Mitochondria (singular, mitochondrion) are oblong shaped organelles that are found in the cytoplasm of every eukaryotic cell. They occur in varying numbers, depending on the cell and its function.
These organelles are the power generators of the cell, converting oxygen and nutrients into ATP (adenosine triphosphate). ATP is the chemical energy "currency" of the cell that powers the cell's metabolic activities. This process is called aerobic respiration and is the reason animals breathe oxygen.
The mitochondrion is different from other organelles because it has its own DNA and reproduces independently of the cell in which it is found; an apparent case of endosymbiosis. Scientists hypothesize that millions of years ago small, free-living prokaryotes were engulfed, but not consumed, by larger prokaryotes; perhaps because they were able to resist the digestive enzymes of the engulfing organism.
The two organisms developed a symbiotic relationship over time, the larger organism providing the smaller with ample nutrients and the smaller organism providing ATP molecules to the larger one. Eventually, the larger organism developed into the eukaryotic cell, the smaller organism into the mitochondrion. Nonetheless, there are a number of prokaryotic traits that mitochondria continue to exhibit. Their DNA is circular, as it is in the prokaryotes, and their ribosomes and reproductive methods (binary fission) are more like those of the prokaryotes.
Mitochondrial DNA can be used study different aspects of inheritance. In most animal species, mitochondria are inherited through the maternal lineage. A sperm carries mitochondria in its tail as an energy source for its long journey to the egg. When it attaches to the egg during fertilization, the tail falls off. Consequently, the only mitochondria the new organism gets are from the egg its mother provided.
Unlike nuclear DNA, mitochondrial DNA doesn't get shuffled every generation, so it is presumed to change at a slower rate. That fact is being used to study human evolution and suggests that modern humans descended from a small group of hominids in Africa around 200,000 years ago.
Mitochondrial DNA is also being 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.
Information provided by: http://micro.magnet.fsu.edu