Eukaryotic cells structure

FIGURE 1-7 Eukaryotic cell structure. Schematic illustrations of the (k) Plant cell... 

Fig. 1. Eukaryote cell structure, (a) Structure of a typical animal cell, (b) structure of a typical plant cell.

Some epigenetic processes (for example protein synthesis and ribosome self-assembly) also take place in prokaryotes, and yet these cells do not give rise to embryos. It is not epigenesis as such, therefore, that accounts for development, but a particular type of epigenesis that prokaryotes do not have. It could be pointed out that prokaryotes lack the complex structures of the eukaryotic cell, but this does not explain their lack of embryonic potential. Protozoa, for example, do have the eukaryotic cell structure but they too are incapable of producing embryos. 

It is not epigenesis as such, nor eukaryotic cell structure, that accounts for embryonic development, and we are bound to conclude that development is based on something which does not exist in... 

Figure 9 Monosaccharide interconversions in eukaryotic cells. Structures in squared boxes serve as exogenous substrates. Structures in rounded boxes are the activated monosaccharide donors. For simplicity, reactions are shown to progress in the forward direction.

The first eukaryotes are believed to have evolved at least 1.5 billion years ago. The transition from ancient prokaryotic to eukaryotic cell structure is arguably the most important one in evolution, except for the origin of life itself. The endosymbiotic hypothesis is an interesting and compelling view of this transition. 

TARGETING Despite the vast complexities of eukaryotic cell structure and function, each newly synthesized polypeptide is normally directed to its proper destination. Considering that translation takes place in the cytoplasm (except for certain molecules that are produced within mitochondria and plastids) and that a wide variety of polypeptides must be directed to their proper locations, it is not surprising that the mechanisms by which cellular proteins are targeted are complex. Although this process is not yet completely understood, there appear to be two principal mechanisms by which polypeptides are directed to their correct locations transcript localization and signal peptides. Each is briefly discussed. 

FIGURE 1-7 Eukaryotic cell structure. Schematic illustrations of the two major types of eukaryotic cell (a) a representative animal cell and (b) a representative plant cell. Plant cells are usually 10 to 100 p,m in diameter—larger than animal cells, which typically range from 5 to 30 p,m. Structures labeled in red are unique to either animal or plant cells. 

In contrast, RNA occurs in multiple copies and various forms (Table 11.2). Cells contain up to eight times as much RNA as DNA. RNA has a number of important biological functions, and on this basis, RNA molecules are categorized into several major types messenger RNA, ribosomal RNA, and transfer RNA. Eukaryotic cells contain an additional type, small nuclear RNA (snRNA). With these basic definitions in mind, let s now briefly consider the chemical and structural nature of DNA and the various RNAs. Chapter 12 elaborates on methods to determine the primary structure of nucleic acids by sequencing methods and discusses the secondary and tertiary structures of DNA and RNA. Part rV, Information Transfer, includes a detailed treatment of the dynamic role of nucleic acids in the molecular biology of the cell. 

Some specialized eukaryotic cells have cilia that show a whiplike motion. Sperm cells move with one flagella, which is much longer than a cilium but has a nearly identical internal structure called axoneme. It is composed of nine doublet MTs that form a ring around a pair of single MTs. Numerous proteins bind to the MTs. Ciliary dynein motors generate the force by which MTs slide along each other to cause the bending of the axoneme necessary for motion. 

Actin is a 42 kDa bent dumbbell-shaped globular monomer which is found in most eukaryotic cells. It is the primary protein of the thin (or actin) filaments. Also, by mass or molarity, actin is the largest constituent of the contractile apparatus, actually reaching millimolar concentrations. Actins from different sources seem to be more similar than myosins from the same sources. Actin binds ATP which is hydrolyzed to ADP, if the monomeric actin polymerizes. The backbone structure of the actin filament is a helix formed by two linear strands of polymerized actins like two strings of actin beads entwined. 

Other antibiotics inhibit protein synthesis on all ribosomes (puromycin) or only on those of eukaryotic cells (cycloheximide). Puromycin (Figure 38—11) is a structural analog of tyrosinyl-tRNA. Puromycin is incorporated via the A site on the ribosome into the carboxyl terminal position of a peptide but causes the premature release of the polypeptide. Puromycin, as a tyrosinyl-tRNA analog, effectively inhibits protein synthesis in both prokaryotes and eukaryotes. Cycloheximide inhibits peptidyltransferase in the 60S ribosomal subunit in eukaryotes, presumably by binding to an rRNA component. 

We shall mainly discuss the membranes present in eukaryotic cells, although many of the principles described also apply to the membranes of prokaryotes. The various cellular membranes have different compositions, as reflected in the ratio of protein to lipid (Figure 41-1). This is not surprising, given their divergent functions. Membranes are asymmetric sheet-like enclosed structures with distinct inner and outer surfaces. 

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. 

The dihydrofolate reductase enzyme (DHFR) is involved in one-carbon metabolism and is required for the survival of prokaryotic and eukaryotic cells. The enzyme catalyzes the reduction of dihydrofolate to tetrahydrofolate, which is required for the biosynthesis of serine, methionine, purines, and thymidylate. The mouse dihydrofolate reductase (mDHFR) is a small (21 kD), monomeric enzyme that is highly homologous to the E. coli enzyme (29% identify) (Pelletier et al., 1998). The three-dimensional structure of DHFR indicates that it is comprised of three structural fragments F[l], F[2] andF[3] (Gegg etal., 1997). 

In the nuclei of all eukaryotic cells, DNA is tightly wrapped around an octamer of histone proteins and is compacted into a dense structure known as chromatin. In order to access the genetic information which is required in numerous essential cellular processes including DNA replication, gene expression and DNA repair, chromatin needs to be partially unwound. One important mechanism to regulate chromatin structure and thus to control the access of the genomic DNA is through histone modifications [1-6]. The histone octamer is composed of two copies of H2A, H2B, H3 and H4 core histone proteins. Their tails, that protrude out of the surface of the... 

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