deDuve_2007_originofeukaryotes_essay_nrg2071 - PERSPECTIVES...

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The origin of nucleated cells has long been an object of wonder and speculation. Three events have marked the modern era of research in this field. First, discoveries in the 1950s and 1960s revealed the intricate organization of eukaryotic cells and the functional specializations of each type of structure. Second, the landmark paper by Lynn Margulis (Sagan at the time) 1 revived the endosymbiotic theory of the origin of mitochondria and plastids, and subsequent experiments confirmed her proposal. Third, with the innovative investigations of Woese and Fox 2 , molecular phylogenies were extended back to the dawn of life, yielding a wealth of new data that prompted a profusion of new hypothetical models. Surprisingly, these new models often focus on a single eukaryotic feature, mostly the nuclear genome or the mitochondria, ignoring several other cell parts of compa- rable importance. My main purpose in this Essay is to restore some balance in the field, reconciling the equally valid demands of cell biology and phylogenies. References to the relevant literature, which has grown to immense proportions, are necessarily selec- tive, but should be sufficient to allow retrieval of further information (see also REF. 3 ). The making of a eukaryote Eukaryotic cells differ from prokaryotic cells by a number of features: a nucleus, fenced off by an envelope and containing elaborately structured chromosomes, along with the main molecular systems responsi- ble for replication and transcription of the chromosomal DNA and for processing of the RNA transcripts; an extensive system of cytomembranes, subdivided into a number of specialized parts; cytoskeletal elements and associated motor systems; peroxisomes and related organelles; mito- chondria and the related hydrogenosomes; and, in phototrophic eukaryotes only, plastids (FIG. 1) . Another distinguishing feature of eukaryotic cells is that they divide by mitosis. When, how and in what order were these various eukaryotic features acquired, and what evolutionary advantages did they provide? Time and setting. A crucial date in the history of eukaryotes lies around 2.4 billion years ago, when molecular oxygen started rising in the Earth’s atmosphere 4 . Oxygen-related organelles, such as peroxisomes, mitochon- dria and plastids, must have been acquired after that date. Other eukaryotic features must likewise have developed under aerobic conditions if their acquisition accompanied or followed that of oxygen-related organelles. If acquired earlier, they could have been developed under anaerobic conditions, a point that is relevant to theories that assume eukaryotic transformation was triggered by the adoption of mitochondria. Such theories imply that eukaryotic cells devel- oped within the period between the rise of atmospheric oxygen and the appear- ance of the first eukaryotic organisms. Unfortunately, estimates of that date vary
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This note was uploaded on 01/25/2011 for the course BIO 1510 taught by Professor Unknown during the Spring '07 term at Georgia Institute of Technology.

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deDuve_2007_originofeukaryotes_essay_nrg2071 - PERSPECTIVES...

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