101

All cells have a plasma membrane and contain genetic material. In some cells, the genetic material is housed in a membrane-bound space called the nucleus. Cells can be divided into two classes based on the absence or presence of a nucleus (Chapter 1). Prokaryotes, including bacteria and archaeons, lack a nucleus; eukaryotes, including animals, plants, fungi, and protists, have a nucleus (Fig. 5.16). The presence of a nucleus in eukaryotes allows for the processes of transcription and translation to be separated in time and space. This separation in turn allows for more complex ways to regulate gene expression than are possible in prokaryotes (Chapter 19).

There are other differences between prokaryotes and eukaryotes. For example, we saw in Chapter 3 that promoter recognition during transcription is different in prokaryotes and eukaryotes. In addition, there are differences in the types of lipid that make up their cell membranes. In mammals, as we have seen, cholesterol is present in cell membranes. Cholesterol belongs to a group of chemical compounds known as sterols, which are molecules containing a hydroxyl group attached to a four-ringed structure. In eukaryotes other than mammals, diverse sterols are synthesized and present in cell membranes. Most prokaryotes do not synthesize sterols, but some synthesize compounds called hopanoids. These five-ringed structures are thought to serve a function similar to that of cholesterol in mammalian cell membranes.

In spite of all of these and other differences between prokaryotes and eukaryotes, it is the absence or presence of a nucleus that defines the two groups. However, from an evolutionary perspective, archaeons and eukaryotes are more closely related to each other than either are to bacteria. For example, archaeons share with eukaryotes many genes involved in transcription and translation, and DNA is packaged with histones in both groups.