Gene cloning refers to the process of making multiple copies of a gene that are perfect replicas of the original gene. The process of cloning an organism is nothing but to make a genetically identical copy of it. Most often researchers in the laboratory perform cloning of a DNA fragment containing a particular gene to characterise it. Therefore, the process needs to be tailored to isolate the specific recombinant DNA containing the particular gene. Gene cloning is distinct yet closely related to genetic engineering since the latter involves the modification of an organism’s genome to achieve or alter a particular trait.
Although the process of cloning depends upon the organism and the gene to be cloned, the process can be generalised as follows:
(i) The section of DNA containing the gene of interest is inserted into a circular DNA (cDNA) which is also referred to as a vector to obtain a recombinant DNA molecule. Vectors can be of different types and the choice of vector is often dictated by the size of the gene that is to be cloned and the application of the cloned gene. The commonly used vectors are plasmids, viral vectors (including phage lambda and single-stranded phages), cosmids, and expression vectors.
(ii) The vector along with the gene gets transported into the host cell such as a bacterium.
(iii) Once inside the host cell, the vector multiples, producing numerous copies of itself, and thereby the gene that it is carrying.
(iv) With the division of the host cell, the vector and the gene of interest is transferred onto its progeny where further replication of the vector takes place.
(v) After several cycles of cell division of the host cell, a colony of identical host cells is obtained. Each cell within the colony has several vectors and multiple copies of recombinant DNA containing the gene that has been cloned.
Gene cloning technique is often used to produce a pure sample of a gene from a mixture of DNA fragments, each containing a different gene. The challenge lies in identifying the particular clone of interest from the ones that contain different genes. The vector used such as a plasmid contains an antibiotic-resistant gene in addition to the gene of interest. This antibiotic-resistant gene allows the bacteria to survive in presence of the specific antibiotic, and thus aid in selecting the particular clone containing the gene of interest. Gene cloning is also used for the production of a particular protein. Once the colony of bacteria has been established with each bacterium containing the particular gene, these bacteria can act as mini-factories producing a large amount of the protein expressed by the gene. For e.g., if the plasmid contains the human insulin gene, then successful transcription of the gene into mRNA followed by its translation within the bacterial colony would produce human insulin protein. Gene cloning finds potential application in the production of biopharmaceuticals, in developing gene therapy for genetic disorders, and gene analysis to understand the functioning of genes.