cloning vectors

Meaning: A cloning vector is a small DNA molecule that replicates in a host cell and carries a foreign DNA insert with it. Plasmids are the usual example; phages and artificial chromosomes (BACs, YACs) hold larger inserts. A useful vector has an origin of replication, a selectable marker such as an antibiotic resistance gene, and a multiple cloning site with several restriction sites. Vector and target DNA are cut with the same restriction enzyme to give complementary sticky ends, then joined by DNA ligase.

On the MCAT: Bacteria are transformed and plated on antibiotic, so only cells carrying the plasmid grow. Many vectors also carry lacZ; with X-gal, an intact lacZ gives blue colonies, while an insert in the cloning site disrupts lacZ and gives white colonies. Questions ask which colonies to pick, or what goes wrong if the wrong enzyme is used.

Common confusion: Growth on antibiotic shows only that a plasmid was taken up, since it may have resealed without the insert. White colonies in blue-white screening suggest an insert is present. Also, a cloning vector simply copies the DNA, while an expression vector adds a promoter so the insert is transcribed and translated in the host.