Genomes are composed of DNA, and a knowledge of the structure of DNA is essential to understand how it can function as hereditary material. DNA is remarkable, breathtakingly simple in its structure yet capable of directing all the living processes in a cell, the production of new cells and the development of a fertilized egg to an individual adult. DNA has three key properties: it is relatively stable; its structure suggests an obvious way in which the molecule can be duplicated, or replicated; and it carries a store of vital information that is used in the cell to produce proteins. The first two properties of DNA are analysed in this unit. This study unit is just one of many that can be found on LearningSpace, part of OpenLearn, a collection of open educational resources from The Open University. Published in ePub 2.0.1 format, some feature such as audio, video and linked PDF are not supported by all ePub readers.