Eukaryotes earliest

Fig. 6.13 The development of aerobic eukaryotic organisms from the earliest anaerobic ancestors via aerobic bacteria, all single cells.

Some of these species can carry out photosynthesis, but they may do so by engulfing symbiotic algae. It has been suggested that Grypania found in iron sediments could be the earliest eukaryotes. 

Kandell and Bernstein published one of the earliest reports to suggest that bile acids also demonstrate DNA-damaging effects in eukaryotic cells. They showed that human foreskin fibroblasts underwent unscheduled DNA synthesis (indicating DNA repair), as measured by tritiated thymidine incorporation when cells were treated with increasing concentrations of sodium deoxycholate or chenodeoxycholate. Utilising mutant Chinese hamster ovary cells deficient in strand rejoining (EM9), the authors were able to demonstrate that the repair of deoxycholate-induced DNA damage was dependent on strand break repair capacity. 

Starting about 1.5 billion years ago, the fossil record begins to show evidence of larger and more complex organisms, probably the earliest eukaryotic cells (Fig. 1-35). 

The other lipid commonly present in eukaryotic membranes is cholesterol, a polycyclic structure produced from isoprene by a complex biosynthetic pathway. It is interesting to ask whether it is conceivable that prebiotically plausible reactions might also produce complex amphiphiles. The earliest investigations aiming to answer this question were carried out by Hargreaves et al. [36], Oro and coworkers [37,38], and, more recently, Ourisson et al. [39] and Conde-Frieboes and Blochliger [40]. In all such reactions,... 

The first attempts to determine the intracellular localization of PolyPs in eukaryotes were made long ago. The earliest investigations aimed at establishing the localization of... 

As the tree shows, the true relatives of the archaea are not the bacteria but the eukaryotes. However, because the archaea branch-off closest to the root of the tree, the suggestion is that they are the most primitive of the three kingdoms of organisms whereas the eukaryotes are the least primitive (or the most derived). Placement of the archaea closest to the universal ancestor is supported by the fact that many archaea inhabit extreme environments involving high temperature, low pH, high salinity, and so on. Thus, archaea may well be evolutionary relics of the Earth s earliest forms of life. 

In the budding yeast, inositol polyphosphate synthesis proceeds via what is likely to be one of the earliest incarnations of a PLC-dependent pathway for higher inositol polyphosphate metabolism in eukaryotes. Early biochemical studies in yeast (and plants) failed to identify a calcium-sensitive Ins(l,4,5)P3 3-kinase activity analogous to that found in mammalian cells. Instead, these studies identified C6-hydroxyl phosphorylation of Ins(l,4,5)P3 and formation of Ins(l,4,5,6)P4 as the most likely first anabolic step in the production of higher inositol polyphosphates (22). Additional biochemical studies identified the sequential phosphorylation of Ins(l,4,5,6)P4 to Ins(l,3,4,5,6)P5 followed by Ins(l,2,3,4,5,6)P6 (23). These findings were interpreted as proof of the existence of disparate pathways in yeast and mammals for the metabolism and functionality of Ins(l,4,5)P3. In contrast, the eventual cloning of the yeast Ins(l,4,5)P3 kinase activity found that mammalian and yeast inositol metabolism were more closely related than initially suspected. 

An upper limit of atmospheric 02 based on BIF deposition is C.0.08PAL. The earliest postulated eukaryotes in the fossil record, Grypania spiralis (believed to be an alga), have an age of 2.1Gyr (Han Runnegar 1992), and so overlap with BIFs. It is assumed they required at least 0.01 PAL of dissolved 02 (Runnegar 1991), while most eukaryotic microbes require at least 0.05 PAL (Jahnke Klein 1979). Although the fossil... 

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