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Practice MCAT Question - Q155 - Answer!

If a substitution occurs in a sequence of 15 nucleotides at the 7th nucleotide, what can we be sure will happen when this sequence is translated?

A.  Only the first amino acid will be correct.
B.  The first two amino acids will be correct.
C.  The first three amino acids will be correct.
D.  The first two amino acids and the last two amino acids will be correct.

The correct answer is (B). Since the substitution is at the seventh nucleotide, the first six nucleotides remain unchanged and will code for the original two amino acids. The substitution is part of the third codon. However, we cannot be sure that the last two amino acids will also be correct (choice D) because the substitution in the third codon may produce early termination.

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