Along with methods to manipulate DNA fragments came the capability of isolating genes from one species and introducing them into another. This type of genetic engineering is called recombinant DNA technology because it literally recombines DNA molecules from two (or more) different sources into a single molecule. Recombinant DNA technology involves cutting DNA by restriction enzymes, isolating them by gel electrophoresis, and ligating them with enzymes used in DNA replication. This technology is possible because the DNA of all organisms is the same, differing only in sequence but not in chemical or physical structure. When DNA fragments from different sources are combined into a single molecule and incorporated into a cell, they are replicated and transcribed just like any other DNA molecule.

Recombinant DNA technology can combine DNA from any two sources, including different species. DNA from one species of bacteria can be combined with another, or a human gene can be combined with bacterial DNA, or the DNA from a plant and a fungus can be combined into a single molecule. These new sequences may be unlike any found in nature, raising questions about their possible effects on human health and the environment.

This section discusses one of the basic methods for producing recombinant DNA, some of the important applications of recombinant DNA technology such as genetically modified organisms, and a new method that can be used to change the DNA sequence of any organism into any other desired sequence.