Lecture 18 - Extranuclear genomes Plastids and Mitochondria...

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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 “prokaryotic”
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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.
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