This third edition has been entirely revised and updated, and expanded to cover new advances in our understanding. It explains, step by step, how DNA forms specific structures, the nature of these structures and how they fundamentally affect the biological processes of transcription and replication. Designed to be essential reading for all molecular biology, biochemistry and genetics students, to newcomers to the field from other areas such as chemistry or physics, and even for seasoned researchers, who really want to understand DNA.