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:
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Nucleotides: 783 bp
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Amino acids: 783 / 3 = 261
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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:
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26 kDa: Using 100 Da/aa average (too low; ignores residue specifics).
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29 kDa: Correct (783/3 × 110 Da).
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39 kDa: Mistake of bp as amino acids (783 Da too simplistic) or 150 Da/aa (free aa mass).
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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
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Confirm single ORF: 783 bp, no introns in coding sequence.
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Codons: 783 / 3 = 261 aa (assumes in-frame start/stop).
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Mass per aa: 110 Da standard.
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Total: 261 × 110 = 28,710 Da = 29 kDa.
Frequent Exam Mistakes and Fixes
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Dividing by 1 bp/aa: Yields ~0.8 kDa (impossible).
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Free aa mass (137 Da): ~36 kDa overestimate.
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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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