page - Eukaryotes(outline What are eukaryotes Shared d...

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1 Eukaryotes (outline) What are eukaryotes Shared derived characters How old are eukaryotes Endosymbiotic origin of eukaryotic life Protistans are diverse and paraphyletic Now considered to represent several kingdoms New ways of moving Amoeboid movement, and the ability to engulf prey Eukaryotic flagella and cilia Sex by meiosis Complex life cycles and alternation of generations Independent evolutions of multicellularity Eukaryotes: Shared-derived characteristics No prokaryotic cell wall – Cytoskeleton Membrane-bound nucleus and organelles Multiple linear chromosomes . bms .ed.ac. uk/research/others/smaciver/A . prot .Loc. mov The oldest fossils of eukaryotic cells date back 2.1 billion years. – This provides a minimum age for eukaryotic origins. Eukaryotic algae, Michigan, southern shore of Lake Superior
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2 Endosymbiotic Origin of Mitochondria and Plastids The theory of endosymbiosis – Proposes that mitochondria and plastids were formerly small prokaryotes living within larger host cells – Plastids are chloroplasts and related organelles The endosymbiotic theory of eukaryotic origins Figure 26.13 Cytoplasm DNA Plasma membrane Ancestral prokaryote Infolding of plasma membrane Endoplasmic reticulum Nuclear envelope Nucleus Engulfing of aerobic heterotrophic prokaryote Cell with nucleus and endomembrane system Mitochondrion Ancestral heterotrophic eukaryote Plastid Mitochondrion Engulfing of photosynthetic prokaryote in some cells Ancestral Photosynthetic eukaryote Lynn Margulis, Originator of the endosymbiotic hypothesis of eukarotic cells Horizontal transfer The tree of life Is not necessarily a neatly bifurcating tree. Figure 25.18 Bacteria Eukarya Archaea 4 Symbiosis of chloroplast ancestor with ancestor of green plants 3 Symbiosis of mitochondrial ancestor with ancestor of eukaryotes 2 Possible fusion of bacterium and archaean, yielding ancestor of eukaryotic cells 1 Last common ancestor of all living things 4 3 2 1 1 2 3 4 0 Billion years ago Origin of life
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3 The evidence supporting an endosymbiotic origin of mitochondria and plastids includes Similarities in inner membrane structures and functions Both have their own circular DNA The genes in organelles are often closer to genes in bacteria than to nuclear genes of eukaryotes The three domain system Secondary endosymbiosis - cells with photosynthetic plastids - red and green algae They themselves were engulfed
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4 Sum - secondary endosymbiosis in eukaryotic evolution The plastid-bearing lineage of protists evolved into red algae and green algae On several occasions during eukaryotic evolution red algae and green algae underwent secondary endosymbiosis, in which they themselves were ingested Symbiotic origins may be (even) more complex Some investigators have speculated that eukaryotic flagella and cilia – Evolved from symbiotic bacteria, based on symbiotic relationships between some bacteria
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