DNA is made of two strands of nucleotides. Each nucleotide has the sugar deoxyribose, a phosphate and one of four bases. The bases in DNA are Adenine (A), Thymine (T), Guanine (G) and Cytosine (C). These letters make up the genetic code.

A pairs with T and G pairs with C in DNA. In RNA the base A pairs with U (Uracil) and G pairs with C.

The DNA in a human cell has over 3 billion nucleotide pairs!