Plasmids are small, extra -chromosomal, double stranded circular forms of DNA that replicate autonomously. They are present in bacterial cells, yeast and animal cells. Plasmids are considered as replicons as they are capable of autonomous replication in suitable host. The most commonly used vectors in r-DNA technology are plasmids as they replicate in E. Coli. Plasmid as a cloning vector should have a replication origin, a marker gene for antibiotic resistance, control elements like promoter, operator, ribosome binding site etc.and a region where foreign DNA can be inserted. Naturally plasmids do not have all these features. Hence, they are constructed by inserting gene for antibiotic resistance. Ti plasmid of agrobacterium tumefaciens is an important vector for carrying new DNA in many plants. It i up tself into the chromosomes of infected plant cells. The transposon, with the new DNA, can be inserted into the host cell's chromosomes. A plant cell containing this DNA, can t...