DNA is a polymer. That means it is a very long molecule made of many small repeating units joined in a chain. The repeating units are called nucleotides. Each nucleotide has a sugar, a phosphate group, and one of four different bases attached to the sugar.
The four bases are adenine (A), thymine (T), cytosine (C) and guanine (G). A DNA molecule is made of two strands that are coiled around each other to form a double helix.
The two strands are linked by pairs of bases that fit together. Adenine always pairs with thymine, and cytosine always pairs with guanine. These are called complementary base pairs. Each pair is joined by weak hydrogen bonds, so the two strands can be separated when the cell needs to use the DNA.
