Which of the following mutations occurred?

A normal peptide has 100 amino acids. Following mutation, it has only 20; 1–10 are the same as the normal peptide; 11–20 differ from the normal peptide. Which of the following mutations occurred?

Nucleotide deletion in the codon for amino acid 11 (codon 11)
Nucleotide deletion in codon 20
Nucleotide deletion in the intervening sequence
Nucleotide substitution in codon 11
Nucleotide substitution in codon 20

Correct answer
Nucleotide deletion in the codon for amino acid 11 (codon 11)

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correct answer: A
Nucleotide deletion in the codon for amino acid 11 (A) would cause a frameshift mutation. Such a mutation could result in a different amino acid at positions 11 and up, with the potential for an aberrant stop codon and early termination, as observed.
Nucleotide deletion in codon 20 (B) would cause a frameshift mutation. This mutation may result in a different amino acid 20, perhaps with termination thereafter. However, this option does not explain the difference observed in amino acids 11–19.

Nucleotide deletion in the intervening sequence (a.k.a. intron) © would not affect peptide sequence because introns are spliced out of the primary transcript, and thus are not translated.

Nucleotide substitution in codon 11 (D) may result in a different amino acid 11, but such a substitution would not change the rest of the nucleic acid sequence.

Nucleotide substitution in codon 20 (E) may result in a different amino acid 20, but such a substitution would not change the rest of the nucleic acid sequence.