OF MITOCHONDRIA AND MEN

The story of humanity is written in our genes, and thanks to modern science and technology, we are finally able to read it, writes J.M. Ledgard in our latest cover story. Geoffrey Carr, science editor of The Economist, explains how this works:

The purpose of sex is to mix up the gene pool. That makes it hard to track particular genes down the generations. But each gender has a small amount of DNA that does not get mixed.

In the case of women, this DNA is found in structures called mitochondria, which are separate from the cell nucleus, where most DNA resides. Mitochondria are the descendants of bacteria that took up residence in the ancestors of human cells about 2 billion years ago, and still have some of their original genes. Their hosts gave them house room because they convert the chemical energy in glucose into a form that a cell can use directly. When a sperm fertilises an egg, only the nucleus penetrates the egg’s outer membrane. The sperm’s mitochondria are left behind. Every human, then, regardless of sex, has only maternal mitochondrial DNA.

Only men, however, have Y chromosomes. These carry the genes that make them men. Part of the Y chromosome corresponds to part of the X chromosome, and swaps genes with it. But part passes from father to son unchanged. The upshot is that the only changes which happen to mitochondrial DNA and unswapped Y-chromosome DNA are those brought about by mutation. Each mutant is like a branch breaking off a tree, and those that have survived to the present day can be used to reconstruct that tree. The branching points can be calculated from which mutants are present and which absent. For example, if modern mitochondria can be found with mutants A, B and C, in the combinations A only, A and C, and A, B and C, but no others, mutation A came first, then mutation C, then mutation B.

In practice, things are a bit more complicated. There are dozens of mutations to deal with, and some combinations have arisen and become extinct—the branch has died and fallen off the tree. But statistics and computers combined can make a pretty accurate reconstruction of what happened.

How long ago a branch split off can be estimated from what is known as a molecular clock. Though mutations happen at random, the average rate over time seems reasonably constant in a given context (eg, a human mitochondrion). Counting the number of mutations between the tip of a twig and a branch point gives an estimate of that branch point’s age. Hence the calculation that the ancestor of all modern human mitochondria was in a woman living 150,000 years ago, and of all modern human Y chromosomes a man who lived 59,000 years ago.

~ GEOFFREY CARR

Picture Credit: mike52ad (via Flickr)