Cytoplasm eukaryotic

Pestova, T. V., Shatsky, I. N., Fletcher, S. P., Jackson, R. J., and Hellen, C. U. (1998). A prokaryotic-like mode of cytoplasmic eukaryotic ribosome binding to the initiation... 

CAP on its 5 end, required for translation, and a poly-adenine tail on its 3 end (Fig. 24.3). Mature mRNA is then exported from the nucleus into the cytoplasm. Eukaryotic genes are always transcribed individually, as operons have not been described in eukaryotes. 

In plants, de novo fatty acid biosynthesis occurs exclusively in the stroma of plastids, whereas, with the exception of plastidial desaturation, modification of fatty acid residues including further desaturation and triacylglrycerol (TAG) assembly are localized in the cytosol/endoplasmic reticulum (ER). The primary fatty acids formed in the plastid (palmitic, stearic, and oleic acid) are used in the plastidic prokaryotic pathway for membrane lipid synthesis or diverted to the cytoplasmic eukaryotic pathway for the synthesis of membrane lipids or storage TAGs (1). Movement of glycerolipids is believed to occur in the reverse direction between the cytosol/ER and the plastids in the highly regulated manner (2). 

While the basic features of the cytoplasmic eukaryotic ribosome resemble those of the prokaryotic type, the eukaryotic ribosome is an 80 S particle composed of 40 S and 60 S subunits. The smaller subunit is constructed from 18 S RNA and about 30 proteins, while the larger subunit is composed of 28 S and 5 S RNA as well as about 40 proteins. Perhaps the most striking difference exhibited on electron microscopy is the attachment of the 40S subunit to the endoplasmic reticulum in the cytoplasm. 

Pyruvate produced by glycolysis is a significant source of acetyl-CoA for the TCA cycle. Because, in eukaryotic ceils, glycolysis occurs in the cytoplasm, whereas the TCA cycle reactions and ail subsequent steps of aerobic metabolism take place in the mitochondria, pyruvate must first enter the mitochondria to enter the TCA cycle. The oxidative decarboxylation of pyruvate to acetyl-CoA,... 

The eukaryotic somatic cell cycle is defined by a sequential order of tasks a dividing cell has to complete it must replicate its DNA, segregate its chromosomes, grow, and divide. The cell cycle can be divided into four discrete phases. DNA replication is restricted to S phase (DNA synthesis phase), which is preceded by a gap phase called G1 and followed by a gap phase called G2. During mitosis (M phase) the sister chromatids are segregated into two new daughter nuclei and mitosis is completed by the division of the cytoplasm termed cytokinesis (Fig. 1). 

There are various protocols of administering eukaryotic expression vectors aiming to deliver (i.e. transfect) the DNA into the cytoplasm of the host cells (see Figure 2). The DNA is subsequently imported into the nucleus of the transfected cells allowing expression of the... 

The cytoplasm of all eukaryotic cells contains a cytoskeletal framework that serves a multitude of dynamic functions exemplified by the control of cell shape, the internal positioning and movement of organelles, and the capacity of the cell to move and undergo division. 

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. 

Fig. 3-8 A thin section showing a eukaryotic cell. Note the nucleus (N) is bound by a nuclear membrane. In the cytoplasm of the cell are many mitochondria (M) and intracytoplasmic membranes. (Reprinted with permission from Richard Rodewald, Univ. of Virginia/Biological Photo Service.)...

As has been reviewed extensively elsewhere (Wyn Jones Pollard, 1983), the evidence from many eukaryotic cells and eubacteria suggests common ionic and osmotic characteristics in the cytoplasm of cells, espe-cialy a high selectivity and similar ionic strength giving... 

Table 2. Estimated cytoplasmic and extracytoplasmic K, Na and Cl concentrations in some eukaryotes...

Much of the RNA synthesized from DNA templates in eukaryotic cells, including mammalian cells, is degraded within the nucleus, and it never serves as either a strucmral or an informational entity within the cellular cytoplasm. 

A large number of discrete, highly conserved, and small stable RNA species are found in eukaryotic cells. The majority of these molecules are complexed with proteins to form ribonucleoproteins and are distributed in the nucleus, in the cytoplasm, or in both. They range in... 

The protein coding regions of DNA, the transcripts of which ultimately appear in the cytoplasm as single mRNA molecules, are usually interrupted in the eukaryotic genome by large intervening sequences of... 

The primary transcripts generated by RNA polymerase II—one of three distinct nuclear DNA-depen-dent RNA polymerases in eukaryotes—are promptly capped by 7-methylguanosine triphosphate caps (Figure 35-10) that persist and eventually appear on the 5 end of mature cytoplasmic mRNA. These caps are necessary for the subsequent processing of the primary transcript to mRNA, for the translation of the mRNA, and for protection of the mRNA against exonucleolytic attack. 

The mechanisms whereby introns are removed from the primary transcript in the nucleus, exons are ligated to form the mRNA molecule, and the mRNA molecule is transported to the cytoplasm are being elucidated. Four different splicing reaction mechanisms have been described. The one most frequently used in eukaryotic cells is described below. Although the sequences of nu-... 

The relationship between hnRNA and the corresponding mature mRNA in eukaryotic cells is now apparent. The hnRNA molecules are the primary transcripts plus their early processed products, which, after the addition of caps and poly(A) tails and removal of the portion corresponding to the introns, are transported to the cytoplasm as mature mRNA molecules. 

It has been estimated that more than a million macromolecules per minute are transported between the nucleus and the cytoplasm in an active eukaryotic cell. 

Nonmuscle cells perform mechanical work, including self-propulsion, morphogenesis, cleavage, endocytosis, exocytosis, intracellular transport, and changing cell shape. These cellular functions are carried out by an extensive intracellular network of filamentous structures constimting the cytoskeleton. The cell cytoplasm is not a sac of fluid, as once thought. Essentially all eukaryotic cells contain three types of filamentous struc-mres actin filaments (7-9.5 nm in diameter also known as microfilaments), microtubules (25 nm), and intermediate filaments (10-12 nm). Each type of filament can be distinguished biochemically and by the electron microscope. 

Microtubules, an integral component of the cellular cy-toskeleton, consist of cytoplasmic tubes 25 nm in diameter and often of extreme length. Microtubules are necessary for the formation and function of the mitotic spindle and thus are present in all eukaryotic cells. They are also involved in the intracellular movement of endocytic and exocytic vesicles and form the major structural components of cilia and flagella. Microtubules are a major component of axons and dendrites, in which they maintain structure and participate in the axoplasmic flow of material along these neuronal processes. 

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