Eukaryotic cells, in general, are bigger and more elaborate than prokaryotic cells, and their genomes are bigger and more elaborate, too. The greater size is accompanied by radical differences in cell structure and function. Moreover, many classes of eukaryotic cells form multicellular organisms that attain levels of complexity unmatched by any prokaryote.
Because they are so complex, eukaryotes confront molecular biologists with a special set of challenges that will concern us in the rest of this book. Increasingly, biologists attempt to meet these challenges through the analysis and manipulation of the genetic information within cells and organisms. It is therefore important at the outset to know something of the special features of the eukaryotic genome. We begin by briefly discussing how eukaryotic cells are organized, how this reflects their way of life, and how their genomes differ from those of prokaryotes. This leads us to an outline of the strategy by which cell biologists, by exploiting genetic and biochemical information, are attempting to discover how eukaryotic organisms work.
eukaryotic cells may have originated as Predators
By definition, eukaryotic cells keep their DNA in an internal compartment called the nucleus. The nuclear envelope, a double layer of membrane, surrounds the nucleus and separates the DNA from the cytoplasm. Eukaryotes also have other features that set them apart from prokaryotes (Figure 1–25). Their cells are, typically, 10 times bigger in linear dimension and 1000 times larger in volume. They have an elaborate cytoskeleton—a system of protein filaments crisscrossing the cytoplasm and forming, together with the many proteins that attach to them, a system of girders, ropes, and motors that gives the cell mechanical strength, controls its shape, and drives and guides its movements (Movie 1.1). And the nuclear envelope is only one part of a set of internal membranes, each structurally similar to the plasma membrane and enclosing different types of spaces inside the cell, many of them involved in digestion and secretion. Lacking the tough cell wall of most bacteria, animal cells and the free-living eukaryotic cells called protozoa can change their shape rapidly and engulf other cells and small objects by phagocytosis (Figure 1–26).
How all of the unique properties of eukaryotic cells evolved, and in what sequence, is still a mystery. One plausible view, however, is that they are all reflections of the way of life of a primordial cell that was a predator, living by capturing other cells and eating them (Figure 1–27). Such a way of life requires a large cell with a flexible plasma membrane, as well as an elaborate cytoskeleton to support
Figure 1–25 The major features of eukaryotic cells. The drawing depicts a typical animal cell, but almost all the same components are found in plants and fungi as well as in single-celled eukaryotes such as yeasts and protozoa. Plant cells contain chloroplasts in addition to the components shown here, and their plasma membrane is surrounded by a tough external wall formed of cellulose.
and move this membrane. It may also require that the cell’s long, fragile DNA molecules be sequestered in a separate nuclear compartment, to protect the genome from damage by the movements of the cytoskeleton.