Figure 2.

Stereoview of the four tetraplexes: Tetraplex 1 (yellow), tetraplex 2 (green), tetraplex 3 (cyan), and tetraplex 4 (red). Each strand sits around the fourfold symmetry axis to form tetraplexes with symmetry-related molecules. The tetraplex molecules stack coaxially in opposite polarity. There are eight Sr²⁺ ions and two Na⁺ ions on the fourfold axis. Notice the 5′-end Us are unstacked with the Gs and protrude into neighboring G tetraplexes, forming octads as indicated by the arrows.