MCAT Question A Day - 5/13/14 - Answer!

If each of the following mRNA nucleotide sequences contains three codons, which one contains a start codon?

A. 3'-AGGCCGUAG-5'

B. 3'-GUACCGAAC-5'

C. 5'-AAUGCGGAC-3'

D. 5'-UAGGAUCCC-3'

B is correct. The start codon is AUG. mRNA is translated 5'->3'. (Note: You did not need to know that to answer this question correctly.) We are looking for AUG 5' .... 3'. Only A, Band C have an AUG sequence. However, if there are three codons in C, they must be: AAU, GCG, and GAe. The three in A must be: GAU, GCC, and GGA.

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