GATE 2019 BT Q.48: solution and answer

GATE 2019 · BTQ.48 · 2 marks · NAT

The molecular mass of a protein is 22 kDa. The size of the cDNA (excluding the untranslated regions) that codes for this protein is _______ kb (rounded off to 1 decimal place).

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Answer and solution

ANSWER

The cDNA (coding region only) size = 0.6 kb

Official answer: 0.6Source: Official answer key, GATE 2019 BT (IIT Madras)

Given: molecular mass of the protein = 22 kDa = 22,000 Da.
Standard approximation used in molecular biology: the average molecular mass of one amino acid residue in a protein ≈ 110 Da.
Number of amino acid residues in the protein $= \frac{22000 Da}{110 Da} = 200$ amino acids.
The genetic code is triplet: each amino acid in the protein is specified by one codon of 3 nucleotides in the coding (mRNA/cDNA) sequence.
Length of the coding sequence (cDNA, excluding untranslated regions) $= 200 \times 3 = 600 nucleotides = 600 bp$ .
Convert to kilobases: $600 bp = 0.6 kb$ (since $1 kb = 1000 bp$ ).
Rounded to 1 decimal place, the size of the cDNA coding region $= 0.6 kb$ .

KEY IDEA

The average molecular mass of an amino acid residue in a protein is taken as ~110 Da; each amino acid is encoded by one codon (3 nucleotides) in the coding (cDNA) sequence.

WATCH OUT FOR

Forgetting to convert the answer from base pairs (600 bp) to kilobases (0.6 kb), or using an incorrect average amino acid residue mass instead of the standard ~110 Da approximation, or forgetting that each amino acid needs 3 nucleotides (one codon).

From the GATE 2019 BT paper. Every question of this paper · Download the full paper · All 21 Gene expression questions, with notes

GATE 2019 BT Q.48: solution and answer

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