Practice question

DNA path around histone octamer dictates DNA topology. Around octamer DNA follows left-handed superhelical ramp of approximately 1.65 turns, compensating positive supercoiling generated ahead of replication fork. Upon nucleosome assembly on closed circular plasmid, agarose electrophoresis shows plasmid acquires more negative writhe, measured as linking number decrease, directly proving left-handed wrapping. Right-handed wrap would produce positive supercoiling opposite to experimental observation. Random binding would produce no systematic change, zig-zag describes fiber arrangement not wrapping direction. Therefore left-handed orientation established by crystallography and topology.