Q. 55 The coding sequence of a gene 𝑿𝑳𝑹𝟏𝟖 has the single ORF of 783 bp . The approximate molecular
weight of the XLR18 protein in kDa is ____ .

The coding sequence of gene XLR18 has a single open reading frame (ORF) of 783 bp, which encodes a protein through translation of its codons. Each codon specifies one amino acid, so divide the ORF length by 3 to find the number of amino acids: 783 ÷ 3 = 261 amino acids. The average molecular weight of an amino acid residue in a protein is approximately 110 Da, accounting for the loss of water during peptide bond formation, yielding a protein mass of about 28,710 Da or 28.7 kDa.​

Exact Answer

The approximate molecular weight of the XLR18 protein is 29 kDa, rounded from the calculation (261 × 110 = 28,710 Da ≈ 29 kDa). This standard approximation suits exam contexts without sequence details.​

Calculation Breakdown

Proteins form by linking amino acids, where each loses H₂O (18 Da) during bond formation, reducing average residue mass from 137 Da (free amino acids) to ~110 Da. For XLR18:​

• Nucleotides: 783 bp

• Amino acids: 783 / 3 = 261

• Mass: 261 × 110 Da = 28,710 Da = 28.7 kDa ≈ 29 kDa​

No post-translational modifications are specified, so use this baseline estimate.

Why This Approximation Works

Exact mass requires the full amino acid sequence via tools like Expasy Compute pI/Mw, but exams use the 110 Da average for simplicity. Variations occur (e.g., glycine 57 Da, tryptophan 186 Da), but 110 Da averages across proteomes accurately for most proteins.​

Common Distractor Options Explained

Typical multiple-choice options test pitfalls:

• 26 kDa: Using 100 Da/aa average (too low; ignores residue specifics).​

• 29 kDa: Correct (783/3 × 110 Da).

• 39 kDa: Mistake of bp as amino acids (783 Da too simplistic) or 150 Da/aa (free aa mass).

• 87 kDa: Dividing by 9 (3 nt/codon misconception) or doubling for subunits.​

Determining protein molecular weight from a gene’s coding sequence is essential for molecular biology exams. For the XLR18 gene with a 783 bp ORF, the protein weighs approximately 29 kDa.

Key Concepts in Protein Mass Estimation

ORFs translate in triplets: 783 bp ÷ 3 = 261 codons = 261 amino acids. Multiply by average residue mass (~110 Da) for total: 28.7 kDa, rounded to 29 kDa. This method skips sequence data, ideal for quick calculations.​

Step-by-Step ORF to kDa Conversion

1. Confirm single ORF: 783 bp, no introns in coding sequence.

2. Codons: 783 / 3 = 261 aa (assumes in-frame start/stop).

3. Mass per aa: 110 Da standard.​

4. Total: 261 × 110 = 28,710 Da = 29 kDa.​

Frequent Exam Mistakes and Fixes

• Dividing by 1 bp/aa: Yields ~0.8 kDa (impossible).

• Free aa mass (137 Da): ~36 kDa overestimate.

• Including UTRs: Inflates beyond ORF.​
  Correct traps ensure 29 kDa.​

Tools for Precise Verification

Online calculators (e.g., Expasy, AAT BioQuest) confirm ~28-29 kDa without sequence, validating the approximation. For sequences, input FASTA for exact Mw.​

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