Screening the Library
The gene cloner must then screen the library in order to discover which bacterial colony is making copies of the one gene they are interested in. Library screening identifies colonies, which have that particular gene. Screening can be based on detecting the DNAsequence of the cloned gene, detecting a protein that the gene encodes, or the use of linked DNA markers. Therefore before library screening can be done, the scientist must know either the DNA sequence of the gene, or a very similar gene, the protein that the gene produces, or a DNA marker that has been mapped very close to the gene. When the bacteria multiply and replicate the recombinant DNA, the number of gene copies also increases, making gene or protein detection easier.
When the bacteria colony containing the desired gene is located, the bacteria can be propagated to make millions of copies of the recombinant plasmid that contains the gene. The plasmids can be extracted for the next steps of genetic engineering, gene modification, and transformation. Gene cloning is also important because copies of a gene are needed for these procedures.
Bacterial plasmids used in gene cloning naturally contain genes that encode some form of antibiotic resistance. As a result of the gene cloning process, the cloned genes that are put into plant cells to make a transgenic plant may also have an antibiotic resistance gene. The antibiotic resistance trait is often used to help scientists determine which bacterium have been transformed. Those that have antibiotic resistance have been transformed and are kept. Some critics of genetic engineering claim that the potential risk of providing an opportunity for organisms in nature to gain antibiotic resistance by taking up the plasmid outweighs the potential benefits of this technology.