© Wikimedia Commons, GNU Free Documentation License, Version 1.2. Author: Zephyris (Richard Wheeler), 4 February 2007.
While the DNA double helix most often appears as the right-handed helix shown in Figure 9, it can take on many different configurations. Here, we see A-DNA (left), B-DNA (center), and Z-DNA (right). B-DNA is the configuration most often found in living cells. A-DNA forms a wider spiral. DNA takes this form in the partially dehydrated samples sometimes studied in the laboratory. Z-DNA is a left-handed double helix with a zig-zag structure not seen in B-DNA. These three configurations are all biologically active and both the B- and Z- configurations have been found in living cells. Which form the molecule takes depends primarily on the level of hydration in the environment surrounding the molecule, and what ions and other organic compounds are present. (Unit: 9)