Screening: Example of The Blue/white Selection
Many general purpose vectors such as pUC19 usually include a system for detecting the presence of a cloned DNA fragment, based on the loss of an easily scored phenotype. The most widely used is the gene coding for E. coli β-galactosidase, whose integrity can easily be detected by the ability of the enzyme it encodes to hydrolyze the soluble, colourless substrate X-gal (5-bromo-4-chloro-3-indolyl-beta-d-galactoside) into an insoluble, blue product (5,5'-dibromo-4,4'-dichloro indigo). Cloning a fragment of DNA within the vector-based gene encoding the β-galactosidase prevents the production of an active enzyme. If X-gal is included in the selective agar plates, transformant colonies are generally blue in the case of a vector with no inserted DNA and white in the case of a vector containing a fragment of cloned DNA.
Read more about this topic: Cloning Vector
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Ten thousand fleets sweep over thee in vain;
Man marks the earth with ruin,—his control
Stops with the shore;”
—George Gordon Noel Byron (1788–1824)
“The Supreme Court would have pleased me more if they had concerned themselves about enforcing the compulsory education provisions for Negroes in the South as is done for white children. The next ten years would be better spent in appointing truant officers and looking after conditions in the homes from which the children come. Use to the limit what we already have.”
—Zora Neale Hurston (1891–1960)
“Every writer is necessarily a critic—that is, each sentence is a skeleton accompanied by enormous activity of rejection; and each selection is governed by general principles concerning truth, force, beauty, and so on.... The critic that is in every fabulist is like the iceberg—nine-tenths of him is under water.”
—Thornton Wilder (1897–1975)