This week in babytalk I'm asking you to remember everything you learned about cells in your biology class. Do you remember mitochondria? Your teacher might have described them as the 'cell batteries' the little organs that keep the cells functioning. When most of us think of human cells and the fertilization process, we naturally think of all the DNA in the nucleus.. the stuff that is important and passed on from both parents to the developing baby but scientists are working on a theory that the presence of mitochondrial DNA in an egg is the key to it's viability and possibly the loss or lack of mitochondrial DNA in an egg cell means it's too old to be fertilized. So how important is mitochondrial DNA and if it is so vital to making babies, what do we need to do to make that knowledge work for us.