity from a fixed gene count, generates multiple related gene products from a single gene, and in plants specifically plays an important role in responding to environmental stress. RNA editing is a further post-transcriptional chemical alteration -- base modification, or nucleotide insertion/deletion -- that changes an mRNA's coding capacity without touching the underlying DNA; in plants it is confined to the mitochondrial and chloroplast (plastid) genomes and coordinates their gene expre …