ent processes. In one of these, called transformation, DNA is released from bacterial cells into the surrounding medium, then taken up and incorporated into the DNA of nearby cells. (The "transforming principle" discovered by Fred Griffith in 1928 and described on pages 12 to 14 was an example of transformation that eventually led scientists to realize that genes are made of DNA.) Another method of DNA transfer involves viruses, which can combine fragments of bacterial DNA with their own. They carry the foreign DNA from one species of bacteria to another when they infect more cells. Researchers have exploited the strategies used in battles between viruses and bacteria to develop a method for making recombinant DNA (that is, novel DNA made by combining DNA fragments from different sources).

Enigmatic entities occupying the borderlines between living and non-living things, viruses are little more than maverick molecules of DNA or RNA housed in protective protein coats. They resemble cells in having genetic instructions for making new versions of themselves, but differ from living organisms in lacking the biochemical machinery needed for their own multiplication.

Left to themselves, viruses do nothing. They can remain unchanging for years, inert as a jar full of pebbles. To reproduce, they must hijack the metabolic apparatus of a living cell, subverting it to manufacture new viruses and often killing the cell in the process.

Viruses that commandeer bacterial cells are named bacteriophages, or simply phages. They settle on bacterial