In living organisms such as humans, DNA exists as a pair of molecules rather than a single molecule. These strands are entwined in the shape of a double helix and the helix is kept stable by hydrogen bonds, which can be found between the bases attached to the two strands. A long polymer, DNA is made up of smaller units called nucleotides. In turn, each nucleotide consists of a phosphate group, a sugar and a nitrogenous base. To get a sense of just how long a DNA polymer is, consider that just one human chromosome is several hundred million base pairs long.