List three main ideas of the cell theory

Concerning the third principle of cell theory (all cells are made from other cells) there is a related theory about the origins of organelles. 

Because mitochandria have seperate DNA from the rest of the cell, it is believed that they were once seperate organisms and were ingested by larger cells. Organelles such as mitochrondria and chloroplast then became mutually beneficial to their host cells and became a permanent structure within the cell. This theory is called The Endosymbiotic Theory.