Discussion
Which of these is an example of a selectable marker used in cloning?
Answer: C
Correct answer: An antibiotic resistance gene
A selectable marker is a gene that allows cells carrying the vector to be identified and grown while those without it are killed, and an antibiotic resistance gene is the standard example. Plating transformed cells on medium containing ampicillin, for instance, means only those that took up an ampicillin-resistance plasmid survive. It solves a real problem: transformation is inefficient, and only a small fraction of cells take up the vector, so without selection the successful ones would be lost among the rest. A second stage is often needed to distinguish cells carrying a vector with an insert from those carrying an empty one. Blue-white screening does this: the cloning site sits within the lacZ gene, so an insert disrupts it, and colonies that cannot cleave the substrate X-gal remain white while empty vectors give blue. The other options are also essential parts of a vector - a restriction site to accept the insert, an origin to replicate, a ribosome binding site to express - but none of them selects.
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