Bacterial Plasmid: Plasmid are widely been used for cloning of foreign DNA into the bacteria as host strain. Before getting into the details of discussing bacterial plasmid we will discuss the basic properties of plasmids.  

Different forms of plasmids: Bacterial plasmid is a double stranded circular DNA exists in 3 different forms (Figure 21.1). If the both strands of circular double strands are intact then it is called as covalently closed circles (CCC) where as if one of the strand has nick, then it acquire the conformation of open circle DNA (OC, DNA). During the isolation of plasmid DNA from bacteria, covalently closed circular DNA losses few number of turns and as a result it acquire supercoiled configuration. The interchange between these different forms are possible under the in-vitro or in-vivo conditions, such as DNA gyrase produces additional turn into the circular DNA to adopt supercoiled conformation. 

 

Figure 21.1: Different forms of plasmids

Features of different plasmids: There are minimum molecular components to assemble bacterial plasmid to perform the function of vector are as follows-

1. Origin of replication-Like any other replicating DNA, plasmid DNA needs its own independent origin of replication to provide replication start site to make more copies. It decides the range of bacterial host strain can be use with the particular plasmid vector. The plasmids containing ori region from Col E1 can be able to grow in limited bacterial species such as E.Coli etc. In contrast, plasmid containing ori from RP4 or RSF1010 can be able to grow in gram (-) bacteria and gram (+) bacteria.

2. Selection marker- Selection marker in the form of either antibiotic resistance gene or enzymatic gene is essential to give phenotypic changes in host after entry of the plasmid.

3. Promoter- Plasmid replication in host is performed by the host provided proteins such as DNA gyrase, helicase, polymerase and DNA ligase. But proteins required for conferring antibiotic resistance or enzyme use for selecting transformed host cells is present on plasmid and a promoter adjacent is required to express genes present on plasmid DNA. In addition, promoter is also needed to express gene present on foreign DNA.