A mutation causing a polypeptide to lack the last three amino acids occurs when a premature stop codon is introduced in the gene, truncating translation early. This defines a nonsense mutation, eliminating options like synonymous or missense that alter codons without truncation.

Option Analysis

Synonymous mutations change a codon but encode the same amino acid due to genetic code degeneracy, leaving the full polypeptide sequence unchanged.
Silent mutations are synonymous variants with no phenotypic effect, preserving the complete protein.
Missense mutations substitute one amino acid for another, but the polypeptide retains its full length.
Nonsense mutations convert a sense codon to a stop codon (UAA, UAG, UGA), halting translation and producing a shortened polypeptide missing C-terminal residues.

Correct Answer

(B) Nonsense mutation. The loss of exactly three amino acids implies the stop codon appears precisely three codons before the natural end, confirming truncation.

Nonsense mutation polypeptide lacks last three amino acids in competitive exams like IIT JAM tests molecular genetics knowledge. This mutation introduces a premature stop codon, truncating protein synthesis early.

Types of Mutations

• Synonymous Mutation: Codon change yields identical amino acid; full protein intact. No truncation occurs.

• Missense Mutation: Single amino acid swap; polypeptide length preserved but function may alter.

• Silent Mutation: Synonymous with no effect on protein or phenotype.

• Nonsense Mutation: Stop codon creation shortens polypeptide, as in lacking last three amino acids.

Mechanism

Translation halts at new stop codon (e.g., CAG to UAG), yielding incomplete chain. This fits IIT JAM biology questions on genetic code degeneracy.

Exam Relevance

For IIT JAM Biotechnology, distinguish truncation (nonsense) from substitutions. Practice identifies premature stops causing C-terminal loss.