A vector is a plasmid or a virus used to transfer foreign genetic material into a cell. A plasmid is a ring of extrachromosomal DNA in the cytoplasm of bacteria. Introduction of foreign DNA into vector DNA to produce rDNA requires two enzymes. It is called restriction enzyme because it restricts growth of viruses. The foreign gene is sealed into the vector DNA by DNA ligase.
Treated cells take up plasmids, and then bacteria and plasmids reproduce.
Eventually, there are many copies of the plasmid and many copies of the foreign gene. In genetic engineering, cloning is production of many identical copies of a gene. Bacterial cells take up recombined plasmids when treated with CaCl 2 to make them more permeable. If the inserted gene is replicated and expressed, we can recover the cloned gene or protein product. Viruses can be a vector to carry rDNA into bacterial cells.