Q32. Approximate molecular weight (kDa) of the product after translation of a 390 bases mRNA will be
To solve this GATE Life Sciences question on mRNA translation and protein molecular weight, we first calculate the number of codons from 390 bases, then estimate the protein’s mass in kDa using the standard average amino acid residue weight of 110 Da.
Step-by-Step Solution
mRNA is translated in groups of 3 bases (codons) per amino acid, with the stop codon not adding an amino acid. For 390 bases:
Number of amino acids = 390 / 3 = 130.
Protein molecular weight ≈ 130 amino acids × 110 Da/amino acid = 14,300 Da = 14.3 kDa (≈14 kDa).
This matches option (D).
Option Analysis
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(A) 48 kDa: Wrong; assumes ~436 amino acids (48,000 / 110), or ~1,308 bases—ignores codon grouping.
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(B) 26 kDa: Wrong; assumes ~236 amino acids (26,000 / 110), or ~708 bases—possible if miscounting without stop codon adjustment.
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(C) 39 kDa: Wrong; assumes ~355 amino acids (39,000 / 110), or ~1,065 bases—perhaps confusing bases with codons directly.
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(D) 14 kDa: Correct, as calculated above using standard 110 Da average (accounts for residue mass after peptide bond water loss).
Introduction to mRNA Translation and Protein Molecular Weight
In molecular biology, determining the approximate molecular weight kDa of the product after translation of a 390 bases mRNA is key for exams like GATE Life Sciences. This involves codon reading (3 bases/amino acid) and average amino acid mass of 110 Da, yielding ~14 kDa for 130 residues.
Why 390 Bases mRNA Gives 14 kDa Protein
Translation reads mRNA in triplets: 390 bases ÷ 3 = 130 codons (one stop codon ends chain, so 129-130 amino acids). Multiply by 110 Da: 130 × 110 = 14,300 Da ≈ 14 kDa. This rule applies universally in eukaryotes/prokaryotes.
Common Errors in Molecular Weight Calculation
Trainees often forget:
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Dividing total bases by 3 for amino acids.
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Using residue mass (110 Da), not free amino acid (~137 Da).
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Stop codon exclusion (minimal impact on approximation).
| Option | Assumed Amino Acids | Bases Needed | Why Incorrect |
|---|---|---|---|
| 48 kDa | ~436 | ~1,308 | Overestimates codons |
| 26 kDa | ~236 | ~708 | Partial sequence error |
| 39 kDa | ~355 | ~1,065 | Direct base-to-aa mix-up |
| 14 kDa | 130 | 390 | Correct |
GATE Life Sciences Exam Tips
Practice with approximate molecular weight kDa formulas: kDa ≈ (bases / 3) × 0.11. Review genetic code, ribosomes, tRNAs for context.
