Selectable markers are a genes introduced into a cell, especially a bacterium or to cells in culture, that confer a traits suitable for artificial selection. They are a type of reporter gene used in laboratory microbiology, molecular biology, and genetic engineering to indicate the success of a transfection or other procedure meant to introduce foreign DNA into a cell. Selectable markers are often antibiotic resistance genes: bacteria that have been subjected to a procedure to introduce foreign DNA are grown on a medium containing an antibiotic, and those bacterial colonies that can grow have successfully taken up and expressed the introduced genetic material. Normally, the genes encoding resistance to antibiotics such as ampicillin, chloramphenicol, tetracycline, kanamycin, etc., are considered useful selectable markers for E. coli.
The non-recombinants are separated from recombinants; that is, an r-DNA is introduced in bacteria, and some bacteria are successfully transformed while some remain non-transformed. When grown on a medium containing ampicillin, bacteria die due to lack of ampicillin resistance. The position is later noted on nitrocellulose paper and separated out to move them to a nutrient medium for mass production of the required product. An alternative to a selectable marker is a screenable marker, which can also be denoted as a reporter gene, which allows the researcher to distinguish between wanted and unwanted cells, such as between blue and white colonies. These wanted or unwanted cells are simply non-transformed cells that were unable to take up the gene during the experiment.[citation needed]
For molecular biology research, different types of markers may be used based on the selection sought. These include:
Examples of selectable markers include:
In the future, alternative marker technologies will need to be used more often to, at the least, assuage concerns about their persistence into the final product. It is also possible that markers will be replaced entirely by future techniques which use removable markers, and others which do not use markers at all, instead relying on co-transformation, homologous recombination, and recombinase-mediated excision.[6]
