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NCERT covers RNA processing in a single paragraph, so a question can quote it almost word for word and change just one word. Our post on DNA replication and transcription gives the whole process in outline. This one stays with what happens after transcription: the three processing steps, how split genes were found, a worked calculation of transcript lengths, and six statements that are easy to get wrong.
What does NCERT say?
The Class 12 chapter Molecular Basis of Inheritance (2026-27 reprint) defines exons as sequences "that appear in mature or processed RNA" and says introns "do not appear in mature or processed RNA". RNA polymerase II "transcribes precursor of mRNA, the heterogeneous nuclear RNA (hnRNA)". The primary transcripts "contain both the exons and the introns and are non-functional". Then:
Splicing: "the introns are removed and exons are joined in a defined order".
Capping: "an unusual nucleotide (methyl guanosine triphosphate) is added to the 5'-end of hnRNA".
Tailing: "adenylate residues (200-300) are added at 3'-end in a template independent manner".
Only "the fully processed hnRNA, now called mRNA" leaves the nucleus for translation. In bacteria, NCERT notes, mRNA "does not require any processing", and transcription and translation can be coupled.
The NEET (UG) 2026 syllabus lists molecular basis of inheritance with "Central dogma; Transcription, genetic code, translation" and gene expression and regulation.
How were split genes discovered?
In 1977 two groups working on adenovirus found that a single mRNA was copied from separate stretches of DNA. Berget, Moore and Sharp hybridised a viral mRNA to viral DNA and saw that 160 nucleotides at its 5' end did not pair with the DNA next to the rest of the message. They proposed that "four segments of viral RNA may be joined together" during maturation of a larger precursor. Chow, Gelinas, Broker and Roberts90180-5) reported the same pattern in Cell under the title "An amazing sequence arrangement at the 5' ends of adenovirus 2 messenger RNA".
The 1993 Nobel Prize in Physiology or Medicine went to Richard J. Roberts and Phillip A. Sharp "for their discoveries of split genes". The Nobel press release describes how, in the electron microscope, the DNA segments with no counterpart in the RNA appeared as loops, and adds that through alternative splicing "the same DNA region can in many cases determine the structure of many different proteins".
Worked example: how long is each RNA?
Sample numbers for a hypothetical gene. From the start to the end of the transcribed region it has: exon 1, 180 nucleotides; intron 1, 420; exon 2, 150; intron 2, 900; exon 3, 270.
Primary transcript (hnRNA): 180 + 420 + 150 + 900 + 270 = 1920 nucleotides.
After splicing: exons only, 180 + 150 + 270 = 600 nucleotides. The introns, 1320 nucleotides, were 68.75% of the transcript.
After capping and tailing: add one cap nucleotide and, say, 250 adenylates (within NCERT's 200 to 300): 600 + 1 + 250 = 851 nucleotides.
Protein length: if 450 of the exon nucleotides are the coding region, including the stop codon, that is 450 ÷ 3 = 150 codons and 149 amino acids, since the stop codon codes for none.
Hybridisation picture: pair this mRNA with the gene's DNA and the two introns, 420 and 900 bases, loop out, just as Roberts and Sharp saw.
Which statements are easy to get wrong?
"The poly-A tail is copied from a poly-T stretch in the gene." False: NCERT says tailing is template independent.
"Introns are removed from the DNA." False: they are removed from the RNA; the gene keeps them.
"Bacterial mRNA is spliced before translation." False: it needs no processing.
"The cap is added at the 3' end." False: cap at 5', tail at 3'.
"RNA polymerase III makes hnRNA." False: polymerase II does; III makes tRNA, 5S rRNA and snRNAs.
"hnRNA is functional." False: NCERT calls primary transcripts non-functional.
When the order of events is hazy, the Fermi Concept Library covers every concept in physics, chemistry, biology, mathematics and computing, each with its own lesson, so you can rebuild the sequence before testing yourself.
A practice routine
Write the processing paragraph from memory, then compare it with NCERT and underline every word you missed.
Redo the worked example with your own exon and intron lengths, and include a tail at both ends of NCERT's range.
Turn each statement in the list above into a true version.
Draw a DNA–mRNA hybrid for a gene with three introns and label every loop.
To follow mRNA into the ribosome, continue with the genetic code and translation. Tonight, do step 1 and correct it against the textbook.
