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MCAT Question A Day - 8/13/13 - Answer!

What would you expect to happen during translation if an exon in the middle of a gene has mutated from GTT to ATT?

A.  The complete protein will be translated normally.
B.  The protein will terminate prematurely.
C.  The protein will never terminate.
D.  The protein will never start.

The correct answer is (B). Exons are protein-coding sequences in the genetic material. The DNA nucleotide sequence, GTT, would be transcribed to produce the mRNA sequence CAA (this codon calls for the amino acid, glutamine, to be inserted into the protein being translated). If the DNA sequence mutated from GTT to ATT, the “new” mRNA codon would be UAA, which is one of the three terminator (STOP!) codons. The protein, therefore, would end prematurely.