Mitochondria are important organelles for generating energy in the cells. They have a unique genome. In some species, the mitochondrial DNA is linear. Whereas in some species it is circular. The endosymbiont theory revealed that the mitochondria and chloroplasts originated as prokaryotes. Later on, they invaded the eukaryotic cells to establish a symbiotic relationship. Mitochondria are involved in cellular respiration which is a process of oxidization of food molecules. Mainly, the oxidization of glucose gives rise to carbon dioxide and water. The energy released in this process is known as ATP (Adenosine Triphosphate). Thus, mitochondria are also known as energy powerhouses. Till now we only knew the genes present in the nucleus. We have studied the structure and expression of the nuclear genes. Years later the discovery of the nuclear genes, the scientists found that there was something else beyond the Mendelian pattern of inheritance. Mitochondria also controlled a few traits. The genes present in the cytoplasmic organelles such as mitochondria and chloroplast are known as extra-nuclear genes. They are alternatively known as Non-Mendelian genes because they do not follow Mendelian inheritance pattern.

The origin of the mitochondrial DNA is of bacterial or prokaryotic origin. Thus, it shows similar replication and gene expression features. A wide structural variation persists in the eukaryotic mitochondrial genome.

The complete sequences of mitochondrial DNA are now known to us. The structure of the mitochondrial genome is highly conserved in the mammals. Though mitochondria have their separate genomes, they are not completely self-operative. In some cases, they may also require nuclear gene products for carrying out their specific functions.