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An insertion or deletion of **one or two** bases in a coding sequence (not a multiple of three) causes:
ANo effect (silent)
BA frame-shift mutation — alters all downstream amino acids
COnly the next amino acid is changed
DReplication arrest only
Answer & Solution
Correct answer: B. A frame-shift mutation — alters all downstream amino acids
Adding/removing a number of bases not divisible by 3 shifts the reading frame from the insertion/deletion point onwards — every downstream codon changes, usually producing a non-functional protein and often an early stop. Insertions of multiples of 3 add/remove whole amino acids without shifting the frame.
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