A plasmid is a small, circular DNA molecule that is different from the cell’s chromosomal DNA. It can replicate independently of the chromosomal DNA and are found in most of the bacteria. Plasmids are generally used as cloning vectors. For being a good cloning vector, a plasmid must have the following characteristics: • An origin of replication or ori from where the replication begins. • A selectable marker that usually has a specific action like antibiotic resistance that enables it to grow on some selective media. • It should itself be a small fragment of DNA that is easy to ligate in a different strand of DNA. • It should have a high copy number.