the source of the DNA. So a fragment sheared from the DNA of a mouse, for example, can be joined to a fragment cut from the DNA of an elephant by the same restriction enzyme.

Compatible fragments of DNA bond together when complementary bases on their sticky ends pair up. These bonds, however, are fairly weak and can easily be broken by such things as heat. The connection is made more secure with the help of a sealing enzyme called a ligase. Ligases are another group of naturally occurring enzymes. They are produced by cells to help synthesize DNA and repair minor damage to the molecule.

The usual reason for making recombinant DNA is to introduce a new sequence into a species where it doesn't normally occur. The added sequence includes a gene that modifies the host cell in some way. For example, scientists may want to introduce a gene for producing an antiviral compound into bacteria, so the bacteria will manufacture this compound for medical use. The challenge is to get the recombinant DNA into the host cells without seriously disrupting their normal functioning.

This is where the plasmids and bacteriophages mentioned earlier come into play. All a researcher has to do is splice the DNA of interest into the DNA of one of these naturally occurring vectors (transmitting agents), then release the vectors with their recombinant DNA in a culture of bacteria and let them do the rest.

The very first transfer of a gene from one organism to another was carried out in this way. In 1973, American