Every cell in the body -- including every egg cell and every sperm cell -- needs a source of energy to power its functions.

The structures responsible for producing this energy are minute organelles inside the cell called mitochondria. A mitochondrion looks, under the electron microscope, like a bacterium (and that's what we think the mitochondrion evolved from, billions of years ago). Every cell in the body contains hundreds of mitochondria, each busy burning up fat and sugar molecules with oxygen to release carbon dioxide as well as usable energy (in the form of high energy phosphate compounds called ATP, which is shifted around in the cell to where energy is needed for some critical action).

What makes these mitochondria fascinating is that each one has its own bit of genetic material -- their own bit of DNA, just like each bacterium has its own DNA. This is the only place outside the cell's nucleus (which is where the stick-like chromosomes are) that DNA is found.

Mitochondrial DNA (mtDNA) is different from nuclear, or chromosomal, DNA in a number of ways.

First, there are two of each particular stick-like chromosome, each with an incredibly long, double-helix of DNA, which codes for different genes along the string. With two such chromosomes in each chromosome pair, there are two such strings, and so two copies of each gene, one maternal and one paternal.

But mtDNA is a circular molecule: the string becomes a ring. The genes are strung around the string -- all 13 of them, coding for proteins unique to mitochondria and their energy-generating apparatus. Moreover there are of thousands of copies per cell.

Second, mutations (mistakes in the sequence contained in the DNA) are more frequent. Repair of the genetic code does not take place, so that as cells get older they gradually accumulate more and more mitochondria that don't work properly. Because of the large number of genes they compensate well for years, but eventually there is a meltdown: age takes its toll.

Third, when you were conceived all of the mitochondria in your body came from the mitochondria in your mother's egg -- none came from the mitochondria in your father's sperm cell that fertilized the egg. So mtDNA is a form of inheritance you get entirely from your mother.

So why do eggs all get produced before birth (for details see WebPage 3)? Why do new sperm cells keep forming, even into a man's old age?

Consider that every time a cell divides the mitochondria will have divided more often -- and every time a mitochondrion divides the several copies of mtDNA it contains divide first. And each time DNA divides there's an opportunity for a mistake, or mutation, in the genetic code to be propagated -- which, in the long run, alters the genome, a phenomenon we call genetic drift.

If egg cells kept dividing through life then the older a woman the more genetic drift there would be in the offspring -- way beyond what nature is prepared to tolerate. That's why nature puts all your eggs into long-term hibernation when you're just a 20 week fetus. (This seems to be such an important principle that it holds true for probably every vertebrate species of the animal kingdom: in all animals studied, all eggs are produced before the animal becomes fertile.)

Genetic integrity for sperm mitochondria? Forget it.

Sperm mitochondria have a different, non-genetic job: providing the energy the sperm cell needs to swim to the egg.

Capacitation of sperm (when they swim fastest to get through the egg's cumulus and zona pellucida) probably represents firing sperm's afterburners, deriving each last ounce of energy from the mitochondria before the successful sperm enters the egg, delivers its chromosomes, and tosses aside the depleted, exhausted, oxidized, maybe electrocuted mitochondria that got it there.

These defunct mitochondria would probably be the last thing you'd choose to deliver pristine mtDNA.

But in none of the body's tissues do mitochondria, and the integrity of their DNA, last forever.

Aging of mitochondria in muscles, for example, accounts for the fact that no amount of training in middle age keeps your muscles as efficient as they were during your youth. Aging of mitochondria in heart muscle starts to happen later, during your forties. Aging of mitochondria in the cortex of the brain, on the other hand, doesn't happen much until the seventies.

When does it happen in eggs? Several studies are starting to show that it occurs in the thirties and forties -- just the time that we know miscarriages get more common and just the time that otherwise unexplainable infertility becomes prevalent.