Question: do you think that mitochondria used to be free living organism?

Caroline Dalton answered on 19 Jun 2012:

Great question! You may have heard of the endosymbiotic theory. This theory proposes that mitochondria and choloplasts (the things in plant cell that do photosynthesis) were originally free living bacteria which then became part of cells in a symbiotic relationship (a symbiotic relationship is one which is mutually beneficial). This theory was proposed because mitochondria and chloroplasts resemble certain types of free living bacteria and, unlike any other organelle, they contain their own DNA which is separate from the nuclear DNA.

Over time mitochondria and chloroplasts have transferred some of their genome to the nucleus and they could no longer survive outside the cell as free living organisms but it is largely believed that this is how they originated 🙂