Mitochondrial DNA.

Mitochondria play an essential role in the generation of energy in eukaryotic cells. Mitochondria are the organelles that are the main “chemical factories” of the cell, where cellular aerobic respiration—using the Krebs (citric acid) cycle and respiratory electron transport to produce NADH (nicotinamide adenine dinucleotide) and ATP (adenosine triphosphate)—occurs. In a light microscope, mitochondria look like short rods or thin filaments about 0.5 to one micrometers long. A mitochondrion is made up of a smooth outer membrane and an inner membrane that is folded into tubular shapes called cristae. Many aerobic respiration reactions are catalyzed by enzymes that are bound to mitochondrial membranes. Other reactions occur in the space between the inner and outer mitochondrial membranes. Cells may contain anywhere from one to ten thousand mitochondria, depending on the organism and the function of the cell. A typical animal cell contains between 750 and two thousand mitochondria, for example, while the human eye contains ten thousand mitochondria per cell. Cells that are dividing and cells that are metabolically active need larger amounts of ATP and usually have large numbers of mitochondria.