In addition to nuclear DNA we also have mitochondrial DNA (mtDNA). This is found in the cytoplasm of cells, and specifically in mitochondria. These are a particular type of cell organelle, and there are about 100 to 1,000 of them in each cell. They are also referred to as little power plants, because they convert sugars and fats into the energy (ATP) that our body needs to function properly.
We only receive mtDNA from our mother; the father's mtDNA is lost during fertilisation because it is broken down. The egg provides the cytoplasm, and this contains the mother's mitochondria.
MtDNA is ring-shaped, and is therefore not organised into chromosomes. It accounts for less than one percent of the total quantity of DNA, with only 16,500 nucleotides, as compared to three billion in nuclear DNA.
Nevertheless, the several hundred mitochondria in each cell each contain many of these circular DNA strands, which have 37 active genes. MtDNA is therefore crucial to our metabolism and (if there is any defect in its structure) it can give rise to complex disorders.