I read this on the PZ Myers's blog (link):
"They also have a peculiar evolutionary history, arising as endosymbionts; their ancestors were independent organisms that took up residence inside eukaryotic cells in a mutually happy and long-lasting relationship. They exhibit some interesting relics of that prior history, as mitochondria have their own private strand of DNA which encodes some of the genes needed for the chemical processes they execute."
Sounds to me like mitochondrial DNA are practicing a little bit of encapsulation. Cell evolutionary biology as an example of "Favor object composition over class inheritance" [GOF]. How's that for a little mind-bending intersection of computer science and real science?