Post-Transcriptional Modifications: Capping, Tailing and Splicing
Biology · Molecular Basis of Inheritance · NEET
In eukaryotes the first RNA made (hnRNA) is not ready to use. It is changed in three steps: splicing (introns are removed and exons are joined), capping (a special nucleotide, methyl guanosine triphosphate, is added to the 5' end), and tailing (about 200-300 adenine residues are added to the 3' end). Memory hook: "5' gets a CAP on its head, 3' gets a TAIL, and introns get SPLICED out."
hnRNA has exons (coding) and introns (non-coding). Splicing removes introns and joins exons. Then a methyl guanosine cap is added to the 5' end (capping) and a poly-A tail to the 3' end (tailing), giving mature mRNA.
Your doubts, answered
Is the cap added at the 5' end or the 3' end?
The cap is added at the 5' end. In capping, an unusual nucleotide called methyl guanosine triphosphate is joined to the 5' end of the hnRNA. NEET loves to swap this to '3' end' as a trap. Remember: Cap first, so it goes on the front (5') end. The tail (adenine residues) goes on the 3' end.
In splicing, are introns or exons removed?
Introns are removed and exons are joined. Exons are the coding (expressed) sequences that stay in the mature mRNA. Introns are the intervening non-coding sequences that are cut out. Memory trick: EXons EXit into the mRNA (they stay); INtrons stay IN the nucleus (they are thrown out).
What is the difference between hnRNA and mRNA?
hnRNA (heterogeneous nuclear RNA) is the primary transcript made by RNA polymerase II. It still has both exons and introns, so it is non-functional. After splicing, capping and tailing, it becomes the fully processed, functional mRNA that leaves the nucleus for translation.
Do prokaryotes (bacteria) do capping, tailing and splicing?
No. In bacteria the mRNA does not need any processing to become active. Bacterial genes are not split (no introns), so there are no spliceosomes and no capping/tailing. This is why spliceosomes are absent in bacteria but present in plants, fungi and animals (all eukaryotes).
What is added in tailing?
In tailing, adenylate residues (about 200 to 300 adenine nucleotides) are added to the 3' end in a template-independent manner. This is the poly-A tail. It is added only after transcription is complete, at the 3' end, not the 5' end.
⚠️ The NEET trap ✗ In capping, methyl guanosine triphosphate is added to the 3' end of hnRNA. ✓ In capping, methyl guanosine triphosphate (an unusual nucleotide) is added to the 5' end of hnRNA. The 3' end gets the poly-A tail (adenylate residues) during tailing. 🧠 5' = Cap, 3' = Tail. NTA flips the end number to trick you. Match the modification to the correct end every single time.
Real NEET questions
2025
Which of the following are the post-transcriptional events in an eukaryotic cell? A. Transport of pre-mRNA to cytoplasm prior to splicing B. Removing of introns and joining of exons C. Addition of methyl group at 5' end of hnRNA D. Addition of adenine residues at 3' end of hnRNA E. Base pairing of two complementary RNAs
A · B, C, E only
B · C, D, E only
C · A, B, C only
D · B, C, D only ✓
Solution: Post-transcriptional processing of hnRNA = splicing (remove introns, join exons = B), capping (methyl guanosine at 5' end = C) and tailing (adenylate residues at 3' end = D). A is false because splicing happens in the nucleus BEFORE transport to the cytoplasm. E (base pairing of two complementary RNAs) is RNA interference, not processing. So B, C, D only.
2021
Identify the correct statement. a. The coding strand in a transcription unit is copied to an mRNA b. Split gene arrangement is characteristic of prokaryotes c. In capping, methyl guanosine triphosphate is added to the 3' end of hnRNA d. RNA polymerase binds with Rho factor to terminate the process of transcription in bacteria
A · The coding strand in a transcription unit is copied to an mRNA
B · Split gene arrangement is characteristic of prokaryotes
C · In capping, methyl guanosine triphosphate is added to the 3' end of hnRNA
D · RNA polymerase binds with Rho factor to terminate the process of transcription in bacteria ✓
Solution: Only (d) is correct: RNA polymerase associates with the termination factor rho to end transcription in bacteria. The others are wrong - the template strand (not coding) is copied, split genes are a eukaryotic (not prokaryotic) feature, and in capping methyl guanosine triphosphate is added to the 5' end (not the 3' end). This is the classic capping-end trap.
2017
Spliceosomes are not found in cells of
A · Plants
B · Fungi
C · Animals
D · Bacteria ✓
Solution: Splicing (removal of introns) occurs only in eukaryotes whose genes are split. Bacteria (prokaryotes) have no introns and their mRNA needs no processing, so spliceosomes are absent. Plants, fungi and animals are all eukaryotes and do have spliceosomes.
Solved Molecular Basis of Inheritance NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Splicing is done by a large complex called the spliceosome, made of snRNPs (small nuclear ribonucleoproteins) which contain snRNAs (small nuclear RNAs). NEET often matches 'Splicing of Exons' with snRNPs.
Why is hnRNA called heterogeneous?
Because the primary transcripts vary greatly in length (they still contain both exons and introns of different sizes), the population is heterogeneous. It is transcribed by RNA polymerase II.
Does splicing prove the RNA world idea?
Yes, NCERT notes that the process of splicing represents the dominance of the RNA world, because RNA molecules (snRNAs) carry out this catalytic cutting and joining, not proteins.
What is the order of the three modifications?
For NEET remember the three events together: splicing (introns out, exons joined), capping (5' end) and tailing (3' end). All happen in the nucleus before the mature mRNA is exported to the cytoplasm.
