Explain the roles of the following with the help of an example each in recombinant DNA technology :
The plasmid is an autonomously replicating, circular, extra-chromosomal DNA present in bacteria.
They usually occur naturally in bacteria and their size varies from 1 to over 1,000 kbp. They carry extra genes which are not essential for bacterial survival but provide their hosts with extra advantages, such as antibiotic resistance.
Plasmids which are used in genetic engineering are called vectors as they are used as a vehicle for transferring a piece of DNA (attached to it) to the host organism. The piece of DNA from the source organism is integrated within the plasmid. The plasmid and the gene insert are cut at specific sites (with the help of restriction enzymes) and are thereafter, joined with the help of DNA ligase. The recombinant plasmid is then transferred inside the host. Once inside the host, the vector can replicate and express the desired gene. An example of such vector is pBR322.