* DNA is a double helix: Imagine a twisted ladder. The sides of the ladder are made of sugar and phosphate molecules. The rungs of the ladder are made of pairs of nitrogenous bases.
* Base pairing: Adenine (A) always pairs with Thymine (T), and Guanine (G) always pairs with Cytosine (C). This is called complementary base pairing.
So, for every molecule of adenine in a DNA molecule, there is one molecule of thymine.
The same applies to guanine and cytosine: for every molecule of guanine, there is one molecule of cytosine.
Important note: This is true for a single DNA molecule. The actual number of adenine and thymine molecules will vary depending on the length of the DNA molecule.
