Look out! Complement is not the same as a compliment, which means saying something nice about someone! Two objects that are complementary fit together to make a whole. So when two nucleotides fit together in a double helix, they are said to be a complementary base pair.

In the cell, DNA forms a double helix made of two chains of nucleotides. In certain combinations, the nucleotides' shapes fit together well and allow them to form hydrogen bonds to each other. [To review, see ChemLink: Hydrogen Bonds].

The following model can show you why the nucleotides fit together well.

What to do: Line up the nucleotides opposite each other to make complementary pairs.

  1. drag the four nucleotides, A, C, G, and T by clicking on them and holding down the mouse button.
  2. Move them close to each other and you will see dotted lines between some of the atoms. The dotted lines represent hydrogen bonds.
  3. Drag each nucleotide near each of the others, one at a time. Which nucleotides form hydrogen bonds with each other? How many hydrogen bonds form between the complementary pairs?

    Note that to simplify the structures, not all hysrogen atoms are shown on the nucleotides.

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