viscosity of its cytoplasm, the rigidity of its cell membrane or cell wall (if it has one), and the physical pressure
exerted on it by the surrounding environment and/or adjoining cells.
Figure 3.37 Eukaryotic cells come in a variety of cell shapes. (a) SpheroidChromulinaalga. (b) Fusiform shaped
Trypanosoma. (c) Bell-shapedVorticella. (d) OvoidParamecium. (e) Ring-shapedPlasmodium ovale.
- Identify two differences between eukaryotic and prokaryotic cells.
Nucleus
Unlike prokaryotic cells, in which DNA is loosely contained in the nucleoid region, eukaryotic cells possess a
nucleus, which is surrounded by a complex nuclear membrane that houses the DNA genome (Figure 3.38). By
containing the cell’s DNA, the nucleus ultimately controls all activities of the cell and also serves an essential role
in reproduction and heredity. Eukaryotic cells typically have their DNA organized into multiple linear chromosomes.
The DNA within the nucleus is highly organized and condensed to fit inside the nucleus, which is accomplished by
wrapping the DNA around proteins called histones.
116 Chapter 3 | The Cell
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