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Our initial discussion of compartmentalization focused on eukaryotic cells. Prokaryotic cells do not have membrane-bound organelles and thus do not exhibit compartmentalization in the same way. However, prokaryotes do have internal regions with specialized structures and functions like DNA replication and protein synthesis. There are different mechanisms for how eukaryotes may have developed internal membrane-bound organelles. One is through the endosymbiosis of prokaryotes. This theory suggests that these prokaryotes, after being engulfed by the cell, developed a symbiotic (mutually beneficial) relationship with the host cell. Over time, these evolved into organelles such as mitochondria and chloroplasts, both of which provide specific and complex functions. This theory is supported by the double-membrane structure of these organelles – one which originated from the prokaryote and the other which originated from the host cell’s plasma membrane as it engulfed the prokaryote. These organelles also contain unique DNA that is different than other genetic material in the cell. Another mechanism that could have created organelles is membrane infolding. In this process, the cell membrane folded inward, like during endocytosis, and created compartments that over time developed into structures like the nucleus, which stores DNA. This theory of common ancestry is also supported by the presence of ribosomes in all cells.
