Q.55. An mRNA of a nuclear encoded plant gene, DSH 20 has an ORF of 1353 nucleotides. Provided
that average molecular weight of amino acid is 110 Dalton (Da), calculated molecular weight of DSH20
protein in kDa (round off to 1 decimal place) is ____
The DSH20 protein from a nuclear-encoded plant gene has an ORF of 1353 nucleotides, and using the given average amino acid molecular weight of 110 Da, the calculated mass is 49.6 kDa. This fill-in-the-blank GATE-style question tests understanding of genetic code translation and protein mass estimation.
Step-by-Step Calculation
An ORF length of 1353 nucleotides encodes amino acids via the triplet genetic code, yielding 451 residues (1353 ÷ 3 = 451). Multiply by 110 Da per amino acid: 451 × 110 = 49,610 Da. Convert to kDa and round to one decimal place: 49.61 kDa becomes 49.6 kDa.
Why Divide by 3?
mRNA ORFs consist of codons, each three nucleotides specifying one amino acid (excluding stop codon in length-based estimates). Here, 1353 / 3 = 451 amino acids exactly, confirming a complete ORF. Standard approximation ignores initiator methionine adjustment for such problems.
Common Mistakes Explained
- Error: Using 1353 amino acids. Treats nucleotides as residues; yields ~149 kDa—incorrect, as code is triplet-based.
- Error: Multiply by 100 Da. Some recall lower average; 451 × 100 / 1000 = 45.1 kDa—wrong with given 110 Da.
- Error: Forgetting kDa conversion or rounding. 49,610 Da is 49.61 kDa; rounds to 49.6, not 50 or 49.
GATE Exam Relevance
This question appears in GATE Life Sciences mock tests for botany, emphasizing molecular biology basics like translation and protein biochemistry. Practice similar calculations boosts scores in genetics sections.
