Those Marvelous Mitochondria

Current theories hold that life began on Earth around 3.5 billion years ago. About a billion years ago, a single celled beast engulfed and absorbed another single celled creature. We are all descended from that hijacking.

The hijacked cell has over time become the mitochondrion. This organelle is responsible for making our energy. But it still has the marks of having once been a free living bacterium.

First off, mitochondria still have remnants of their old DNA. There isn’t much there in human mitochondria but there is enough to still get us into trouble. A big part of aging might be due to damage to this mitochondrial DNA (mtDNA). Some genetic diseases are also caused by mutations in mtDNA.

The DNA in mitochondria is also much more like bacterial DNA compared to the rest of our DNA. In fact, the mitochondrion has its own bacteria-like machinery for reading its DNA. This means that mitochondria can’t read the genes in our nucleus and vice versa. Mitochondria are so similar to bacteria that some antibiotics can damage them too.

Even though it was once free living, the mitochondrion doesn’t have a lot of its original DNA left. Over time, most of our mitochondrion’s original genes have traveled to the nucleus. These genes now work in the nucleus to make most of a mitochondrion’s proteins which are then transported back to the mitochondrion.

After all these years, human mtDNA is now only around 16,000 bases long and has only 37 genes left. This is a far cry from even the simplest of bacteria, Mycoplasma genitalium, with its 582,970 bases and 521 genes.

Humans are not unique in having mitochondria. Every plant, animal, and fungus cell in the world that has been looked at has mitochondria. But the DNA in these mitochondria is all wildly different.

The size of mtDNA can range from just 6000 base pairs all the way up to 2 million base pairs. Sometimes the mtDNA is a circle like ours. Sometimes it is spread out over lots of little circles. Sometimes it is one long, linear piece of DNA. Sometimes it is lots and lots of little pieces of linear DNA. And sometimes it is too weird to describe in a short blog like this.

Mitochondria from different species also have different numbers of genes. Some species have mitochondria with nearly 100 genes. While others have as few as 5.

With up to 2000 mitochondria/cell, evolution has had a free hand in tinkering with mtDNA. If a mutation or change in mtDNA causes a problem, that mitochondrion simply goes away. If there is some advantage to the new DNA structure, it is free to sweep through and take over. It is amazing what evolution has done to this bacterium!

Of course, evolution has made the mitochondrion a shell of what it once was. But we could argue that it is one of the most successful beasts ever.

It has gone from humble bacterium to being part of every eukaryote in the world. If humans die out, mitochondria will still be around somewhere else. Mitochondria will outlive us all.

More information on mitochondrial genomes: http://dx.doi.org/10.1016/j.tig.2003.10.012