THE STRUCTURE OF SUPERCOILED INTERMEDIATES IN DNA-REPLICATION

We studied the structure of replication intermediates accumulated by T us-induced arrest of plasmid DNA replication at termination sites. For intermediates generated both in vitro with purified components and in vivo, superhelical stress is distributed throughout the entire partia lly replicated molecule; daughter DNA segments are wound around each o ther, and the unreplicated region is supercoiled. Thus, unlinking of p arental DNA strands by topoisomerases can be carried out both behind a nd in front of the replication fork. We explain why previous studies w ith prokaryotic and eukaryotic replication intermediates discerned onl y supercoiling in the unreplicated portion.