Microbiology, 2021

3.4 • Unique Characteristics of Eukaryotic Cells 107
Paramecium typically 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 called coenocytes.
The nucleus is bound by a complex nuclear membrane, often called the nuclear envelope, that consists of
two distinct lipid bilayers that are contiguous with each other (Figure 3.38). 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, which are large, rosette-shaped protein complexes that
control the movement of materials into and out of the nucleus. The overall shape of the nucleus is determined
by the nuclear 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 3.38
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.
Eukaryotes are able to multiply through asexual reproduction, during which a single parent cell becomes two
identical daughter cells. This process of clonal reproduction is called mitosis. Although mitosis may sound
similar to asexual binary fission in prokaryotes, the processes are very different. In contrast to the single
chromosome in most prokaryotes, eukaryotic cells possess multiple chromosomes that must be replicated and
strategically divided between daughter cells. Therefore, mitosis is a much more complex cellular process than
binary fission.
The eukaryotic cell cycle is an ordered and carefully regulated series of events involving cell growth, DNA
replication, and cell division to produce two clonal daughter cells. One “turn” or cycle of the cell cycle consist of
two general phases: interphase and the mitotic phase. (Figure 3.39). During interphase, the cell is not dividing,
but rather is undergoing normal growth processes and DNA is replicated preparing for cell division. The three
stages of interphase are called G1, S, and G2.