Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

24 Chapter 1: Cells and Genomes
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
microtubule
centrosome with
pair of centrioles
5 µm
chromatin (DNA)
extracellular matrix
nuclear pore
nuclear envelope
vesicles
lysosome
actin
filaments
peroxisome
nucleolus
ribosomes
in cytosol
Golgi apparatus
intermediate
filaments
plasma membrane
nucleus
endoplasmic
reticulum
mitochondrion
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.