CHAPTER 7
CELL STRUCTURE AND FUNCTION
As discussed briefly in Chapter 1, prokaryotic cells differ in a number of major waysfrom eukaryotic cells (Table 7-1). Lower eukaryotes (protozoa, algae, and fungi) lacksome of the distinguishing features of metazoan cells. Nevertheless, all eukaryotic cellsappear to be similar versions of the same overall plan. In the past, some cytologistsconsidered that bacterial cells were merely smaller replicas of the cells of higher formsand that failure to distinguish comparable subcellular structures in bacteria could beattributed to the limitations of the microscopic and cytological techniques available.This may prove to be the case, since many recent investigations with highly improvedtechniques and equipment have shown that there may be a closer similarity betweeneukaryotes and prokaryotes than previously thought. It has also become apparent thatthe cells of archaebacteria (Archaea) display important structural differences from thecells of either eubacteria or eukaryotes.
THE EUKARYOTIC NUCLEUS
Eukaryotes (Eukarya) display a cytologically distinct nucleus, the organizational andregulatory center for virtually all of the biochemical and hereditary processes of thecell. With the aid of the electron microscope it has been possible to demonstrate manystructural details of the eukaryotic nucleus. Figure 7-1 shows a mammalian cell witha well-defined nuclear membrane composed of at least two distinct layers. The outersurface contains pores with tubules that transcend both membrane layers. Amoebae,protozoa, algae, and fungi (yeasts and molds) also contain a discrete membrane-boundnucleus (Fig. 7-2). The myxomycete Acyria cinerea contains a double-layered nuclearmembrane that exhibits pores (Fig. 7-3).
277
Microbial Physiology. Albert G. Moat, John W. Foster and Michael P. SpectorCopyright 2002 by Wiley-Liss, Inc.
ISBN: 0-471-39483-1
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278
BACTERIAL NUCLEOIDS 279
Fig. 7-1. Electron micrograph of a portion of a mammalian cell. The region of this pancreaticexocrine cell between the nucleus (n) on the lower right and the plasmalemma on the lowerleft is occupied by numerous cisternae of the rough endoplasmic reticulum (rer) and a fewmitochondria (m). Prominent pores can be seen in the nuclear membrane. The ribosomes appearas small black dots lining the rer. The nucleolus is not visible in this photograph. Bar equals1 m. (Source: From Palade, G. E. Science 189:347, 1975.)
BACTERIAL NUCLEOIDS
In most bacteria the DNA-containing region of the cell (the chromosome) is folded intoa cytologically distinct region that does not appear to be bound by a nuclear membraneand is generally termed the nucleoid to distinguish it from the eukaryotic nucleus.
280 CELL STRUCTURE AND FUNCTION
CW
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1 m
(a) (b)
Fig. 7-2. Fine structure of cells of lower eukaryotes. (a) General view of a section of a cellof Saccharomyces cerevisiae. Prominent features include the cell wall, the nucleus with distinctpores in the nuclear membrane, mitochondria with cristae, and intermembranous structures.Bar equals 100 nm. (Source: From Avers, C. J., et al., J. Bacteriol. 100:1044, 1969.) (b) Finestructure of a hyphal element of the fungus Mucor genevensis. Nuclei (N) with pores, vacuoles(V), mitochondria (M), endoplasmic reticulum (ER), cell wall (CW), and dark bodies (DB) arevisible. Bar equals 1 m. (Source: From Clark-Walker, C. D. J. Bacteriol. 109:299, 1972.)
The compaction process is aided by a number of DNA-binding proteins, sometimescalled histone-like proteins, since they resemble the histones found in the nucleusof eukaryotic cells. Most of the DNA is in the form of supercoils (see Chapter 2)constrained by these binding proteins. Interaction between the DNA supercoils andcrowding within the cell is presumed to lead to further compaction and subsequentphase separation between the nucleoid and the cytoplasm.
As shown in Figure 7-4 (a, b, and c), growth in normal or high salt mediumand fixation with OsO4 results in visualization of a condensed nucleoid, whereasglutaraldehyde fixation (Fig. 7-4d) results in a dispersed appearance of the nucleoid.The development of confocal scanning light microscopy made it possible to observethe shape and substructure of the nucleoid and to compare these images with phase-contrast and electron microscope images. In Figure 7-5, a confocal scanning lightmicroscope image of an OsO4-fixed cell is compared with an electron micrographand a reconstruction model based on a study of serial sections. Some of thedifferences observed might reflect the presence of transcriptiontranslation complexeswith ribosomes and proteins in the more dense preparations observed with OsO4fixation.
The nucleoid is observed as a coralline (coral-like) shape as shown in Figure 7-6a.The branches of the coralline nucleoid spread far into the cytoplasm and throughout theentire interior of the cell. By viewing serial sections, it has been possible to reconstructthe ribosome-free area of the nucleoid. Figure 7-6b presents a three-dimensional modelof how the coralline nucleoid may appear.
BACTERIAL NUCLEOIDS 281
1m
Fig. 7-3. Interphase nucleus of the myxomycete Arcyria cinerea. Prominent pores interruptthe typical nuclear envelope. The nucleolus near the center is well defined, while the chromatinis randomly dispersed throughout the nucleus. (Source: From Mims, C. W. J. Gen. Microbiol.71:53, 1972.)
Fast-growing cells harvested and fixed for observation under the electron microscopeoften display multiple nucleoids. This multinucleate condition occurs becausefast-growing cells contain DNA i
