What are Mitochondria?

Mitochondria: Power houses, factories, waste disposal and recycling centres, and assassins...

What are mitochondria and what do they do?

The “powerhouses of the cell”, that’s how many people know mitochondria. The parts of cells that turn sugars, fats and proteins that we eat, into forms of chemical energy that the body can use to carry on living.

Every living thing is made of cells: tiny compartments contained by a membrane. Cells are the smallest things that can reproduce themselves. When we look inside cells, we see that they have sub-compartments that are smaller still, known as “Organelles” which perform different functions that are essential for the cell to live.

Mitochondria are organelles found in the cells of every complex organism.  They produce about 90% of the chemical energy that cells need to survive. No energy; no life! So it's easy to see why when mitochondria go wrong, serious diseases are the result, and why it is important we understand how mitochondria work.

However, mitochondria do much more than just produce energy. They also produce chemicals that your body needs for other purposes, break down waste products so they’re less harmful, and recycle some of those waste products to save energy.

Mitochondria also have a special role in making cells die (apoptosis). This may sound strange, but it is vital for the processes of growth and development. Sometimes cells don’t die when they should, and start to grow uncontrollably. This is how a tumour starts to grow, so you shouldn’t be surprised that mitochondria play an important part in cancer and are seen as targets for anti-cancer drugs.

To produce all of that energy, mitochondria require oxygen. Mitochondria effectively burn your food in a carefully controlled way to produce that chemical energy by a process called “oxidative phosphorylation”. And just as a fire goes out without oxygen, if mitochondria lack oxygen, they also stop working => No energy; No life!

During a heart attack, or a stroke, the blood stops delivering oxygen to the heart and brain. These two organs do a lot of work and need a lot of energy. Without oxygen, the mitochondria stop working, and the cells in the brain or heart are damaged or even die. Perversely, if the oxygen does return, then the mitochondria get overwhelmed and produce a lot of “free radicals”. These are very reactive chemicals which cause a lot of additional damage - called “Reperfusion injury”.

If you look at mitochondria in detail, they look a lot like miniature cells themselves, so how did they arise?   We know that mitochondria were originally bacteria. About 1,500,000,000 years ago, a bacterial cell was engulfed by another cell, but rather than killing each other, the two cells worked together, probably because it was beneficial to each cell.

Mitochondria have their own DNA

One reason we know that mitochondria came from bacteria is that they still contain a tiny amount of DNA that is similar to bacterial DNA. Mitochondrial DNA is about 16,000 bases long and has 37 genes (in humans). The DNA in the nucleus - sequenced during the human genome project - is 3,000,000,000 bases long and has about 25,000 genes. So only about 0.1% of your genes are in your mitochondria. But the mitochondrion needs more than the 37 genes on the mitochondrial genome to work. We think about another 1,500 genes are needed, and they are on the nuclear genome.

Here’s another strange fact about mitochondria - You only get them from your mother. This is because when sperm fertilise an egg, they only pass on the DNA from their nucleus, not their mitochondria. The embryo has all its mitochondria from the mother’s egg. This means that mitochondrial diseases due to mutations on the mitochondrial DNA are only passed on by the mother - they can affect both her sons and daughters - but it will be only her affected daughters who may pass the disease on to their children. However, if the mutations are on the nuclear DNA, then they can be inherited from both the mother and the father.