Z-DNA: a prodrome for the 1990s.

Two duplexes could become unwound locally in homologous regions which, it might be argued, could then rewind to form regions of hybrid DNA. The difficulty with this scheme is that, without breaks, each right-hand turn of the hybrid helix generated in this way must be compensated by a left-hand turn. No one who has attempted to build a stable left-handed double helix with molecular models has succeeded.' Meselson (1967)