Mitochondria are the tiny compartments inside nearly every cell of the body that convert food into useable energy. But genetic defects in the mitochondria mean the body has insufficient energy to keep the heart beating or the brain functioning.
The structures are passed down only from the mother and have their own DNA, although it does not alter traits including appearance or personality.
The technique, developed in Newcastle, uses a modified version of IVF to combine the healthy mitochondria of a donor woman with DNA of the two parents. It results in babies with 0.1% of their DNA from the second woman and is a permanent change that would echo down through the generations.
The Timetable is this: