cloning vector

If you want to carry a fragile note safely through a crowded city and have a copy machine make thousands of duplicates of it, you need a courier — something that knows the route and triggers the copier. In molecular cloning, that courier is the vector: a DNA molecule engineered to carry your gene of interest into a host cell and get it copied there.

The workhorse cloning vector is a plasmid — a small, circular DNA that bacteria naturally carry separate from their chromosome. An engineered plasmid vector has three essential parts. First, an origin of replication, the sequence the host's machinery recognizes to start copying the plasmid, so it multiplies inside the cell. Second, a selectable marker, usually an antibiotic-resistance gene, so you can selectively grow only the cells that took up the vector. Third, a multiple cloning site, a short stretch packed with unique restriction sites where you cut open the vector and ligate your insert. You cut both vector and insert, ligate them, and put the recombinant plasmid into bacteria, which then copy it millions of times as they divide.

Different jobs need different vectors. Ordinary plasmids carry small inserts (up to roughly 10,000 base pairs) and are perfect for a single gene. To carry much larger pieces of DNA you switch to high-capacity vectors built from bacteriophages, cosmids, BACs, or YACs. And if your goal is not just to store a gene but to make its protein, you use an expression vector, which adds a promoter and other signals so the host actually transcribes and translates the gene.