The mitochondria are normal structures or organelles in cells. They are located outside the nucleus in the cell's cytoplasm. The mitochondria are responsible for energy production. They consist of two sets of membranes, a smooth continuous outer coat and an inner membrane arranged in tubules or in folds that form plate-like double membranes (cristae). The mitochondria are in fact the principal energy source of the cell (thanks to the cytochrome enzymes of terminal electron transport and the enzymes of the citric acid cycle, fatty acid oxidation, and oxidative phosphorylation). The mitochondria convert nutrients into energy as well as doing many other specialized tasks.
Each mitochondrion has a chromosome that is made of DNA (mitochondrial DNA or mtDNA) but is otherwise quite different from the far better known chromosomes in the nucleus of cells. It is much smaller and it is round (whereas the chromosomes in the nucleus are shaped like rods). There are also many copies of the mitochondrial chromosome in every cell (whereas there is normally only one set of chromosomes in the nucleus). Mitochondrial DNA contains 37 genes which all are essential for normal function of the mitochondria.
Because of the peculiarities of mitochondria, mitochondrial inheritance does not obey the classic rules of Mendelian genetics. Mitochondrial DNA is always passed from the mother to the offspring in humans. Many genetic conditions are related to changes in particular mitochondrial genes. Persons with a mitochondrial disease may be male or female but they are always related to one another in the maternal line. No male with a mitochondrial disease can transmit it to his children. A female with a mitochondrial disease will transmit it to all of her children.