DNA has a double helix structure composed of nucleotides.

DNA is a macro molecule that consists of many subunits connected together. The subunits are called nucleotides. Each nucleotide has three parts; a sugar, a phosphate, and a base. The sugar and phosphate molecules are linked together in two long chains. The bases are linked to the sugar-phosphates. Bases of one strand are bound to those of the other strand by hydrogen bonds making what is called a base pair. The structure of DNA is a double helix which allows it to perform the functions of replication and information storage. Base pairs look similar to rungs on a ladder. In fact, the DNA structure could be described as a long ladder twisted into a spiral.

Nucleotides

There are only 4 nucleotides in DNA, Adenine (A), Guanine (G), Thymine (T), and Cystosine (C). The chemical structures of Thymine and Cytosine are smaller, while those of Adenine and Guanine are larger. Size and structure of the specific nucleotides cause Adenine and Thymine to always pair together while Cytosine and Guanine always pair together. Therefore the two strands of DNA are considered complimentary.