.) ▁▁ 3 0 - nan ometer ▁chrom atin ▁fi ber ▁ ▁With ▁addition ▁of ▁H 1 , ▁the ▁be ads - on - a - string ▁structure ▁in ▁turn ▁co ils ▁into ▁a ▁ 3 0   nm ▁diameter ▁hel ical ▁structure ▁known ▁as ▁the ▁ 3 0   nm ▁fib re ▁or ▁fil ament . ▁The ▁precise ▁structure ▁of ▁the ▁chrom atin ▁fi ber ▁in ▁the ▁cell ▁is ▁not ▁known ▁in ▁detail , ▁and ▁there ▁is ▁still ▁some ▁debate ▁over ▁this . ▁ ▁This ▁level ▁of ▁chrom atin ▁structure ▁is ▁thought ▁to ▁be ▁the ▁form ▁of ▁heter och rom atin , ▁which ▁contains ▁mostly ▁trans cription ally ▁silent ▁gen es . ▁EM ▁( elect ron ▁micro sc opy ) ▁studies ▁have ▁demonstrated ▁that ▁the ▁ 3 0   N M ▁fi ber ▁is ▁highly ▁dynamic ▁such ▁that ▁it ▁unf olds ▁into ▁a ▁ 1 0   nm ▁fi ber ▁(" be ads - on - a - string ") ▁structure ▁when ▁trans vers ed ▁by ▁an ▁R NA ▁polym er ase ▁engaged ▁in ▁trans cription . ▁ ▁The ▁existing ▁models ▁commonly ▁accept ▁that ▁the ▁nucle os om es ▁lie ▁per pend icular ▁to ▁the ▁axis ▁of ▁the ▁fib re , ▁with ▁linker ▁hist ones ▁arranged ▁internally . ▁A ▁stable ▁ 3 0   nm ▁fib re ▁re lies ▁on ▁the ▁regular ▁position ing ▁of ▁nucle os om es ▁along ▁DNA . ▁Link er ▁DNA ▁is ▁relatively ▁resist ant ▁to ▁b ending ▁and ▁rotation . ▁This ▁makes ▁the ▁length ▁of ▁linker ▁DNA