Mitochondria: Structures located in the cell's cytoplasm outside the nucleus. Mitochondria are responsible for energy production. Each consists 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 mitochrondria are the principal energy source of the cell. They not only convert nutrients into energy but also perform many other specialized tasks. Each mitochondrion has a chromosome that is made of DNA but is otherwise quite different from the better-known chromosomes in the nucleus. The mitochondrial chromosome is much smaller than other chromosomes. It is round, whereas the chromosomes in the nucleus are shaped like rods. There are many copies of the mitochondrial chromosome in every cell, whereas there is normally only one set of chromosomes in the nucleus. All mitochondrial chromosomes are inherited from the mother.
REFERENCE: NIH, US Library of Medicine, Genetics Home Reference. Mitochondrial DNA.