Plasmids

Plasmids:

1. replicons
2. stably inherited
3. extrachromosomal
4. double-stranded circular DNA (mostly)
5. relaxed covalently closed circles (CCC DNA), open circles (OC DNA) or supercoiled DNA
6. plasmids with unknown phenotypic traits are called cryptic plasmids
7. conjugative or non-conjugative - depending upon whether or not they carry a set of transfer genes, called the tra genes, which promote bacterial conjugation. 
8. multiple copies per cell (relaxed plasmids) or a limited number of copied per cell (stringent plasmids). 

Host range

1. Encoding only a few of the proteins required for their own replication; all the other proteins required for replication are provided by the host cell. 
2. Plasmid-encoded replication proteins are located very close to the ori (origin of replication).
3. Other parts of the plasmid can be deleted and forein sequences can be added to the plasmid and the replication can occur.  
4. The host range of a plasmid is determined by its ori region. Plasmids with a broad host range encode most, if not all, of the proteins required for replication.