Four Ways to Insert Foreign DNA Into Cells

The techniques for inserting foreign DNA into cells depend on the type of cell in question. Bacteria can take up foreign DNA through processes called transformation, conjugation, and transduction. A process called transfection is used to insert foreign DNA into mammalian cells. All four of these techniques are important to modern research in molecular biology and medicine.

The simplest way bacteria can take up foreign DNA is through transformation, a technique used very frequently in molecular biology labs. In this technique bacteria take up purified DNA through chemical and heat shock. Bacterial cell walls do not normally allow DNA in, and must be made "competent" to take up DNA. Treating bacteria with calcium chloride makes them take up water and DNA, and a subsequent heat shock activates genes that help bacteria recover from the calcium chloride treatment.

In the process of bacterial conjugation, bacteria transfer native plasmids -- small circular pieces of DNA -- to other bacteria. The plasmid contains a special sequence known as the F-factor, and one bacterium forms a threadlike structure called a pilus to transfer a copy of its F-factor-containing plasmid to another bacterium. The plasmid containing the F-factor can have other genes besides the F-factor, and in this way various genes can be inserted from one bacterium to another. This technique can be harnessed in the molecular biology laboratory to insert a DNA sequence of interest into bacteria.

Like animal cells, bacteria have viruses that can infect them. Bacterial viruses are called bacteriophages. The life cycle of a bacteriophage involves the insertion of the bacteriophage's DNA into the host cell DNA, followed by removal of the bacteriophage DNA when the phage is ready to reproduce itself. Bacteriophage insertion is known as transduction. Generalized transduction is used to insert foreign DNA into bacterial cells in laboratories. The process takes advantage of bacteriophage reproduction, in which they sometimes package bacterial DNA into their phage particles that is later transferred to a separate bacterium.

Mammalian cells can be made to take up foreign DNA through two methods - both of which are called transfection. Chemical transfection involves treating cells with a chemical like calcium phosphate, which allows them to take up DNA by a mechanism that isn't understood. Another method involves using viruses to insert foreign DNA, similar to bacterial transduction. This method is called virus-mediated transfection, or simply transduction. At the time of publication, it is of limited use in research because viruses have a limited capacity for carrying DNA.