This model of the double helix shows a small part of DNA. The thymines (T) and adenines (A) are linked to each other, and so are the guanines (G) and cytosines (C). When DNA replicates itself these links give way and the two strands separate.
Adapted from National Human Genome Research Institute (2004). The National Human Genome Research Institute of the USA was established in 1989 as the National Center for Human Genome Research (NCHGR) to carry out the contribution of the USA to the International Human Genome Project, which was completed in April 2003.
The photo below shows the demonstration model constructed by Watson and Crix in 1953. The scale is in Ångstrom (10-10 m). Adapted from Watson (1968).
National Human Genome Research Institute (2004) DNA double helix. http://www.genome.gov/11006909 (accessed 1 October 2004)
Watson, J. D. (1968) The double helix, a personal account of the discovery of the structure of DNA. Weidenfeld and Nicolson, London.
Image and photo: public domain (US government)