Double helix is the description of the structure of a DNA molecule. A DNA molecule consists of two strands that wind around each other like a twisted ladder. Each strand has a backbone made of alternating groups of sugar (deoxyribose) and phosphate groups. Attached to each sugar is one of four bases: adenine (A), cytosine (C), guanine (G), or thymine (T). The two strands are held together by bonds between the bases, adenine forming a base pair with thymine, and cytosine forming a base pair with guanine.
A double helix has become the icon for many, many kinds of discussions about where science has been and where it's going. This really is an amazing structure. You can't stare at the double helix for very long without having a sense of awe about the elegance of this information molecule DNA, with its double helical form basically being the way in which all living forms are connected to each other, because they all use this same structure for conveying that information. Of course, this is Watson and Crick's incredible realization back in 1953, but it will stand in history as probably one of the most significant scientific moments of all time.- Francis S. Collins, M.D., Ph.D.