Editing Genetic transformation (section)

===Selection and screening in plasmid transformation===
Because transformation usually produces a mixture of relatively few transformed cells and an abundance of non-transformed cells, a method is necessary to select for the cells that have acquired the plasmid.<ref name="bimboim">{{cite journal | vauthors = Birnboim HC, Doly J | title = A rapid alkaline extraction procedure for screening recombinant plasmid DNA | journal = Nucleic Acids Research | volume = 7 | issue = 6 | pages = 1513–23 | date = November 1979 | pmid = 388356 | pmc = 342324 | doi = 10.1093/nar/7.6.1513 }}</ref> The plasmid therefore requires a [[selectable marker]] such that those cells without the plasmid may be killed or have their growth arrested. [[Antibiotic resistance]] is the most commonly used marker for prokaryotes. The transforming plasmid contains a gene that confers resistance to an antibiotic that the bacteria are otherwise sensitive to. The mixture of treated cells is cultured on media that contain the antibiotic so that only transformed cells are able to grow. Another method of selection is the use of certain [[auxotrophy|auxotrophic]] markers that can compensate for an inability to metabolise certain amino acids, nucleotides, or sugars. This method requires the use of suitably mutated strains that are deficient in the synthesis or utility of a particular biomolecule, and the transformed cells are cultured in a medium that allows only cells containing the plasmid to grow.

In a cloning experiment, a gene may be inserted into a plasmid used for transformation. However, in such experiment, not all the plasmids may contain a successfully inserted gene. Additional techniques may therefore be employed further to screen for transformed cells that contain plasmid with the insert. [[Reporter gene]]s can be used as [[marker gene|markers]], such as the ''[[lac operon|lacZ]]'' gene which codes for [[Beta-galactosidase|β-galactosidase]] used in [[blue-white screen]]ing. This method of screening relies on the principle of α-[[Complementation (genetics)|complementation]], where a fragment of the ''lacZ'' gene (''lacZα'') in the plasmid can complement another mutant ''lacZ'' gene (''lacZΔM15'') in the cell. Both genes by themselves produce non-functional peptides, however, when expressed together, as when a plasmid containing ''lacZ-α'' is transformed into a ''lacZΔM15'' cells, they form a functional β-galactosidase. The presence of an active β-galactosidase may be detected when cells are grown in plates containing [[X-gal]], forming characteristic blue colonies. However, the [[multiple cloning site]], where a gene of interest may be [[DNA ligase|ligated]] into the plasmid [[Vector (molecular biology)|vector]], is located within the ''lacZα'' gene. Successful ligation therefore disrupts the ''lacZα'' gene, and no functional β-galactosidase can form, resulting in white colonies.  Cells containing successfully ligated insert can then be easily identified by its white coloration from the unsuccessful blue ones.

Other commonly used reporter genes are [[green fluorescent protein]] (GFP), which produces cells that glow green under blue light, and the enzyme [[luciferase]], which catalyzes a reaction with [[luciferin]] to emit light. The recombinant DNA may also be detected using other methods such as nucleic acid hybridization with radioactive RNA probe, while cells that expressed the desired protein from the plasmid may also be detected using immunological methods.