Prokaryotic nucleus

Eukaryote Organism whose cells have a discrete nucleus and other subcellular compartments (cf. prokaryote)... 

Prokaryotic cells have only a single membrane, the plasma membrane or cell membrane. Because they have no other membranes, prokaryotic cells contain no nucleus or organelles. Nevertheless, they possess a distinct nuclear area where a single circular chromosome is localized, and some have an internal membranous structure called a mesosome that is derived from and continuous with the cell membrane. Reactions of cellular respiration are localized on these membranes. In photosynthetic prokaryotes such as the cyanobacteria,... 

Eukaryotic ceils possess a discrete, membrane-bounded nucleus, the repository of the cell s genetic material, which is distributed among a few or many chromosomes. During ceil division, equivalent copies of this genetic material must be passed to both daughter ceils through duplication and orderly partitioning of the chromosomes by the process known as mitosis. Like prokaryotic... 

In terms of evolutionary biology, the complex mitotic process of higher animals and plants has evolved through a progression of steps from simple prokaryotic fission sequences. In prokaryotic cells, the two copies of replicated chromosomes become attached to specialized regions of the cell membrane and are separated by the slow intrusion of the membrane between them. In many primitive eukaryotes, the nuclear membrane participates in a similar process and remains intact the spindle microtubules are extranuclear but may indent the nuclear membrane to form parallel channels. In yeasts and diatoms, the nuclear membrane also remains intact, an intranuclear polar spindle forms and attaches at each pole to the nuclear envelope, and a single kinetochore microtubule moves each chromosome to a pole. In the cells of higher animals and plants, the mitotic spindle starts to form outside of the nucleus, the nuclear envelope breaks down, and the spindle microtubules are captured by chromosomes (Kubai, 1975 Heath, 1980 Alberts et al., 1989). 

The prokaryotes are simple cells without a nucleus. Until about 25 years ago, this term was used for all bacteria. 

The distribution of elements in single-cell non-photosynthetic eukaryotes is probably best seen in terms of the well-defined compartments of yeast. The central cytoplasmic compartment containing the nucleus has many free element concentrations, only somewhat different from those in all known aerobic prokaryotes (Figure 7.7). (The nuclear membrane is a poor barrier to small molecules and ions and so we include the nucleus with the cytoplasm.) We do not believe in fact that the free cytoplasmic values of Mg2+, Mn2+, Fe2+, Ca2+, and possibly Zn2+, have changed greatly throughout evolution. As stressed already there are limitations since free Mg2+ and Fe2+ are essential for the maintenance of the primary synthetic routes of all cells, and changes in other free metal ions could well have imposed... 

Eukaryote Cell or organism with membrane-bound, structurally discrete nucleus and other well-developed subcellular compartments. Eukaryotes include all organisms except viruses, bacteria, and blue-green algae (see also prokaryote). 

The protein synthesis machinery reads the RNA template starting from the 5 end (the end made first) and makes proteins beginning with the amino terminus. These directionalities are set up so that in prokaryotes, protein synthesis can begin even before the RNA synthesis is complete. Simultaneous transcription-translation can t happen in eukaryotic cells because the nuclear membrane separates the ribosome from the nucleus. 

Simple single-cell organisms, such as bacteria and blue-green algae, are called prokaryotes (see Fig. A2.2). Prokaryotes do not have a well-defined nucleus. 

Prokaryotes Single-ceUed organisms that lack a nucleus, includes bacteria, archaeans, and viruses. 

There are two general kinds of cells those having a membrane-bounded nucleus called eukaryotic cells, and those without a nuclear envelope called prokaryotic cells. Humans have eukaryotic cells. All eukaryotic cells contain a nucleus that contains the genome, the complete set of genes. Unless noted otherwise, our discussion will be restricted to eukaryotic cells. 

In eukaryotic cells transcription and translation occur in two distinct temporal and spacial events, whereas in prokaryotic cells they occur in one step. As humans have eukaryotic cells, we will look at this process. Transcription occurs on DNA in the nucleus and translation occurs on ribosomes in the cytoplasm. 

The topoisomerases enzymes are essential in prokaryotic and eukaryotic cell visibility. Most quinolones have a characteristic core planar heterocyclic nucleus. 

The 1980 view assumed that the prokaryote-to-eukaryote transition occurred via gradualist mechanisms such as point mutation and hence did not involve symbiosis at all (van Valen and Maiorana 1980 Doolittle 1980) and culminated with a cell that possessed a nucleus, but lacked mitochondria. This is what Doolittle (1998) has called the standard model . In this view, mitochondria are interpreted as a small tack-on to, and mechanistically unrelated to, the process that made eukaryotic cells nucleated and complex (Cavalier-Smith 2002). In the standard model, mitochondria (and chloro-plasts) are descended from endosymbionts, but the nuts-and-bolts of the prokaryote-to-eukaryote transition (the origin of eukaryote-specific traits) was seen as having occurred independently from, and prior to, the origin of mitochondria. The paper by van Valen and Maiorana (1980) expresses this view in clear physiological terms the host was assumed to be an amoeboid, anaerobic, fermenting cell related to archaebacteria, the advantage of the mitochondrial endosymbiont was to supply ATP. 

Fe- and Mn-containing superoxide dismutases have, moreover, been isolated. They show striking sequence homologies. (Fe)-SOD has mainly been found in prokaryotes. (Mn)-SOD occurs in prokaryotes, (20,000), in the matrix of mitochondria, although encoded by the nucleus, and also in the cytosol of e.g. liver cells (human, chicken in contrast with rat)... 

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