Functional analysis of minichromosome replication: bidirectional and unidirectional replication from the Escherichia coli replication origin, oriC.

Replicating molecules of minichromosomes pCM959 and pOC24 were analyzed by electron microscopy. Replication of pCM959 proceeded bidirectionally from the replication origin, oriC, in about 60% of the molecules; the rest of the molecules replicated unidirectionally in either direction. pOC24, in which deoxyribonucleic acid to the right (clockwise) of the oriC segment is deleted, seemed to replicate predominantly unidirectionally counterclockwise from oriC.