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mitochondrion (mì´te-kòn´drê-en) noun
plural mitochondria (-drê-e)
A spherical or elongated organelle in the cytoplasm of nearly all eukaryotic cells, containing genetic material and many enzymes important for cell metabolism, including those responsible for the conversion of food to usable energy. Also called chondriosome.

[New Latin : Greek mitos, warp thread + Greek khondrion, diminutive of khondros, grain, granule.]
- mi´tochon´drial (-drê-el) adjective

Mitochondrion

Mitochondrion, small, membrane-bound cellular structure responsible for converting nutrients into the energy-yielding compound adenosine triphosphate (ATP) to fuel the cell's activities. This function, known as respiration, is why the mitochondrion is frequently referred to as the "powerhouse" of the cell. Mitochondria are found in eukaryotic cells (cells with a nucleus contained within a membrane). The number of mitochondria in a cell depends on the cell's function. Cells with particularly heavy energy demands, such as muscle cells, have more mitochondria than other cells.

The mitochondrion, which can range from 0.5 micrometer (0.00005 in) to 1 micrometer (0.0001 in) in length, has a double-membrane coat. The smooth outer membrane is separated from the inner membrane by liquid. The inner membrane surrounds a liquid matrix containing a large number of enzymes, or biological catalysts. Within the liquid matrix lies mitochondrial deoxyribonucleic acid (mtDNA), which contains the information to direct protein synthesis.

Mitochondria are used to trace the ancestry of organisms that contain eukaryotic cells. Among mammals, mitochondria tend to follow a pattern of maternal inheritance. This maternal inheritance creates a family tree that is not affected by the typical shuffling of genes that occurs between a mother and father. A recent comparison of samples of human mtDNA suggests that humans have descended from a woman who lived in Africa 140,000 to 290,000 years ago. There were likely many other women alive at the time of the so-called mitochondrial Eve, but their lines of maternal inheritance have died out. This commonly occurs when one generation in a family fails to have a daughter. Another use of mtDNA analysis is in forensic science. The identity of the skeletons alleged to be those of Czar Nicholas II, the last Russian czar, and his family were established using mtDNA.

Why are there only two sexes?
by BBC Science's Corinne Podger

Scientists in Britain believe that the reason there are only two sexes is due to a bacterial infection our ancestors caught about two billion years
ago.

There is no end to the diversity of life on Earth, so why most species have only two sexes has been puzzling scientists around the world for many years.

Mushrooms have as many as thirty-six thousand sexes, and a strange growth called slime mould has about thirteen. But these are rare exceptions to the almost universal rule that life on earth is divided into two sexes.
 

An evolutionary mystery

This presents an evolutionary mystery; If we had a hundred sexes, and could mate with any one of them, our chance of finding a partner in our
surrounding environment would be ninety-nine percent.

Professor Laurence Hurst, of Bath University, England, explains the problem: "For example, imagine that you are in a disco and the lights have gone out. You're looking for somebody to go home with and the law is the first person you bump into is the person you choose.

If there are only two sexes, then on average, half the time the person you meet is not going to be a potential mate."

Bacteria in the genes

So the question is why we only have two sexes, if it appears to make the survival of species more rather than less difficult. Professor Hurst believes it's all down to how we inherit a particular set of genes, called mitochondrial genes.

Unlike the genes carried in the nucleus or centre of our cells, mitochondria are found in material outside the nucleus. And unlike genes in the nucleus, mitochondrial DNA can copy itself very rapidly. Professor Hurst, speaking at the Festival of Science in Sheffield, believes mitochondrial genes are the remains of a bacterial infection caught by our distant ancestors.

"It looks as though there used to be bacteria where mitochondria come from. So we think that we got them about two billion years ago from bacteria taken into ourselves. So they became part of us, and that ability to replicate at will is left over from their bacterial ancestry."

Fewer mutations

Because mitochondrial DNA can reproduce so quickly, any mutation in it could spread rapidly if 99 percent of a population could mate with any other members. If the mutation was damaging the result could be catastrophic.

Instead, almost every species on Earth only inherits mitochondria from its mothers. Exceptions like mushrooms have evolved to avoid exchanging
mitochondrial DNA when they reproduce.

For the rest of us, finding a partner may be a little difficult, but in evolutionary terms, this is set against the benefits of fewer mutations.

• minimal techno track _Mitochondria_ MP3 (192k) by Theorem vs. Sutekh off of _THX: Experiments In Synchrnonicity_ compilation CD on Minus #010 (2002)
• The fictional "midichlorians" described in the film _Star Wars I: The Phantom Menace_ (vhs/ntsc)are analogous to real-life mitochondria.

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