The word plasmid was originally used by J. Lederberg (1952) for any extrachromosomal hereditary determinant. This term is now used in a more restricted sense for those accessory DNA circles, which are found in bacteria (or in cell organelles like mitochondria in eukaryotes), in addition to the main chromosome. Since in bacteria the main chromosome is considered to be the nucleus, plasmids can be regarded as extranuclear genetic material. Therefore, sometimes plasmids are included in a treatment of extranuclear genetic systems which mainly deal with cell organelles like chloroplasts and mitochondria. However, since in eukaryotes, a cell is divided into nucleus and cytoplasm and the organelles are located in the cytoplasm, a parallelism between the cytoplasmic organelles and plasmids can not be easily established. Plasmids have, however, been studied most extensively using modern techniques of molecular genetics and, therefore, deserve a separate detailed treatment.
Plasmids differ among themselves in their characteristics and share the following general properties : (i) they are genetic elements made up of DNA; (ii) they are smaller than and separate from the main chromosome and (iii) they are capable of replication. There are other properties of plasmids, which are found in some plasmids and not in others. These properties which are found only in some plasmids like sex factor, include : (i) control of bacterial conjugation; (ii) reversible insertion in bacterial chromosome and (iii) transferability from one bacterium to another. Phages which are included in the category of episomes are sometimes excluded from the category of plasmids, since they exhibit unrestricted replication, leading to lysis. However, some phages like λ (lambda) share most of the properties of plasmids and, therefore, are grouped with them by some workers.
Although plasmids are often regarded as autonomous, they can be so regarded only in so far as properties of plasmid DNA depend on pre-existing plasmids. Replication of plasmid DNA, however, partly depends on the main bacterial chromosome also.