# Complementary Base Pairing --- **Complementary base pairing** describes how [[nucleotide|nucleotides]] in double-stranded [[DNA]] and [[RNA]] (or one DNA strand and one RNA strand), fit together. Each different [[nucleotide|nucleotides]] naturally only forms a [[hydrogen bond|bond]] with a specific other nucleotide. 6 billion base pairs in every human cell, each of them is (probably) matched to their complement. If there *is* a miss-match in base pairing, this is a type of [[point mutation]]. *Guanine* always matches with *adenine*, and vice versa. *Cytosine* always matches with *thymine* in [[DNA]] and *uracil* in [[RNA]]. ![[complementary base pairing.png]] ## Purines vs Pyrimidines There are two shape-based classes of nitrogenous bases, **pyrimidines** and **purines**. The pyrimidine bases have one ring, and they are *cytosine*, *thymine* and *uracil*. Purines have two rings attached together and they are *guanine* and *adenine*. A pyrimidine always pairs with a purine, and vise versa. This is not to say that *any* pyrimidine matches with *any* purine, but it is important because otherwise it would not be able to forms bonds at a consistent width all the way down the strands. ![[purine and pyrimidine.png]] ___