Q.20 During translation, the directionality of polypeptide synthesis is ?
1. C-tenninal to N-terminal direction.
2. N-tenninal to 3′ direction.
3. 5′ to C-terminal direction.
4. N-tenninal to C-terminal direction.
The correct answer is option 4: N-terminal to C-terminal direction. During translation, ribosomes synthesize polypeptides by adding amino acids sequentially to the growing chain’s carboxyl (C-terminal) end, starting from the amino (N-terminal) end.
Option Analysis
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Option 1: C-terminal to N-terminal direction: Incorrect; this reverses the actual polarity, as peptide bonds form by linking the N-terminal of a new amino acid to the C-terminal of the existing chain.
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Option 2: N-terminal to 3′ direction: Incorrect; confuses polypeptide ends with mRNA polarity (mRNA is read 5′ to 3′, but synthesis direction refers to protein terminals).
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Option 3: 5′ to C-terminal direction: Incorrect; mixes nucleic acid directionality (5′ end of mRNA) with protein synthesis, which is defined by N- to C-terminus regardless of mRNA reading frame.
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Option 4: N-terminal to C-terminal direction: Correct; the first amino acid (Met) forms the free N-terminus, and subsequent amino acids attach via peptide bonds at the C-terminus.
The directionality of polypeptide synthesis during translation follows a strict N-terminal to C-terminal pattern, a fundamental aspect of protein biosynthesis in molecular biology. This ensures proper folding and function of proteins, as ribosomes link amino acids by forming peptide bonds between the amino (N-) terminus of the incoming residue and the carboxyl (C-) terminus of the chain.
Translation Process Overview
Translation occurs on ribosomes in three stages: initiation (start codon AUG binds Met-tRNA, forming N-terminus), elongation (tRNAs deliver amino acids, extending the chain C-terminally), and termination (stop codon releases the completed polypeptide). mRNA is read 5′ to 3′, but the polypeptide grows N- to C-terminally due to the chemistry of peptide bond formation.
Why N-to-C Direction?
This directionality arises because tRNA anticodons match mRNA codons sequentially, and the ribosome’s peptidyl transferase catalyzes bonds adding new amino acids to the chain’s C-end. Reversing it would disrupt universal protein structures.
Exam Relevance
Common in NEET, GATE Life Sciences, and CSIR-NET, such questions test central dogma details. Key tip: Protein synthesis polarity is always N- to C-terminal, distinct from nucleic acid 5′-3′.
