recombinant DNA

Meaning: Recombinant DNA is a DNA molecule built by joining sequences from different sources, typically a gene of interest placed into a vector such as a plasmid. A restriction enzyme cuts both the vector and the source DNA, often leaving complementary sticky ends that base-pair, and DNA ligase seals the sugar-phosphate backbone with phosphodiester bonds. The recombinant plasmid is introduced into host cells, usually bacteria, by transformation, and the host copies it as it divides, producing many copies of the gene or its protein.

On the MCAT: Passages show a plasmid map with restriction sites, an antibiotic resistance gene, and a marker such as lacZ. You may be asked which enzyme to use, what fragment sizes appear on a gel, or which colonies carry the insert. Cells growing on antibiotic took up a plasmid; white colonies in blue-white screening contain an insert that disrupted lacZ. To express a eukaryotic gene in bacteria, cDNA made by reverse transcriptase is used, because bacteria cannot splice out introns.

Common confusion: Growth on antibiotic does not prove the insert is present, since a plasmid with no insert also confers resistance. The insert is shown by a disrupted marker, or by PCR, gel analysis or sequencing.