3.15 describe a DNA molecule as two strands coiled to form a double helix, the strands being linked by a series of paired bases; adenine (A) with thymine (T), and cytosine (C) with guanine (G)

a molecule of DNA is made up of two strands that are coiled together and held in place by chemicals known as bases. the four bases are as follows... adenine, thymine, cytosine and guanine. The bases are paired together. Adenine ALWAYS pairs with thymine and cytosine ALWAYS pairs with guanine. This ensures that you will always have an equal amount of adenine to thymine and cytosine to guanine. This diagram may help...


NOTE: A good way to remember which base pairs with which is with the rhyme 'Apples on a Tree, Cars in a Garage' (Apple = Adenine, Tree = Thymine. Cars = Cytosine, Garage = Guanine)