MCAT Biology Question #5: Answer and Rationale

5. Starting with the translation initiation codon, the following sequence encodes a polypeptide of how many amino acids? 

5'-CUGCCAAUGUGCUAAUCGCGGGGG-3'

A) 2

B) 3

C) 6

D) 8

Answer Analysis

The correct answer is A.

Rationale: This is a scientific reasoning question, in which you must use your knowledge about transmission of genetic information to solve a problem. You are told that the sequence contains codons and you can see that the sequence includes U, or uracil; Uracil is a base found in RNA, but not DNA, so you can deduce that you are looking at a sequence of RNA. The cell reads RNA sequences three bases at a time, with each group of three bases constituting a codon that will code for one amino acid. You must recall the sequence for the start codon, AUG, and that there are three stop codons in the genetic code: UAG, UAA, and UGA. In this sequence, the stop codon UAA appears two codons after the start codon, so you can conclude that the polypeptide generated from this RNA sequence will be comprised of only two amino acids.

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