Expert: Dana Krempels, Ph.D. - 7/8/2009

Question
Hi Dana,

I understand that mitochondria are independent organisms living within our cells.  They are not created by our DNA, and do not evolve with our evolution.  That's why we can trace female genetic lineage: we trace the mtDNA (the mitochondria's own DNA).  

So my question is this:

How do mitochondria come to be within our cells?  Our DNA doesn't have the genes to create them?  How do they get passed thru the female line, if it's not our DNA making them?

Thanks for your time, Dana!

With Blessings,

erin

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Answer
Dear Erin,

All current evidence suggests that mitochondria evolved from early prokaryotic organisms that took up residence in larger ones.  This is known as the Endosymbiont Model of eukaryotic cell evolution.

Today's mitochondria are not independent organisms living within our cells, though their very ancient ancestors might have been.  You can get a more complete overview here:

http://www.bio.miami.edu/dana/160/160S09_9.html

Mitochondria and chloroplasts (both derived from ancient endosymbionts) have their own DNA which encodes the instructions for their own reproduction via binary fission (which is similar to the way their prokaryotic ancestors reproduced).  There is *some* input from the nuclear DNA, and the cell and mitochondria do interact genetically to some degree.

As mitochondria reproduce during the cell cycle, about half of them are transported to each new daughter cell as mitosis occurs, just as the other organelles are duplicated and passed along.

The reason we inherit them from our maternal parent is that sperm are very "stripped down" in terms of cytoplasm. The head of the sperm is pretty much just a nucleus with a bit of cytoplasm and membrane around it, and it's only the head of the sperm--not the tail or the midpiece (containing mitochondria)--that fuse with the ovum to make a zygote.

Hope that helps.

Dana