Figure 3.38 Eukaryotic cells have a well-defined nucleus. The nucleus of this mammalian lung cell is the large, dark,
oval-shaped structure in the lower right.
Although most eukaryotic cells have only one nucleus, exceptions exist. For example, protozoans of the genus
Parameciumtypically have two complete nuclei: a small nucleus that is used for reproduction (micronucleus) and a
large nucleus that directs cellular metabolism (macronucleus). Additionally, some fungi transiently form cells with
two nuclei, called heterokaryotic cells, during sexual reproduction. Cells whose nuclei divide, but whose cytoplasm
does not, are calledcoenocytes.
The nucleus is bound by a complexnuclear membrane, often called thenuclear envelope, that consists of two
distinct lipid bilayers that are contiguous with each other (Figure 3.39). Despite these connections between the
inner and outer membranes, each membrane contains unique lipids and proteins on its inner and outer surfaces. The
nuclear envelope contains nuclear pores,whicharelarge,rosette-shaped protein complexes that control themovement
of materials into and out of the nucleus. The overall shape of the nucleus is determined by thenuclear lamina,
a meshwork of intermediate filaments found just inside the nuclear envelope membranes. Outside the nucleus,
additional intermediate filaments form a looser mesh and serve to anchor the nucleus in position within the cell.
Figure 3.39 In this fluorescent microscope image, all the intermediate filaments have been stained with a bright
green fluorescent stain. The nuclear lamina is the intense bright green ring around the faint red nuclei.
Nucleolus
Thenucleolusis a dense region within the nucleus where ribosomal RNA (rRNA) biosynthesis occurs. In addition,
the nucleolus is also the site where assembly of ribosomes begins. Preribosomal complexes are assembled from rRNA
and proteins in the nucleolus; they are then transported out to the cytoplasm, where ribosome assembly is completed
(Figure 3.40).
Chapter 3 | The Cell 117