Some viruses such as SV40 contain closed circular DNAs carry

Some viruses, such as SV40, contain closed circular DNAs carrying nucleosomes. If the SV40 viral chromosome is treated with topoisomerase, and the histone is then removed, it is still supercoiled. If histones are removed before topoisomerase treatment, however, the DNA is relaxed. Explain.

Solution

Histone protien is an octameric, made up of positively charged amino acid around that negatively charged dna IS WRAPPED. So if we apply topoisomerase teratment before removal of histone then warpped DNA around it would not be able to release its coil tension of 146 bp (one turn around histone). Due to this reason if we apply topoisomerase later after removal of histone DNA will be relaxed.