| dc.description | The transition between prokaryotic and eukaryotic cellular architecture has been examined with the aim of clarifying the nature of the ancestor of all extant eukaryotes. Specifically, the origin of DNA replication, endomembrane signalling and protein turnover processes were examined, along with structures such as the cytoskeleton and mitochondrion. One strategy used was to identify genes involved in these processes in the most ancient eukaryotic phyla, as the presence of these genes implies that the process or structure predates the divergence of all extant eukaryotes. Calmodulin, ubiquitin, E2 ubiquitin-conjugating enzyme and alpha-tubulin genes were isolated from a variety of taxa for this purpose, for the most part demonstrating that these genes predate extant eukaryotes. Another strategy was to identify an archaebacterial analogue of a eukaryotic process to determine the state of their common ancestor. An archaebacterial chromosomal replication origin was characterised to better define the DNA replication system in the ancestor of eukaryotes, but since no definite conclusions about this locus' activity could be made, such inferences about the ancestral state are not possible. Lastly, the origin of the mitochondrion was examined by identifying a gene, triosephosphate isomerase, which appears to be of mitochondrial origin, but whose product functions in the cytosol. The presence of this gene in deeply-branching amitochondrial protists suggests that these taxa may have had a mitochondrion which they secondarily lost. This would mean that the mitochondrion, contrary to the current conventional hypothesis, was also present in the ancestor of extant eukaryotes. | en_US |
