Eukaryotic cell properties

FIGURE 11.24 The properties of mRNA molecules in prokaryotic versus eukaryotic cells during transcription and translation. 

A three-dimensional meshwork of proteinaceous filaments of various sizes fills the space between the organelles of all eukaryotic cell types. This material is known collectively as the cytoskeleton, but despite the static property implied by this name, the cytoskeleton is plastic and dynamic. Not only must the cytoplasm move and modify its shape when a cell changes its position or shape, but the cytoskeleton itself causes these movements. In addition to motility, the cytoskeleton plays a role in metabolism. Several glycolytic enzymes are known to be associated with actin filaments, possibly to concentrate substrate and enzymes locally. Many mRNA species appear to be bound by filaments, especially in egg cells where they may be immobilized in distinct regions thereby becoming concentrated in defined tissues upon subsequent cell divisions. 

In addition to the RC there are two protein complexes, REGajS and REGy, and a single polypeptide chain, PA200, that bind the 20S proteasome and stimulate peptide hydrolysis but not protein degradation. Like the RG, proteasome activators bind the ends of the 20 S proteasome and, importantly, they can form mixed or hybrid 26S proteasomes in which one end of the 20S proteasome is associated with a 19S RC and the other is bound to a proteasome activator [147-150]. This latter property raises the possibility that proteasome activators serve to localize the 26S proteasome within eukaryotic cells. 

Actin, the most abundant protein in eukaryotic cells, is the protein component of the microfilaments (actin filaments). Actin occurs in two forms—a monomolecular form (C actin, globular actin) and a polymer (F actin, filamentous actin). G actin is an asymmetrical molecule with a mass of 42 kDa, consisting of two domains. As the ionic strength increases, G actin aggregates reversibly to form F actin, a helical homopolymer. G actin carries a firmly bound ATP molecule that is slowly hydrolyzed in F actin to form ADR Actin therefore also has enzyme properties (ATPase activity). 

Like bacteria, eukaryotes have several types of DNA polymerases. Some have been linked to particular functions, such as the replication of mitochondrial DNA. The replication of nuclear chromosomes involves DNA polymerase a, in association with DNA polymerase S. DNA polymerase a is typically a multisubunit enzyme with similar structure and properties in all eukaryotic cells. One subunit has a primase activity, and the largest subunit (Afr -180,000) contains the polymerization activity. However, this polymerase has no proofreading 3 —>5 exonuclease activity, making it unsuitable for high-fidelity DNA replication. DNA polymerase a is believed to function only in the synthesis of short primers (containing either RNA or DNA) for Okazaki fragments on the lagging strand. These primers... 

The cytoplasm of eukaryotic cells contains a complex network of slender rods and filaments that serve as a kind of internal skeleton. The properties of this cytoskeleton affect the shape and mechanical properties of cells. For example, the cytoskeleton is responsible... 

The first evidence showing that plant polyprenols involved in glycosyl-transfer reactions are not allylic, but a-saturated, like animal dolichol (2), came from Pont Lezica and coworkers.23 The authors postulated that the presence of sugar acceptors having the properties of a-saturated polyprenyl phosphates may be a general feature of eukaryotic cells, in contrast to the a-unsaturated polyprenyl phosphates characteristic of prokaryotic cells. 

Lipids have several important functions in animal cells, which include serving as structural components of membranes and as a stored source of metabolic fuel (Griner et al., 1993). Eukaryotic cell membranes are composed of a complex array of proteins, phospholipids, sphingolipids, and cholesterol. The relative proportions and fatty acid composition of these components dictate the physical properties of membranes, such as fluidity, surface potential, microdomain structure, and permeability. This in turn regulates the localization and activity of membrane-associated proteins. Assembly of membranes necessitates the coordinate synthesis and catabolism of phospholipids, sterols, and sphingolipids to create the unique properties of a given cellular membrane. This must be an extremely complex process that requires coordination of multiple biosynthetic and degradative enzymes and lipid transport activities. 

It is most likely that the protoeukaryote host and enslaved purple symbiont were both facultative aerobes, able to live under anaerobic or aerobic conditions (Cavalier-Smith 2002b) this makes their coming together easy to understand and fits all we know of the diversity of mitochondrial/hydrogeno-somal properties as well as of the rest of the eukaryotic cell, especially in the basal kingdom Protozoa. Given that most actinobacteria are aerobes, it is... 

Compared with eukaryotes, prokaryotes have a much simpler internal structure, but are nevertheless biochemically diverse and complex. Table 1.1 compares some general properties of prokaryotic and eukaryotic cells. The gramnegative bacterium Escherichia coli is the best understood prokaryote both bio-... 

The comparison of exopolyphosphatases from different cell compartments of the yeast S. cerevisiae suggests that they are a typical example of compartment-specific enzymes. The latter differ from each other in their physico-chemical properties, substrate specificity, response to changing cultivation conditions, and presumably, in the functions and ways of regulation. The compartment-specificity of exopolyphosphatases should be taken into account in the study of PolyP metabolism in the eukaryotic cell. 

Secondary metabolites with similar structural types and pharmacophoric groups can be seen in several bacteria (where they are often termed antibiotics if they have antimicrobial or cytotoxic properties). Since eukaryotic cells had taken up a-proteobacteria (which became mitochondria) and cyanobacteria (which became chloroplasts), they also inherited a number of genes that encode enzymes for pathways leading to secondary metabolites. Therefore, we may speculate that early plants already had the capacity of building defense compounds and that alkaloids were among the first. Since the numbers and types of herbivores and other enemies have increased within the last 100 million years, angiosperms have had to face more enemies and as a consequence have developed a more complex pattern of defense and signal compounds. 

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