Question
Describe the process of translation (protein synthesis). Explain the roles of mRNA, tRNA, and ribosomes in the three stages: initiation, elongation, and termination.
(NCERT Class 12, high-frequency NEET question)
Solution — Step by Step
- mRNA: Carries the genetic code from DNA. The code is read in triplets called codons (e.g., AUG = methionine).
- tRNA: The adaptor molecule. One end has an anticodon (complementary to the mRNA codon), and the other end carries the corresponding amino acid. Each tRNA is “charged” by a specific aminoacyl-tRNA synthetase.
- Ribosome: The molecular machine where translation occurs. Has a small subunit (reads mRNA) and a large subunit (catalyses peptide bond formation). Contains three sites: A site (aminoacyl), P site (peptidyl), E site (exit).
- The small ribosomal subunit binds to the mRNA near the 5’ end and scans for the start codon (AUG).
- The initiator tRNA carrying methionine (fMet in prokaryotes) binds to the AUG codon at the P site.
- The large ribosomal subunit joins, forming the complete initiation complex.
- Initiation factors (IF1, IF2, IF3 in prokaryotes) assist in this assembly. GTP hydrolysis provides energy.
This is a repetitive cycle with three sub-steps:
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Codon recognition: A charged tRNA with the correct anticodon enters the A site and pairs with the mRNA codon. GTP hydrolysis by elongation factors (EF-Tu) drives this step.
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Peptide bond formation: The ribosome’s peptidyl transferase activity (catalysed by the rRNA of the large subunit — a ribozyme) transfers the growing polypeptide from the P-site tRNA to the amino acid on the A-site tRNA.
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Translocation: The ribosome shifts one codon along the mRNA in the 5’ to 3’ direction. The tRNA in the A site moves to the P site, the P site tRNA moves to the E site (and exits), and a new codon is exposed at the A site.
This cycle repeats for each codon, adding one amino acid at a time.
- Elongation continues until the ribosome reaches a stop codon (UAA, UAG, or UGA) on the mRNA.
- No tRNA has an anticodon for stop codons. Instead, release factors (RF1, RF2) bind to the A site.
- Release factors trigger hydrolysis of the bond between the polypeptide and the last tRNA, releasing the completed protein.
- The ribosome dissociates into its subunits, ready for another round.
Why This Works
Translation converts the language of nucleic acids (4-letter code: A, U, G, C) into the language of proteins (20 amino acids). The genetic code is read as non-overlapping triplets — each codon specifies one amino acid. With 64 possible codons and 20 amino acids, the code is degenerate (multiple codons can code for the same amino acid).
The ribosome moves along the mRNA like a reading head, ensuring each codon is read exactly once and in order. The tRNA acts as the translator, matching each codon to the correct amino acid.
For NEET, memorise these codons: AUG = start codon (methionine), UAA, UAG, UGA = stop codons (no amino acids). Also know that the genetic code is universal (same in almost all organisms) and unambiguous (each codon codes for only one amino acid, though the reverse is not true).
Common Mistake
Students often write that the enzyme “peptidyl transferase” catalyses peptide bond formation. While technically correct, the crucial detail is that this activity comes from the rRNA (23S rRNA in prokaryotes), NOT a protein enzyme. This makes it a ribozyme. NEET has specifically tested whether peptidyl transferase is a protein or RNA.
Another error: confusing the direction of translation. The ribosome reads mRNA in the 5’ to 3’ direction, and the polypeptide is synthesised from the N-terminus to the C-terminus. Getting either direction wrong leads to incorrect answers.