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Ch. 28 Eukaryotic Domain

Ch. 28 Eukaryotic Domain - Eukaryotic Diversity Protists...

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Protists 1 BIOL 112 Eukaryotic Diversity, Protists: Protozoa, Slime Molds and Algae Chapter 28 BIOL 112 How Many Kingdoms? In recent years, major efforts to revise systematics of the “lower” eukaryotes Revisions have been stimulated by a mass of new data, especially from molecular analyses of common gene sequences (e.g., rRNA) The interpretation of these new data in the context of systematics schemes is the subject of active debate/discussion among scientists Campbell presents a “work in progress:” one possible way to organize phylogeny of eukaryotes BIOL 112 Recent Historical Trends Continued elevation of lower taxa to higher levels of the Linnaean scheme (“Phylum Protozoa” “Kingdom Protista” Several Kingdoms of Protists) Proliferation of taxa. More and more groups with fewer species in each. (Consequence of efforts to avoid polyphyletic taxa) Movement away from morphological criteria to define taxa towards molecular/biochemical/dynamic criteria Result has been development of a series of hypothesized schemes BIOL 112 Distinguishing Characteristics Eukaryotic Cell Organization No more than 1 tissue type in cell mass (no true multicellularity) Most species are single-celled and microscopic — bigger than most prokaryotes Diverse lifestyles, nutrition, motility At least 120,000 species; perhaps as many as 200,000. Many not yet discovered or described Fossil record indicates appearance about 1.5–2 x 10 9 years bp BIOL 112 BIOL 112
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Protists 2 BIOL 112 Origin of Eukaryotes? Basic cell organization is so distinct from that of prokaryotes, so how evolve from prokaryotic to eukaryotic forms? Current model formulated/refined by Lynn Margulis in 1970s Endosymbiont Theory Certain major organelle types in modern eukaryotic cell have separate ancestral lineages, distinct from that of other cell parts These organelles were once independent organisms, but now are in a tight mutualistic/symbiotic relationship comprising modern eukaryote Thus, eukaryotic cell is a collection of original cell types living in relation to one another BIOL 112 Which Organelles? Very likely: Chloroplasts (Plastids) Mitochondria Possibly: Nucleus; endomembranes Centrosomes/Basal Bodies Microbodies/Peroxysomes BIOL 112 Evidence for Endosymbiosis Chloroplasts and mitochondria have their own distinct inheritance patterns and their own genetics Morphology and size similar to bacteria; similar to gram negative forms New chloroplasts and mitochondria arise only from the division/replication of existing organelles. No de novo organelle formation Both chloroplasts and mitochondria contain their own DNA. Molecular analyses show that mtDNA and chDNA sequences are distinct from each other and from sequences in the cell nucleus. Both organelles also have full set of genetic expression enzymes and cofactors DNA in these organelles is circular (in most species) — like that of prokaryotic DNA and unlike that of eukaryotic chromosomes Both chloroplasts and mitochondria possess ribosomes more like bacterial
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Ch. 28 Eukaryotic Domain - Eukaryotic Diversity Protists...

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