The pairs of nitrogenous bases in a DNA double helix are held together by hydrogen bonds.

The geometry of the bases are the basis of base-pairing rules.

• Adenine (A) pairs with Thymine (T) by forming two bonds,

• Cytosine (C) pairs with Guanine (G) by forming three bonds.

• When DNA base-pairs with RNA:
  • A pairs with U.