Imat 2011 q54 [dna components]

Which of the components of DNA listed below are found on the outside of a DNA double helix?

  1. Pentose sugar
  2. Phosphate
  3. Purine bases
  4. Pyrimidine bases

A. 1 and 2 only

B. 3 and 4 only

C. 1, 2 and 3 only

D. 2, 3 and 4 only

E. 1, 2, 3 and 4


“Which of the components of DNA” (We know DNA is made from a phosphate backbone, connected to a pentose, connected to a nitrogenous base). To remember which items are found outside of a DNA double helix, we need to remember the following diagram:

Pentose sugar: The middle building block of a single DNA molecule (A good tip for the IMAT is to remember that each DNA double helix is comprised of two very long molecules attracted to each other by hydrogen forces), from the diagram above you can see that the pentose sugar is not in the middle.

Phosphate: As you can see by the diagram, DNA has a “backbone” made of phosphate groups. As the name suggests, the backbone can be found outside of the molecules.

Purin and Pyrimidine bases – both are the gene sequence itself. As we are talking about a double strand, the bases are always in the middle due to the strong intermolecular forces between them.

The answer is A


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