Getting DNA into a host

Once the recombinant DNA is made, it must get into the host cell. Bacteria do this naturally in three ways:

• Transformation: a cell takes up naked DNA directly from its surroundings. Cells that have done so are transformants.
• Transduction: a phage carries the DNA. When a phage that had joined the host chromosome (in lysogeny) leaves, it sometimes takes a piece of bacterial DNA with it, and delivers it to the next cell it infects (transductants).
• Conjugation: two bacteria touch, and DNA, usually an F plasmid, passes from the donor to the recipient through a pilus. An enzyme, relaxase, nicks one strand of the plasmid; that strand is transferred, and both cells then make the missing partner strand.

In the laboratory, the entry of DNA is helped along by chemical methods:

• Calcium chloride: Ca²⁺ ions bind the DNA to the cell surface; after a sudden heat shock, the cells take it in.
• Lipofection: the DNA is wrapped in tiny fat bubbles, liposomes, which fuse with the cell's phospholipid membrane and release the DNA inside.

And by physical methods:

• Electroporation: a short, mild electric pulse opens temporary micropores in the membrane, through which the DNA enters.
• Microinjection: the DNA is injected straight into the nucleus with a very fine glass needle.
• Biolistics, the gene gun: tiny particles of gold, nickel or tungsten, coated with DNA, are fired into cells at high speed. It is especially useful for plant cells, with their tough walls.