Professor Sykes explains what Mitochondrial DNA is

Professor Sykes:
Now DNA, the code, is absolutely enormous. In humans this code is three thousand million letters long. So I had to choose a particular type of DNA to look at, I couldn't possibly read through three thousand million bits of spelling if you like.

So I chose a piece of DNA that is very special in that it's inherited only through the Mother. It's got a long name. It's called Mitochondrial DNA, but we'll call it M-DNA for short.

I got the DNA out of the bones and then actually looked at the sequence of this M-DNA. And, I mean, so what? Well, what I was able to do was then look at other people's DNA - modern people, people like you and I, and look at the gene M-DNA and look at the sequence there and sometimes, usually it was very different from the old bones.