Finally, you can construct two complete base pair layers of DNA.
There are now four bases involved which you can switch by clicking the corresponding "Change Base" button.
Also, to see better into the core of the molecule, you can temporarily omit a base from any position with that same button.
When you get the pairings right, the yellow hydrogen bonds will form between the bases.
And you can then rotate the whole structure by slowly dragging your mouse across the structure a few times.
If you want to learn more about this subject, take a look at our Nanoscience class website including its lecture note set on DNA Self-Assembly.