In eukaryotic cells, chromosomes are made up of DNA. DNA can also be found in the chloroplasts and mitochondria, and inside the cytoplasm in the form of plasma. DNA molecules consist of nucleotides that are linked to each other in a chain. Each nucleotide has three components: one sugar (deoxyribose), one phosphate, and one of the four bases– adenine (A), cytosine (C), guanine (G), and thymine (T). All nucleotides contain the same sugar and phosphate backbone. The genetic information comes from the order in which the nucleotides appear.
In 1953, Watson and Crick came up with the standard double-helix model of DNA. Two twisted polynucleotide chains create a double helix held together by nucleotide base pairs. Periodal polyesther chains between phosphates and the sugar backbone create the outer structure of the double helix from attraction and repulsion from intermolecular forces in the molecule. Between the strands, either adenine and thymine is paired by two hydrogen bonds, or cytosine and guanine are paired by three hydrogen bonds. Adenine can only pair up with thymine and cytosine can only pair up with guanine.
As a result, the base sequence along one polynucleotide strand determines the base sequence of the other strand. The strands of the double helix are therefore complimentary, not identical, and oriented in opposite directions.
DNA needs not only to store genetic information, but also to identically reproduce this information in order to pass it on to the next generation. Republication of the information happens during interphase (the time period between two cell divisions).
Reference: Hoffmann, Gudrun. The Science Book. Washington: National Geographic, 2008