Eukaryotic cells have organelles, membrane-bound structures that perform different functions in the cell, and prokaryotic cells do not. Some typical eukaryotic organelles include the nucleus, mitochondria, ribosomes, Golgi apparati and chloroplasts.
Eukaryotic cells are more organized and complex than prokaryotic cells. Eukaryotic DNA is housed in the nucleus, whereas prokaryotic DNA floats freely in the cytoplasm.
In the 1970s, Dr. Lynn Margulis theorized that the organelles of eukaryotic cells were at one time prokaryotes that were engulfed by larger cells. The larger cells became the hosts, protecting the smaller cells, while the smaller cells provided food to their hosts. In time, neither could exist without the other. This endosymbiotic theory was supported by the evidence that organelles such as mitochondria and chloroplasts resemble modern-day bacteria.