Molecular cloning

Molecular cloning, also called gene cloning or DNA cloning, means isolating a particular segment of DNA and making many identical copies of it so it can be studied or used. It is different from cloning a whole organism, such as Dolly the sheep. The usual vehicle is a plasmid, a small circle of bacterial DNA that copies itself independently of the chromosome and acts as a vector to carry the foreign fragment.

In a typical experiment, the DNA of interest and the plasmid are cut with the same restriction enzyme so their sticky ends match, and DNA ligase joins them into a recombinant plasmid. Bacteria are then made to take up the plasmid, a step called transformation. The plasmid carries an antibiotic resistance gene, so growing the bacteria on a medium containing that antibiotic kills cells without it. Blue-white screening can then separate colonies whose plasmid gained the insert, which disrupts a gene called lacZ and leaves them white, from blue colonies with an empty plasmid. Each colony grows from a single cell, so all its plasmids carry the same fragment.

Cloned genes are used to sequence DNA, study gene function and make recombinant proteins. Today the fragment to be cloned is often produced by PCR.