A recombinant DNA molecule is produced by joining together two or more DNA segments usually originating from different organisms. More specifically, a recombinant DNA molecule is a vector) e.g. a plasmid, phage or virus) in to which the deserted DNA fragment has been inserted to enable its cloning in an appropriate host. This is achieved by using specific enzymes for cutting the DNA (restriction enzymes) into suitable fragments and then for joining together the appropriate fragments (ligation).
In this manner, a recombinant DNA molecule may be produced which contains a gene from one organism joined to regulatory sequences from another organism, such a gene is called chimeric gene. Clearly, the capability to produce recombinant DNA molecules has given man the power and opportunity to create novel gene combination to suit specific needs.
Recombinant DNA molecules are produced with one of the following three objectives: (1) to obtain a large number of copies of specific DNA fragments, (2) to recover large quantities of the protein produced by the concerned gene, or (3) to integrate the gene in question into the chromosomes of a target organism where it expresses itself.
Even for the latter two objectives, it is essential to first obtain a large number of the concerned genes. To achieve this, the DNA segments are integrated into a self-replicating DNA molecule called vector; most commonly used vectors are either bacterial plasmids or DNA viruses.
All these steps concerned with piecing together DNA segments of diverse origin and placing them into a suitable vector together constitute recombinant DNA technology.
The vectors containing DNA segments to be cloned, called DNA inserts (chimaerie vectors) are then introduced into a suitable organism, usually a bacterium, this organism is called host, while the process is called transformation. The transformed host cells are selected and cloned.
The vector present in such clones would replicate either in synchrony with or independent of the host. The gene present in the vector may or may not express itself by directing the synthesis of concerned polypeptide. The step concerned with transformation of a suitable host with a chimaeric vector, and cloning of the transformant cells is called DNA cloning or gene cloning. However, often DNA or gene cloning is taken to include both the development of chimaeric vectors as well as their cloning in a suitable host.
A clone consists of asexual progeny of a single individual or cell, while the pr5ocess / technique of producing a clone are called cloning. As a result, all the individuals of a clone have the same genotype which is also identical with that of the individual from which the clone was derived. Therefore the genomes present in members of a single clone are also identical this applies to the recombinant DNA as well. Therefore, gene or DNA cloning produces large numbers of copies of the gene / DNA being cloned.
Similarly, often the term recombinant DNA technology is used as a synonym for DNA or gene cloning used in the broader sense. A rather popular term for these activities is genetic engineering.