BIOL 1020U Lecture Notes - Lecture 27: Lynn Margulis, Symbiogenesis, Chloroplast Dna
Eukaryotic Cells: Origins and Diversity Lecture
Chapter 27
Textbook Reference: Chapter 27 in Biology How Life Works (2nd ed.)
Sections 27.2 – 27.3; pp.557-572
What is endosymbiosis?
Eukarya have distinctive cellular organization when compared to Bacteria and
Archaea
Many cases of symbiosis in nature human microbiome – corals – giant clams
Why is it thought that endosymbiosis may have played a part in the evolution of
the eukaryotic cell?
Endosymbiosis and Origin of Chloroplasts
•Konstantin Sergeevich Merezhkovsky (Russian botanist) proposed:
•Chloroplasts once free-living cyanobacteria that became permanently
incorporated into their host cell - endosymbiosis
•American biologist Lynn Margulis resurrected his hypothesis in 1967
•She looked at:
•Structural similarities in photosynthetic membranes of cyanobacteria,
algae, and chloroplasts
•She found:
•Chloroplasts have 2 membranes; inner from cyanobacteria; outer from
engulfing cell
•Chloroplasts & cyanobacteria show similar biochemistry during
photosynthesis
•Further support for the endosymbiotic hypothesis:
Discovery of chloroplast DNA
Chloroplast DNA is single circular chromosome similar to that of bacterial cells
Molecular sequence of chloroplasts and cyanobacteria genes very similar –
very different from genes found in nucleus of the eukaryotic host in which
chloroplast was found
**strong
-endosymbiosis not only significant event that occurs in origin of chloroplasts
-cyanobacteria most closely related to chloroplasts 2000 to 3000 genes
In photosynthetic eukaryotes 60 to 200 chloroplast genes
Origin of Mitochondria
What gave rise to mitochondria?
Originated as endosymbiotic bacteria
Evidence:
Closely resemble free-living bacteria in organization & biochemistry
Contain own DNA and have small genome
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Origin of Eukaryotic Cell
Hypothesis 1
Ancestral archaeon plasma membrane in-folds; cytoskeleton increases in complexity
regions of in-folded membrane enclose DNA = nucleus precursor endosymbiosis of
facultatively aerobic proteobacterium heterotrophic eukaryote results (w/
mitochondria)
Hypothesis 2
Ancestral archaeon proteobacterium engulfed via endosymbiosis mitochondrion
evolved within archaeon host in-folding of membranes; DNA surrounded by
membranes heterotrophic eukaryote results (within mitochondria)
What is the main difference between the hypotheses?
Eukaryotic Tree of Life
•The phylogenetic tree of eukaryotes:
•Seven superkingdoms (molecular data) but
tree is unrooted – why?
•Could not find the ancestor that all 7
superkingdoms had in common
•Opisthokonts
•Amoebozoans
•Archaeplastids
•Stramenopiles
•Alveolates
•Rhizarians
•Exacavates
•What group are animals in? opisthokonta
•What group are plants in? Archaeplastida
Opisthokonta
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What percentage of described species fall
into this group?
75% fall into this group of 1.8 million species
What types of organisms are opisthokonts?
Includes 1.3 million animal species (mostly
insects and their relatives), fungi (75,000
species), and “protists” (choanoflagellates)
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Document Summary
Textbook reference: chapter 27 in biology how life works (2nd ed. ) Eukarya have distinctive cellular organization when compared to bacteria and. Many cases of symbiosis in nature human microbiome corals giant clams. Chloroplast dna is single circular chromosome similar to that of bacterial cells. Molecular sequence of chloroplasts and cyanobacteria genes very similar very different from genes found in nucleus of the eukaryotic host in which chloroplast was found. Endosymbiosis not only significant event that occurs in origin of chloroplasts. Cyanobacteria most closely related to chloroplasts 2000 to 3000 genes. In photosynthetic eukaryotes 60 to 200 chloroplast genes. Closely resemble free-living bacteria in organization & biochemistry. Ancestral archaeon plasma membrane in-folds; cytoskeleton increases in complexity. Regions of in-folded membrane enclose dna = nucleus precursor endosymbiosis of facultatively aerobic proteobacterium heterotrophic eukaryote results (w/ mitochondria) Ancestral archaeon proteobacterium engulfed via endosymbiosis mitochondrion evolved within archaeon host in-folding of membranes; dna surrounded by membranes heterotrophic eukaryote results (within mitochondria)
