text: Even once the DNA duplex structure was solved, it was initially an open question whether additional DNA structures were needed to explain its overall topology. there were initially questions about how it might affect DNA replication. In 1963, autoradiographs of the E. coli chromosome demonstrated that it was a single circular molecule that is replicated at a pair of replication forks at which both new DNA strands are being synthesized. The two daughter chromosomes after replication would therefore be topologically linked. The separation of the two linked daughter DNA strands during replication either required DNA to have a net-zero helical twist, or for the strands to be cut, crossed, and rejoined. It was this apparent contradictions that early non-helical models attempted to address until the discovery of topoisomerases in 1970 resolved the problem.
  In the 1960s and 1970s, a number of structures were hypothesised that would give a net-zero helical twist over the length of the DNA, either by being fully straight throughout or by alternating right-handed and left-handed helical twists. For example, in 1969, a linear tetramer structure was hypothesised, and in 1976, a structure with alternating sections of right-handed and left-handed helix was independently proposed by two different groups. The alternating twists model was initially presented with the helicity changing every half turn, but later long stretches of each helical direction were later proposed. However, these models suffered from a lack of experimental support. Under torsional stress, a Z-DNA structure can form with opposite twist to B-form DNA, but this is rare within the cellular environment. The discovery of topoisomerases and gyrases, enzymes that can change the linking number of circular nucleic acids and thus "unwind" and "rewind" the replicating bacterial chromosome, solved the topological objections to the B-form DNA helical structure. Indeed, in the absence of these topology-altering enzymes, small circular viral and plasmid DNA are inseparable supporting structure whose strands are topologically locked together.
  Non-helical DNA proposals have therefore dropped from mainstream science.