Microbiology, 2021

106 3 • The Cell
Figure 3.36 Eukaryotic cells come in a variety of cell shapes. (a) Spheroid Chromulina alga. (b) Fusiform shaped Trypanosoma. (c) Bellshaped
Vorticella. (d) Ovoid Paramecium. (e) Ring-shaped Plasmodium ovale. (credit a: modification of work by NOAA; credit b, e:
modification of work by Centers for Disease Control and Prevention)
CHECK YOUR UNDERSTANDING
• 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 3.37). 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.
Figure 3.37 Eukaryotic cells have a well-defined nucleus surrounded by a nuclear membrane. The nucleus of this mammalian lung cell is
located in the bottom right corner of the image. The large, dark, oval-shaped structure within the nucleus is the nucleolus.
Although most eukaryotic cells have only one nucleus, exceptions exist. For example, protozoans of the genus
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