It refers production of multiple copies of a particular gene by using techniques of genetic engineering.
The term gene cloning is also called DNA cloning or recombinant DNA technology
Principle of gene cloning:
In this technique, a particular gene or sequence of DNA (by treatment with endonuclease enzymes) is isolated from an organism.
The isolated genome segment is combined with the vector genome, forming recombinant DNA.
The recombinant DNA is transferred to the host cell usually a bacteria, within which the recombinant DNA replicates as the bacteria cell divides, thus forming multiples copies of the recombinant DNA.