Lecture 20 - Origin of Euks

Lecture 20 - Origin of Euks - 1 Lecture 19 - Origin of...

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1 Lecture 19 - Origin of Eukaryotes I. LUCA and the Tree (?) of Life - Previously, discussed (a) model of the origin of life and the origin of the genetic code. Briefly discussed some properties of the LUCA. The goal of systematics is to understand the history of life on earth, i.e., the phylogeny of all organisms, extant and extinct. Historically, organisms have been divided into two fundamental types, Prokaryotes and Eukaryotes. Eukaryotes were essentially defined by the presence of a nucleus, linear chromosomes and mitotic cell division, and prokaryotes defined by the absence of those things. With the advent of molecular systematics, it became apparent that there were two lineages of prokaryotes, called eubacteria and archaebacteria, or archaea. - The evolutionary relationship between the three domains of life - archaea, eubacteria, and eukarya, is controversial. - The problem is difficult because to resolve the relationship you need a rooted tree. Normally, a tree is rooted by comparing to an outgroup, but there is no outgroup when the ingroup is all life on earth. - Use a trick: compare duplicate genes whose duplication pre-dates the LUCA (e.g., EF- tu and EF-G). Duplicate genes are called paralogs. Thus, the paralog is the outgroup. (Barton Fig. 5.20). Early studies with rRNA revealed that Eukaryotes and Archaea were sister taxa. However, as more data were collected, it was shown that different genes gave different results. * "Information" genes (transcription, translation) group Eukarya with Archaea. (importantly, includes rDNA) * "Operational" genes (metabolism and biosynthesis) group Eukarya with Eubacteria - One possibility is lateral (horizontal) transfer. Prokaryotes have several ways of undergoing recombination that are much more promiscuous than eukaryotic sexual
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2 reproduction, e.g., uptake of DNA from the environment (transformation), phage mediated recombination (transduction), conjugation. See Barton Fig. 5.23. - Many biologists attribute the apparent conflict in the tree of life to LGT. * But, the implication is that for some reason there was wholesale LGT of either archaebacterial information genes or of eubacterial operational genes to an ancient eukaryote. - A second possibility is that the stem eukaryote was a hybrid, formed by a fusion of two prokaryotes. There is overwhelming evidence that eukaryotic organelles (e.g., mitochondria, chloroplasts) are derived from ancient symbioses. - - Predicted phylogenetic reconstruction of a hybrid gives conflicting results (~ half
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Lecture 20 - Origin of Euks - 1 Lecture 19 - Origin of...

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