Origins, eukaryotic nucleus

Whatever the explanation of the stem eukaryote, the eukaryote organelles, both mitochondria and chloroplasts, are best explained as symbiont bacteria. Explanations of the mitochondrial symbiosis mostly invoke an early Archean stem that incorporated a bacterial symbiont. One explanation of the mitochondrion is that the origin of the mitochondrion was simultaneous with the origin of the eukaryote nucleus (Grey et al, 1999). In the hydrogen hypothesis (Martin and Muller, 1998), the symbiosis is seen as the end product of a tight physical association between anaerobic... 

Takemura M. Poxvimses and the origin of the eukaryotic nucleus. J Mol Evol. 2001 52 419-25. 

Universality and homology. The rRNAs are found in Archaebacteria, Eubacteria, and in Eukaryotes (nucleus and organelles) and consequently are universal. The homology of rRNA genes whatever their origin is not questionable. The secondary structure of the core of the molecule is very conservative and the function of the rRNA is the same everywhere. 

Lake JA (1988) Origin of the eukaryotic nucleus determined by rate-invariant analysis of rRNA sequences. Nature 331 184-186... 

With these considerations in mind, we summarize evidence to explain ontogeny of modern eukaryotic endomembrane systems and address two major evolutionary enigmas 1) the origin of the eukaryotic nucleus and 2) the origin of the modern eukaryotic plasma membrane. 

The problem of endomembrane origins is considerably simplified by the presence of a eukaryotic nucleus. Once a cell derived an eukaryotic nucleus with a nuclear envelope, other endomembrane components (endoplasmic reticulum, Golgi apparatus, lysosomes, etc.) were likely acquired in a manner not unlike their present-day derivations (i.e., formation by processes of membrane flow and membrane differentiation (Table II) beginning with endoplasmic reticulum-like extensions of the nuclear envelope (Table 1)). 

Figure 2. Hypothetical scheme to account for acquisition of a eukaryotic nucleus with double membrane, pores and duplicate sets of genetic material by endosymbiotic invasion of one procaryotic cell by a primitive flagellate. The resultant endosome would have a double membrane. The pores and pore apparatus could derive from multiple fusions of the flagellar membranes with the outer membrane of the endosome. The primitive motile apparatus of the invading flagellate could have contributed as well to the origins of the mitotic apparatus of the resulting endosymbiont.

In spite of recent progress toward elucidation of the mechanism of nuclear envelope formation In modern eukaryotes, evolutionary origins of the eukaryote nucleus remain obscure. Even the concept of nuclear envelope organization around annulae derived from preexisting annulae offers little to help select among the various possibilities. Yet, similarities of Inner membrane of the nuclear envelope and the procaryotic cell membrane argue for a common parallel evolution of these two membrane types. For example, the Inner nuclear membrane Is associated with DNA (i) and perhaps even with DNA replication 21, 23) as In bacterial cells (J). There was even a report that It contained cytochrome oxidase 20), an enzyme associated with the plasma membrane of prokaryotes and critical to the development of aerobic metabolism. 

Each eukaryotic chromosome contains one linear molecule of DNA having multiple origins of replication. Bidirectional replication occurs by means of a pair of replication forks produced at each origin. Completion of the process results in the production of two identical linear molecules of DNA. DNA replication occurs in the nucleus during the S phase of the eukaryotic cell cycle. The two identical sister chromatids are separated om each other when the ceU divides during mitosis. 

Eukaryotes are differentiated from the Archaea and Eubacteria by the possession of a nucleus in the cell enclosed by a membrane as well as by membrane-enclosed subcellular organelles. The nucleus houses the basic genetic information of these organisms, their genomes, as I will describe in chapter 14. The eukaryotes are a diverse set of species, including but not limited to all plants and animals. Remarkably, the Archaea are more closely related to Eukarya than they are to the Eubacteria. This reflects a striking origin of the eukaryotic cell. 

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. 

The outcome of, or perhaps the support for, the endosymbiont transferring its genes to the nucleus was the evolution of new machinery in the eukaryotic cell to send the nuclear-encoded proteins back to the degenerate endosymbiont to allow the latter to function. Moreover, it is of note that the large majority of extant mitochondrial proteins are not of endosymbiotic or a-proteobacterial origin. These proteins have either been recruited... 

Research work with large genomes and the associated need for high-capacity cloning vectors led to the development of yeast artificial chromosomes (YACS Fig. 9-8). YAC vectors contain all the elements needed to maintain a eukaryotic chromosome in the yeast nucleus a yeast origin of replication, two selectable markers, and specialized sequences (derived from the telomeres and centromere, regions of the chromosome discussed in Chapter 24) needed for stability and... 

The existence of mitochondrial DNA, ribosomes, and tRNAs supports the hypothesis of the endosymbiotic origin of mitochondria (see Fig. 1-36), which holds that the first organisms capable of aerobic metabolism, including respiration-linked ATP production, were prokaryotes. Primitive eukaryotes that lived anaerobically (by fermentation) acquired the ability to carry out oxidative phosphorylation when they established a symbiotic relationship with bacteria living in their cytosol. After much evolution and the movement of many bacterial genes into the nucleus of the host eukaryote, the endosymbiotic bacteria eventually became mitochondria. 

MEIOSIS The process by which a eukaryotic cell nucleus (in which the DNA has been replicated only once) undergoes two coordinated divisions that yield four cells, each having ploidy half that of the original cell in higher animals, meiosis provides for the production of haploid sperms or eggs from diploid spermatocytes or oocytes. (See also MITOSIS)... 

MITOSIS The process by which the nucleus of a eukaryotic cell (in which the DNA has been replicated) divides, providing for the exact division of the cell to produce two daughter cells of the same ploidy as the original cell. (See also MEIOSIS)... 

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