Lecture 18 - Extranuclear genomes: Plastids and...

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Extranuclear genomes: Plastids and Mitochondria
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3-2 Evolution of the eukaryotic cell
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Mitochondrial / Chloroplast Evolution Endosymbiotic theory – Lynn Margulis 1966 The Origin of Mitosing Eukaryotic Cells. The progenitors of mitochondria and chloroplasts arose independently about 2 billion years ago as free- living prokaryotes Primitive eukaryotes engulfed the prokaryotes… endosymbionts Relationship ultimately changed to that of an organelle Organelles have circular DNA Most genes moved to the host nuclear genome (<10% remain) Expression - targeting peptides Organelle-encoded genes/expression still
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Initial Evidence Supporting the Endosymbiotic Origin of Plastids Host associations between green algae and protists are known Both plastids and bacteria divide by binary fission Ribosomes in bacteria and plastids are similar Number of plastid membranes - 2, 3, or 4 Metabolic processes similar to cyanobacteria Photosynthetic pigment similarity Similarity in translation machinery
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Endosymbiosis and Eukaryotic Organelles Both mitochondria and chloroplasts are thought to have arisen from a prokaryotic ancestor that were incorporated into eukaryotes. α -proteobact eria were the ancestors of mitochondria This lineage includes some bacteria that are intracellular parasites or bacterial symbionts of eukaryotes. Although most mitochondria are highly reduced in gene content, one eukaryote with a mitochondrial genome resembling a eubacterial genome has been found. This is a protozoan called Reclinomonas americana ; 69 kb genome encoding 97 genes . Cyanobacteria were the ancestors of chloroplasts. Some chloroplasts appear to reflect secondary endosymbiosis - a eukaryote that engulfed another eukaryote. These secondary plast ids have more than two membranes. There are two unrelated types of algae that have a second highly reduced nucleus (called the NUCLEOM ORPH ) that remains associated with the secondary plastid.
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Primary secondary Prim ary endosym biosis of an α - proteobact eria led to the establishment of mitochondria. Prim ary endosym biosis of a cynanobact erium led to green algae and plants, red algae, and glaucophytes. Glaucophytes have cyanelles , which have retained peptidoglycan (in membranes). There have been at least three secondary endosym bioses. Led to three distinct lineages of protists (chromalveolates, from red algal plastids, and to euglenophytes and chlorarachniophytes, both from green algal plastids).
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Photosynthetic protists and plants gained their plastids independently of their mitochondria at a later point in evolutionary history.
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Secondary Endosymbiosis (of plastids) This figure shows the process of secondary endosym biosis. Notice the
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This note was uploaded on 01/08/2010 for the course BIO 51405 taught by Professor Ruhlman during the Fall '09 term at University of Texas at Austin.

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Lecture 18 - Extranuclear genomes: Plastids and...

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