Our bodies contain many different organs, each carrying out a special and important function. Similarly, all our cells contain internal structures called organelles that have specific roles to play.
Mitochondria are organelles whose job is to generate power. Ultimately, the energy-rich molecules in the food we eat are burned in the mitochondria, providing the power to drive other biochemical reactions and cellular processes. This method of energy release is called respiration.
Mitochondria are unique organelles in animal cells because they contain their own DNA. The human mitochondrial genome is a small circular DNA molecule 16 568 bp in length containing 37 genes. It was sequenced in 1981, ten years before the Human Genome Project got underway.
Why do mitochondria contain DNA? It may seem difficult to believe, but these organelles were once independent living cells similar to today’s bacteria. Millions of years ago, the bacteria invaded primitive amoeboid cells and established a mutually beneficial (symbiotic) relationship. Rather like the lichen we see growing on trees, which are really harmonic associations between algae and fungi, our cells probably evolved as symbiotic cellular communities. Over millions of years, redundant genes were lost from the bacteria and they became entirely dependent on their hosts, ceasing to exist as independent life forms.
What does the mitochondrial genome do? Twenty-four of the genes specify RNA molecules involved in protein synthesis while the remaining 13 encode proteins required for the biochemical reactions that make up respiration. A number of rare diseases are caused by mutations in mitochondrial DNA, and the tissues primarily affected are those that most rely on respiration, i.e. the brain and nervous system, muscles, and the kidneys and liver.
Mitochondrial diseases include Leber's hereditary optic neuropathy, in which there is loss of vision often combined with cardiac arrhythmia, and Kearns-Sayre syndrome, which involves paralysis of the eye muscles, dementia and seizures.
Interestingly, all mitochondrial diseases are maternally inherited, i.e. passed exclusively from mothers to their children.
Feature: Mitochondrial DNA and human history
This is because all the mitochondria in the developing human embryo come from the egg. There are no mitochondria in the sperm head, which carries the paternal half of the nuclear genome.
Anderson S, et al. Sequence and organization of the human mitochondrial genome. Nature 1981 290: 457-65. Abstract