Page "DNA" Paragraph 6

In living organisms DNA does not usually exist as a single molecule, but instead as a pair of molecules that are held tightly together.

These two long strands entwine like vines, in the shape of a double helix.

The nucleotide repeats contain both the segment of the backbone of the molecule, which holds the chain together, and a nucleobase, which interacts with the other DNA strand in the helix.

A nucleobase linked to a sugar is called a nucleoside and a base linked to a sugar and one or more phosphate groups is called a nucleotide.

Polymers comprising multiple linked nucleotides ( as in DNA ) are called a polynucleotide.