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MCAT Biology Question — Nucleotide Structures

DNA and RNA are composed of long chains of monomers known as nucleotides. On the guanosine monophosphate molecule shown below, which arrow best indicates the 5’ end of the subunit?

guanine molecule


  1. I
  2. II
  3. III
  4. IV
Click for Explanation

This question is testing your familiarity of nucleotide structures found in DNA and RNA. Nucleotides are monomers, or subunits, that are the building blocks of DNA and RNA molecules. Each nucleotide consists of a phosphate group, a pentose ring, and a nitrogenous base. In the case of the guanosine monophosphate nucleotide shown, the 5’ end corresponds to the phosphate group attached to the fifth carbon on the pentose ring, indicated by the arrow labeled IV. This phosphodiester bond represents the 5’ of a nucleotide making choice D the best answer. The arrow labeled III corresponds to the third carbon in the pentose ring. The hydroxyl group attached to the third carbon represents the 3’ end of a nucleotide. The arrow labeled II represents the first labeled carbon. Lastly, arrow I points to the nitrogenous base of guanine.

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