Practice question

Linking number change measured by topoisomerase I relaxation assay using closed circular plasmids indicates each nucleosome reduces linking number approximately minus 1.2 to minus 1.26. Core particle alone contributes minus 1.0, but additional crossing of linker DNA constrained by H1 adds extra writhe bringing total to minus 1.2. This value originates from Simpson and colleagues SV40 minichromosome studies. Positive 1 would indicate overwinding opposite to nucleosome direction, 0.5 or minus 0.5 would underestimate DNA turns observed by crystallography. Topological storage underlies compaction and provides energy for unwrapping during transcription elongation by polymerase.