Genes can be inserted into prokaryotic or eukaryotic cells

• Bacteria are easily grown and manipulated in the laboratory. Much of their molecular biology is known, especially for well-studied bacteria such as E. coli. Furthermore, bacteria contain plasmids, which are easily manipulated to carry recombinant DNA into the cell. But since the processes of transcription, translation, and posttranslational modification proceed differently in prokaryotes than they do in eukaryotes, bacteria are not always well suited as hosts to express eukaryotic genes.

• Yeasts such as Saccharomyces cerevisiae are commonly used as eukaryotic hosts for recombinant DNA studies. The advantages of using yeasts include rapid cell division (a life cycle completed in 2–4 hours), ease of growth in the laboratory, and a relatively small genome size (see Table 17.2). In addition, yeast cells have most of the characteristics of other eukaryotic cells, with a notable exception being those associated with multicellularity.

• Plant cells are good hosts because of their ability to make totipotent *stem cells from mature plant tissues. The unspecialized cells can be transformed with recombinant DNA and then studied in culture, or grown into new plants. There are also methods for making whole transgenic plants without going through the cell culture step. These methods result in plants that carry the recombinant DNA in all their cells, including the germ line cells.

• Animal cells grown in lab culture can be used to study expression of human or animal genes, for example for medical purposes. Whole transgenic animals can also be made by inserting new DNA into egg cells.