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Eukaryotic Cells: Origins and Diversity LectureChapter 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
Why is it thought that endosymbiosis may have played a part in the evolution of
the eukaryotic cell?
Many cases of symbiosis in nature – human microbiome, corals, giant clams
Endosymbiosis and Origin of Chloroplasts
• Konstantin Sergeevich Merezhkovsky (Russian botanist) proposed:
• Chloroplasts were once free-living cyanobacteria that became permanently
incorporated into their host
• American biologist Lynn Margulis resurrected his hypothesis in 1967
• She looked at: Structural similarities in photosynthetic membranes of
cyanobacteria, algae, chloroplasts
• She found:
• Chloroplasts have 2 membranes; inner from cyanobacterium; outer from
engulfing cell
• Chloroplasts and cyanobacteria show similar biochemistry during
photosynthesis
• Further support for the endosymbiotic hypothesis:
• Discovery of chloroplast DNA
• Chloroplast DNA is a single circular chromosome similar to that of a bacterial cell
• The molecular sequence of chloroplasts and cyanobacteria genes very similar
Very different from the genes found in nucleus of eukaryotic
host in which the chloroplast was found
Page | 1
, Origin of Mitochondria
What gave rise to mitochondria?
Like chloroplasts, mitochondria closely resemble free-living bacteria in organization and
biochemistry
Evidence:
Contain their own DNA (similar to that of proteobacteria)
Have small genome
Origin of Eukaryotic Cell
Hypothesis 1
Hypothesis 2
What is the main difference between the hypotheses?
Page | 2
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