**
Ever since Rosalind Franklin made those fateful X-ray crystallographs, we’ve all been indoctrinated to the fact that DNA in our cells exists as a Double Helix, per Watson and Crick. A binds T, C binds G, end of story. Until I was in graduate school, taking a class in biophysical chemistry, it never even occurred to me that other forms of DNA could exist and that other base pairings were possible. For example, with enough energy, T-T dimers are possible using UV light, and the basis of some skin cancer mutations.
Well brief interludes of a very different structure of DNA, the I-motif, formed when C binds a quadrahelix making little cross structures. You can also have a G quadrahelix as well, which is more of a box. Unlike the thiamine dimers, these are not mistakes, but are hypothosized to be prevalent in promoter regions. It would make sense, as it would give the enzyme a “handle” for grabbing on and opening up the tightly wound DNA coil.
Really cool stuff!

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Nature Chemistry iMotif DNA