Microbiology, 2021

3.2 • Foundations of Modern Cell Theory 81
Wallin’s ideas regarding the endosymbiotic hypothesis were largely ignored for the next 50 years because
scientists were unaware that these organelles contained their own DNA. However, with the discovery of
mitochondrial and chloroplast DNA in the 1960s, the endosymbiotic hypothesis was resurrected. Lynn
Margulis (1938–2011), an American geneticist, published her ideas regarding the endosymbiotic hypothesis of
the origins of mitochondria and chloroplasts in 1967. 12 In the decade leading up to her publication, advances
in microscopy had allowed scientists to differentiate prokaryotic cells from eukaryotic cells. In her publication,
Margulis reviewed the literature and argued that the eukaryotic organelles such as mitochondria and
chloroplasts are of prokaryotic origin. She presented a growing body of microscopic, genetic, molecular
biology, fossil, and geological data to support her claims.
Again, this hypothesis was not initially popular, but mounting genetic evidence due to the advent of DNA
sequencing supported the endosymbiotic theory, which is now defined as the theory that mitochondria and
chloroplasts arose as a result of prokaryotic cells establishing a symbiotic relationship within a eukaryotic host
(Figure 3.7). With Margulis’ initial endosymbiotic theory gaining wide acceptance, she expanded on the theory
in her 1981 book Symbiosis in Cell Evolution. In it, she explains how endosymbiosis is a major driving factor in
the evolution of organisms. More recent genetic sequencing and phylogenetic analysis show that
mitochondrial DNA and chloroplast DNA are highly related to their bacterial counterparts, both in DNA
sequence and chromosome structure. However, mitochondrial DNA and chloroplast DNA are reduced
compared with nuclear DNA because many of the genes have moved from the organelles into the host cell’s
nucleus. Additionally, mitochondrial and chloroplast ribosomes are structurally similar to bacterial ribosomes,
rather than to the eukaryotic ribosomes of their hosts. Last, the binary fission of these organelles strongly
resembles the binary fission of bacteria, as compared with mitosis performed by eukaryotic cells. Since
Margulis’ original proposal, scientists have observed several examples of bacterial endosymbionts in modernday
eukaryotic cells. Examples include the endosymbiotic bacteria found within the guts of certain insects,
such as cockroaches, 13 and photosynthetic bacteria-like organelles found in protists. 14
Figure 3.7
According to the endosymbiotic theory, mitochondria and chloroplasts are each derived from the uptake of bacteria. These
(2000):951–961.
12 L. Sagan. “On the Origin of Mitosing Cells.” Journal of Theoretical Biology 14 no. 3 (1967):225–274.
13 A.E. Douglas. “The Microbial Dimension in Insect Nutritional Ecology.” Functional Ecology 23 (2009):38–47.
14 J.M. Jaynes, L.P. Vernon. “The Cyanelle of Cyanophora paradoxa: Almost a Cyanobacterial Chloroplast.” Trends in Biochemical
Sciences 7 no. 1 (1982):22–24.