A selectable marker is a gene introduced into a cell, especially a bacterium or to cells in culture, that confers a trait 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, chloroamphenicol, tetracycline or kanamycin, etc., are considered useful selectable markers for E.coli. Modus Operandi- The non recombinants are separated from recombinants. i.e. A r-DNA is introduced in a bacteria, some bacterias are successfully transformed some remains non-transformed. When grown on 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 nutrient medium for mass production of required product. An alternative to a selectable marker is a screenable marker, which allows the researcher to distinguish between wanted and unwanted cells, e.g. between blue and white colonies. It is based on the fact that non recombinat have z-gene intact thus they produce blue colonies. On the other hand recombinants produces colourless colonies due to insertional inactivation of lac-z-gene. Lac-z-gene produces beta-galactosidase enzyme, which is responsible for the colour of colonies. Examples of selectable markers include:
- Beta-lactamase which confers ampicillin resistance to bacterial hosts.
- Neo gene from Tn5, which confers resistance to kanamycin in bacteria and geneticin in eukaryotic cells
- Mutant FabI gene (mFabI) from E. coli genome, which confers triclosan resistance to the host.
- Callmigration.org: Gene targeting
- Jang, Chuan-Wei; Magnuson, Terry (20 February 2013). "A Novel Selection Marker for Efficient DNA Cloning and Recombineering in E. coli". PLoS ONE 8 (2): e57075. doi:10.1371/journal.pone.0057075. PMC 3577784. PMID 23437314.
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