IMAT 2016 Q26 [DNA and tRNA]

Which row of the table correctly indicates features that are found in both pure extracts of DNA and tRNA molecules?

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A. row 4
B. row 1
C. row 5
D. row 3
E. row 2

The correct answer is A.

Let’s review what DNA and tRNA molecules are made of.

Pentose sugar, adenine and uracil

  • First, both DNA and RNA are made of ribose (or deoxyribose in the case of DNA), which is a type of sugar. Ribose is a pentose sugar : it is made of 6 carbon atoms.

    Therefore, both molecules contain a pentose sugar.

  • First, there are 4 types of nucleotides (or bases) : adenine, cytosine, thymine and guanine. Each of them are represented by a letter, respectively A, C, T, G.

    These bases form pairs and are the building blocks of nucleic acids, such as DNA or RNA. In the case of RNA, thymine is replaced by uracil (U), through a process of demethylation.

    Uracil is NOT present in both molecules, but adenine is.

Hydrogen and phosphodiester bonds

  • Like we’ve said previously, nucleic acids are made of nucleotides. They are linked together by hydrogen bonds :
  1. Adenine and thymine (or uracil) can form pairs and are bonded by 2 hydrogen bonds

  2. Guanine and cytosine can form pairs and are bonded by 3 hydrogen bonds

  • Now onto phosphodiester bonds : it is the element that ensures the integrity of nucleic acid strands. It links the 3’ carbon atom of a molecule A and the 5’ carbon atom of a molecule B.

Therefore, row 4 is the one that best describes the features present in pure DNA and tRNA molecules.