All plasmids identified so far are small circular DNA molecules. The DNA circles may be single or multiple structures linked together in chains. The size of these DNA molecules vary from 30 μm (e.g. F')in a plasmid of E. coli, making 20% of the main chromosome (same size in R plasmids also) to less than 1 μm as in P16C of E. coli. Colicin plasmids Col E₁, E₂ and E₃ are usually small having a length of 2.3 μm. The number of plasmids in a cell varies from a few in case of F and R plasmids to as many as 20-30 in case of Staphylococcus plasmids.

The DNA plasmids replicate in semi-conservative manner. It has been shown that while initiation of replication' is controlled by plasmid genes, other stages of replication i.e. polymerization (elongation) and termination are controlled by bacterial genes. When a plasmid is inserted in the bacterial chromosome, bacterial control on plasmid replication is absolute, but when free, the control is less that absolute.

In E. coli, transfer is controlled by plasmid genes tra-A, -B, -C, etc. and mutations in these genes lead to failure of transfer. However, when two plasmids having mutations for different tra-genes are introduced in the same cell, pilus may be formed and transfer may occur due to complementation between mutants. When transfer occurs, one of the two strands of plasmid DNA is nicked (broken) at a site called origin of transfer (Ori-T)arid the linear strand thus formed moves to the recipient bacteria. The single strands in the donor as well as in recipient cell will then synthesize complementary strands, so that duplex DNA plasmids are reconstituted in both the cells (Fig. 15.1). Sometimes due to insertion of plasmid, the transfer process may involve transfer of bacterial genes also, which are carried with plasmid DNA. Plasmid DNA may undergo recombination either with another plasmid DNA or with bacterial DNA. In the bacterial chromosome there are insertion sequences (IS), which are homologous in plasmids and bacteria. These sequences help in recombination. There are atleast 17 such sites in E. coli chromosome. Mapping of genes in plasmid DNA has also been achieved. Methods employed in mapping were not mainly recombination, but complementation (as described above) between strains having overlapping deletions. For more detailed description, the reader may refer to various reviews (Clowes, R.C., 1972, Bacterial Rev., 36 : 361-405; Clowes R.C., 1973, Sci. Amer. April 1974, 228 : 18-27). A condensed account of plasmids is also available in the book Extranuclear Genetics, written by G. Beale and J. Knowles.