The termination region for SV40 DNA replication directs the mode of separation for the two sibling molecules

Separation of the two newly replicated chromosomes in simian virus 40 late replicating intermediates (RI*) occurred by two pathways. The parental DNA strands were completely unwound, releasing circular DNA monomers with a gap in the nascent strand (Form II*), or duplex DNA in the termination region was not unwound, resulting in formation of catenated dimers. Under optimal conditions, both products were transient intermediates in replication, although Form II* was predominant. However, in hypertonic medium both RI* and catenated dimers accumulated, and Form II* was not observed. Hypertonic medium appeared to inhibit both DNA unwinding in the termination region and separation of catenated dimers. When the size of the genome or the position of the origin of replication was changed, termination occurred at sites other than that of wild-type SV40. Neither catenated dimers nor RI* DNA accumulated at these sites. Instead, RI* separated into Form II*. Unwinding parental DNA was more difficult at some termination regions than others. Therefore, although completion of DNA replication does not require a unique termination sequence, this sequence can determine the mode of separation for sibling molecules.