Non-helical models of DNA structure refer to molecular models that do not follow the traditional double-helical structure of DNA. Common non-helical models include triple helix DNA, quadruplex DNA, and left-handed DNA. Triple helix DNA occurs when a third strand of DNA binds to an existing double helix, resulting in a unique, three-stranded structure. Quadruplex DNA, on the other hand, occurs when four strands of DNA form a stable structure, commonly found at the ends of chromosomes. Lastly, left-handed DNA is a mirror image of the right-handed double helix, where the two DNA strands twist in the opposite direction. These variations in DNA structure play a crucial role in regulating gene expression and understanding the mechanisms of genetic diseases.