In Biotechnology the gene is the cornerstone of most molecular biology studies. The study of genes can be facilitated by isolation and amplification of gene of interest. Cloning is one method used for isolation and amplification of gene of interest. The gene is cloned by inserting it into another DNA molecule which acts as vehicle or vector that will replicate in living cells. As the two DNA molecules of different origin are combined, the resulting DNA is known as recombinant DNA molecule. The term "gene cloning," "DNA cloning," "molecular cloning," and "recombinant DNA technology" all refer to same technique: Insertion of DNA fragment of interest from one organism into a vector which is a self- replicating genetic element inside a living cell. Gene cloning processes include removal of DNA from the cell, carrying out the DNA manipulations in test vial and, transformation of constructed DNA molecule back into the cells.

The first step in cloning is to prepare large amount of the vector and chromosomal DNAs. To carry the gene or the desired DNA fragment to the cell there is a need of a vector molecule. All cloning vectors are carrier DNA molecules. These carrier molecules host few common features in general such as; all vectors are self replicating in the cell, they contain a number of unique restriction enzyme cleaving sites that are present only once in the vector, they carry the selectable marker gene which is useful in selection of clone (usually an antibiotic resistance gene that is absent in the host cell) and, they can be very easily isolated from host cell. Depending on the purpose of cloning there are many vectors available. For use in the bacterial host E. coli system a greatest variety of cloning vectors have been developed. Thus, the first thing in cloning that a molecular biologist requires is to grow pure culture and isolate the cloning vector from the cells.

Choice of vector is dependent on insert size and application

The most commonly used cloning vectors include plasmids and bacteriophages (phage λ ) beside all the other available vectors (Table 1). The cloning vectors are limited to the size of insert that they can carry. Depending on the size and the application of the insert the suitable vector is selected. The different types of vectors available for cloning are plasmids, bacteriophages, bacterial artificial chromosomes (BACs), yeast artificial chromosomes (YACs) and mammalian artificial chromosomes (MACs).

Plasmids: Plasmids are extra chromosomal circular double stranded DNA replicating elements present in bacterial cells. Plasmids show the size ranging from 5.0 kb to 400 kb. Plasmids are inserted into bacterial calls by a process called transformation. Plasmids can accommodate an insert size of upto 10 kb DNA fragment. Generally plasmid vectors carry a marker gene which is mostly a gene for antibiotic resistance; thereby making any cell that contains the plasmid will grow in presence of the selectable corresponding antibiotic supplied in the media.