During enzymatic replication of plasmids containing the origin of the Escherichia coli chromosome, oriC, formation of an active initiation complex consisting of dnaA, dnaB, dnaC, and HU proteins, requires a supercoiled DNA template. Relaxed covalently closed plasmids are active only if supercoiled by gyrase prior to initiation; nicked and linear DNAs are inactive. Semi-conservative replication proceeds via delta structure as intermediates. Daughter molecules include nicked intermediates. Daughter molecules include nicked monomers and catenated pairs. Elongation is rapid, but late replicative intermediates accumulate because the final elongation and termination steps are slow. Production of covalently closed circular daughter DNA molecules requires removal of ribonucleotide residues (primers) by DNA polymerase I, assisted by ribonuclease H, gap filling, and ligation of nascent strands by ligase. Reconstitution of a complete cycle of oriC plasmid replication, beginning and ending with supercoiled molecules, has been achieved with purified proteins.