According to the theory of endosymbiosis, describe how mitochondria came to exist within our modern-day eukaryotic cells.

The endosymbiotic theory explains that mitochondria and chloroplasts descended from the same type of bacteria. Symbiosis is an interaction between two organisms of different species, and endosymbiosis is a type of symbiosis in which one organism lives inside the other one. According to the endosymbiotic theory, a host cell ingested symbiotic bacteria, which specialized into mitochondria or chloroplasts. The evidence for this theory is that mitochondria and chloroplasts, like bacteria, have their own circular DNA. They also have their own transcriptional and translational machinery.