ulation ▁and ▁R NA ▁polym er ase ▁control ▁by ▁chrom atin ▁structure . ▁ ▁DNA ▁structure ▁ ▁In ▁nature , ▁DNA ▁can ▁form ▁three ▁structures , ▁A -, ▁B -, ▁and ▁Z - D NA . ▁A - ▁and ▁B - D NA ▁are ▁very ▁similar , ▁forming ▁right - hand ed ▁hel ices , ▁whereas ▁Z - D NA ▁is ▁a ▁left - hand ed ▁hel ix ▁with ▁a ▁z ig - z ag ▁ph osph ate ▁back bone . ▁Z - D NA ▁is ▁thought ▁to ▁play ▁a ▁specific ▁role ▁in ▁chrom atin ▁structure ▁and ▁trans cription ▁because ▁of ▁the ▁properties ▁of ▁the ▁j unction ▁between ▁B - ▁and ▁Z - D NA . ▁ ▁At ▁the ▁j unction ▁of ▁B - ▁and ▁Z - D NA , ▁one ▁pair ▁of ▁bases ▁is ▁f li pped ▁out ▁from ▁normal ▁bond ing . ▁These ▁play ▁a ▁dual ▁role ▁of ▁a ▁site ▁of ▁recognition ▁by ▁many ▁prote ins ▁and ▁as ▁a ▁sink ▁for ▁t ors ional ▁stress ▁from ▁R NA ▁polym er ase ▁or ▁nucle os ome ▁binding . ▁ ▁N uc le os om es ▁and ▁be ads - on - a - string ▁Main ▁articles : ▁N uc le os ome , ▁Ch rom atos ome ▁and ▁Hist one ▁ ▁The ▁basic ▁repeat ▁element ▁of ▁chrom atin ▁is ▁the ▁nucle os ome , ▁inter connected ▁by ▁sections ▁of ▁linker ▁DNA , ▁a ▁far ▁shorter ▁arrangement ▁than ▁pure ▁DNA ▁in ▁solution . ▁ ▁In ▁addition ▁to ▁the ▁core ▁hist ones , ▁there