This scanning tunnelling electron micrograph shows part of a DNA molecule magnified 17 million times. DNA is the main component of chromosomes and bears the genes that determine the individual characteristics of all organisms. Its structure is a double helx, like a twisted rope-ladder, each 10 nanometers long. When James Watson and Charles Crick worked out the DNA's structure in 1953, it led to an understanding of how this molecule could encode genetic information that could be copied and passed on from one generation to the next.