In support of the endosymbiotic theory, scientists in the 1960s discovered that mitochondria and chloroplasts contain 70S ribosomes, which are structurally similar to bacterial ribosomes rather than the eukaryotic ribosomes of their host cells.

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The modern endosymbiotic theory states that mitochondria and chloroplasts arose as a result of prokaryotic cells establishing a symbiotic relationship within a eukaryotic host. Mounting genetic evidence, particularly from DNA sequencing, supports this theory by demonstrating similarities between these organelles and bacteria.

Endosymbiotic Theory

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Microbiology covers the scope and sequence requirements for a single-semester microbiology course for non-majors. The book presents the core concepts of microbiology with a focus on applications for careers in allied health. The pedagogical features of the text make the material interesting and accessible while maintaining the career-application focus and scientific rigor inherent in the subject matter. Microbiology’s art program enhances students’ understanding of concepts through clear and effective illustrations, diagrams, and photographs.

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American geneticist Lynn Margulis ($${}1938$$–$${}2011$$) published her ideas regarding the endosymbiotic hypothesis of the origins of mitochondria and chloroplasts in $${}1967$$. Benefiting from advances in microscopy that allowed scientists to differentiate prokaryotic from eukaryotic cells, Margulis reviewed the literature and argued that these eukaryotic organelles are of prokaryotic origin. To support her claims, she presented a growing body of microscopic, genetic, molecular biology, fossil, and geological data.

Lynn Margulis

The endosymbiotic theory is supported by the discovery in the 1960s that mitochondria possess their own genome, which was sequenced in 1976 and found to be bacterial. Genetic sequencing and phylogenetic analysis show that mitochondrial and chloroplast DNA are closely related to bacterial counterparts in sequence and structure, though organelle DNA is reduced due to gene transfer to the host nucleus.

Genetic Evidence for Endosymbiotic Theory

Ribosomal Evidence for Endosymbiotic Theory

Supporting the endosymbiotic theory, the binary fission of mitochondria and chloroplasts strongly resembles the binary fission of bacteria, as compared with the mitosis performed by eukaryotic cells.

Binary Fission Evidence for Endosymbiotic Theory

Since Lynn Margulis' original proposal of the endosymbiotic theory, scientists have observed several modern examples of bacterial endosymbionts living within eukaryotic cells. Examples include endosymbiotic bacteria found in the guts of certain insects, like cockroaches, and photosynthetic bacteria-like organelles within some protists.

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