If you have a polypeptide of size 11 kilo Daltons, what would be the mass of
the open reading frame that codes for the polypeptide? The average mass of
an amino acid and a nucleotide is considered to be 110 Daltons and 330
Daltons, respectively.
99990
99000
90900
99900
An 11 kilodalton (kDa) polypeptide has a mass of 11,000 Daltons. Dividing by the average amino acid residue mass of 110 Daltons yields exactly 100 amino acids (11,000 / 110 = 100). Each amino acid requires a codon of 3 nucleotides in the ORF, so the coding sequence contains 300 nucleotides (100 × 3). Multiplying by the average nucleotide mass of 330 Daltons gives the ORF mass: 300 × 330 = 99,000 Daltons. Thus, the correct answer is 99,000.
Why 99,000 is Correct
The calculation assumes a standard genetic code without start/stop codons in the basic ORF mass estimate, focusing on coding nucleotides. This matches common biochemistry approximations where protein mass directly scales to residue count, and DNA mass follows triplet codons. Real-world tools and references confirm the average residue mass near 110 Da after peptide bond water loss.
Explanation of Incorrect Options
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99,990: Likely from miscalculating nucleotides as 100 × 3 = 300, then 300 × 333 (an off average) or adding extraneous mass; ignores precise 330 Da input.
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90,900: Assumes ~90 amino acids (11,000 / ~122 Da, wrong average) or 275 nucleotides (e.g., 91.67 × 3); underestimates residues.
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99,900: Close but erroneous—perhaps 303 nucleotides (101 amino acids from 11,110 Da) × 330 / slight adjustment; overcounts amino acids.
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This method applies to molecular biology estimates for gene sizing from protein data, aiding research in protein engineering and genomics.
