Principle of DNA cloning:

Suppose an experimental target that is to make huge amounts of a particular DNA fragment in pure form from a combination of DNA fragments. While the DNA fragments can be introduced into bacterial cells the most or all will lack the ability for self-replication and will fast be lost. Moreover, two kinds of DNA molecule are known that can replicate autonomously in bacterial cells; plasmids and bacteriophages. Plasmids are little circular double-stranded DNA molecules which exist free inside bacterial cells, frequently carry particular genes which confer drug resistance and are self-replicating of the genes. If a recombinant DNA molecule is establish through joining a foreign DNA fragment to plasmid or bacteriophage DNA then the foreign DNA is replicated when the plasmid or phage DNA is replicated. In that role the plasmid or phage DNA is called as a vector. Todays, a population of recombinant DNA molecules can be build, each recombinant molecule containing one of the foreign DNA fragments in the original combination. This can then be introduced into a population of bacteria like which each bacterial cell contains, in common, a different kind of recombinant DNA molecule. If we can recognize the bacterial cell which contains the recombinant DNA bearing the foreign DNA fragment we need, the cell can be multiplied in large and culture amounts of the recombinant DNA isolated. Foreign DNA can then be recovered from this in pure form; it is then said to have been cloned. The vector which was used to achieve this cloning is known as a cloning vector. The Vectors are not limited to bacterial cells; plant and animal viruses can also act as vectors.