THE ENDOSYMBIOSIS HYPOTHESIS:
Evaluation of the Evidence
The modern version of the endosymbiosis hypothesis was developed and promoted by Dr. Lynn Margulis in 1970. The term endosymbiosis comes from “endo” meaning “within,” and symbiosis which occurs when two different species benefit from living and working together (Genetic Science Learning Center). This theory proposes that the organelles of eukaryotic cells, specifically the mitochondria and chloroplasts, were once free-floating bacteria that were ingested by larger, prokaryotic bacteria by means of endocytosis. The host bacteria benefitted from the engulfment of the organelles as they were able to carry out functions, such as cellular respiration, more efficiently than the host itself could. Therefore, the free-living bacteria were not digested, and consequently over time their DNA was combined with the host bacterial cell’s DNA to eventually develop a new form of cells called eukaryotes. One of the most eminent pieces of evidence that supports the hypothesis is that although prokaryotic cells and eukaryotic cells are quite distinctive in physiological characteristics, some of the organelles of eukaryotes share a number of staggering similarities with prokaryotes. Like prokaryotes, the mitochondrion and chloroplast both replicate through means of binary fission, unlike the eukaryotic cells that contain them, which undergo mitosis. Moreover, prokaryotes and the abovementioned eukaryotic organelles have circular DNA, whereas eukaryotes have linear DNA. The size of the DNA is also a factor in the theory as the prokaryotic DNA and the DNA of the organelles is similar in size, and much smaller than that of eukaryotic cells. The dissimilarities between the physical characteristics of the DNA in the eukaryotic nucleus and the DNA in the organelles lead to the impression that the organelles were once bacterial (prokaryotic) symbionts (IUPUI Department of Biology). However, since the DNA in the nucleus and...
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