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Mitochondria are organelles (distinguishable parts of cells) in animals (and nearly all other eukaryotes). One of their functions is the production of ATP, which is the "fuel" of animal cells. Enzymes convert glucose, oxygen and AMP or ADP to ATP, water and carbon dioxide, which is the reason why animals have to breathe air and exhale carbon dioxide. The metabolic rate of this process is so high that in a day, about half of the body weight in ATP is being synthesized. Some poisons can block this process and thus lead to "internal asphyxiation". Other than it is often assumed (and as it is seen in cuts), mitochondria are not many single organelles, but instead form a mostly contiguous "network" inside a cell, which can split off and join parts.
Mitochondria are thought to be "immigrated" bacteria — similarly to the plants' chloroplasts, which are thought to be immigrated algae. This idea is termed the endosymbiotic hypothesis, or symbiogenesis. It is is supported by the fact that mitochondria have their own DNA, and, like prokaryotic genomes, those of mitochondria contain a high proportion of coding DNA in relation to non-coding DNA, are also circular, and are genetically quite similar to those of Rickettsial bacteria. Additional support is found in the character of the mitochondrion's inner membrane, as well as the fact that new mitochondria are generated only from existing mitochondria by binary fission. Additionally, some bacteria are known to live parasitically on other bacteria (TM7 phylum) or even within amoebae (TM6 phylum).
In most animal species, mitochondria appear to be primarily inherited through the maternal lineage, though some recent evidence suggests that in rare instances mitochondria may also be inherited via a paternal route. Typically, a sperm carries mitochondria in its tail as an energy source for its long journey to the egg. When the sperm attaches to the egg during fertilization, the tail falls off. Consequently, the only mitochondria the new organism usually gets are from the egg its mother provided. Therefore, unlike nuclear DNA, mitochondrial DNA doesn't get shuffled around by meiosis (and sex!) every generation, so it is presumed to change at a slower rate, which is useful for the study of human evolution, where it has been used to estimate the age of mitochondrial Eve at somewhere between 100,000 and 200,000 years ago, well before the age of the humanity put forth by young earth creationists. Mitochondrial DNA is also used in forensic science as a tool for identifying corpses or body parts, and has been implicated in a number of genetic diseases, such as Alzheimer's disease and diabetes.
Mutations in the mitochondrial genome can lead to hereditary diseases, most of which are thought to be lethal early on. An embryo with defective mitochondria cannot develop. Some, however, are survivable, and often result in symptoms like paralysis, exercise intolerance or degeneration of muscles or the nervous system, since muscles and the nervous system are the primary consumers of energy in an animal's body. Therapy is thought to be impossible currently.
Current genetic analysis is optimised for nuclear (i.e. non-mitochondrial) DNA and RNA. This means that the identification and analysis of mitochondrial conditions is currently more difficult and consequently less studied.
Cells do not simply decay when they "die" of age or stress. Actually, they decompose themselves in a process called apoptosis (or programmed cell death), which breaks the cell up into useful parts that can be recycled to build new cells. Unprogrammed cell death (e.g. in gangrene) leaves no useful remains and can be a disease in itself. Mitochondria appear to trigger and control the process of apoptosis. So it appears natural to assume, that cancer, in which apoptosis does not happen as it should, might be — in part — a mitochondrial disease. Cancer cells need to process a lot more of ATP than normal cells. Some drugs are thought to intervene in this process and trigger apoptosis when the metabolism is extremely high. This might turn out to be a useful form of therapy.
The nutty stuff
Since mitochondria are a legitimately important yet complex part of a functioning eukaryotic cell, they serve as a natural source of technobabble for alt-medders, who make various vague claims about the little buggers in order to sell whatever they're pushing, most commonly excessive quantities of some oh-so-great new antioxidant or supplement.
- Microbial matter comes out of the dark: Scientists identify bacteria that defy rules of biochemistry by Laura Beil (7:00am, September 7, 2016) Science News.
- Cultivation of a human-associated TM7 phylotype reveals a reduced genome and epibiotic parasitic lifestyle by Xuesong He et al. Proc. Natl. Acad. Sci. U. S. A. 2015 Jan 6; 112(1): 244–249.
- "You Are Energy Personified"
- "Mitochondria Resuscitation: The Key to Healing Every Disease" Yes, that really is the title, and yes, it really does describe the article.