In addition to the variety of verified DNA structures, there have been a range of proposed DNA models that have either been disproven, or lack evidence. Some of these structures were proposed during the 1950s before the structure of the double helix was solved, most famously by Linus Pauling. Non-helical or "side-by-side" models of DNA were proposed in the 1970s to address what appeared at the time to be problems with the topology of circular DNA chromosomes during replication (subsequently resolved via the discovery of enzymes that modify DNA topology). These were also rejected due to accumulating experimental evidence from X-ray crystallography, solution NMR, and atomic force microscopy (of both DNA alone, and bound to DNA-binding proteins). Although localised or transient non-duplex hel