Human mitochondria contain their own DNA.

Most human DNA is stored in the nucleus, but a small fraction resides in mitochondria, structures in the cytoplasm. Their DNA is organized as a circular genome, providing a second genetic compartment inside the same cell rather than a second complete set of human chromosomes.

Mitochondrial DNA contains instructions needed for normal mitochondrial function. Some genes encode components involved in oxidative phosphorylation, the process that produces much of the cell's ATP. Other mitochondrial genes encode RNA molecules needed to assemble proteins within mitochondria.

Mitochondrial inheritance differs from ordinary inheritance of nuclear chromosomes. Children normally receive mitochondrial DNA from their mother. A father can pass nuclear genes involved in mitochondrial function to children, but he does not normally pass on his mitochondrial genome.

MedlinePlus Genetics describes human mitochondrial DNA as about 16,500 base pairs long. Base pairs are paired DNA building blocks, so this number describes genome length, not the number of genes. The mitochondrial genome contains 37 genes, making it tiny compared with the genetic material stored in the nucleus.

Of those 37 genes, 13 provide instructions for proteins involved in oxidative phosphorylation. The remaining genes produce transfer RNAs and ribosomal RNAs. These RNA molecules help translate genetic instructions into proteins. The genome therefore carries both some protein instructions and parts of the machinery needed to use those instructions.

Mitochondria convert energy from food into a form cells can use. ATP, or adenosine triphosphate, is a central carrier of that usable energy. Describing mitochondria as energy producers is useful shorthand, but they transform chemical energy rather than create energy from nothing. Their work depends on coordinated biochemical reactions.

Having their own DNA does not make mitochondria independent organisms inside the body. Many proteins required for mitochondrial structure and activity are encoded by nuclear genes. The nuclear and mitochondrial genomes must therefore work together. A disorder affecting mitochondria can originate in either genetic compartment, not only in mitochondrial DNA.

Maternal inheritance has a recognizable family pattern. A mother can pass her mitochondrial DNA to sons and daughters, while daughters are normally the ones who pass that mitochondrial lineage to the next generation. This is different from saying that all traits, or all energy-related conditions, come only from mothers.

Cells can carry a mixture of mitochondrial DNA variants, a situation called heteroplasmy. The proportion and distribution of a harmful variant can influence its effects, so mitochondrial inheritance does not guarantee identical symptoms among relatives. Clinical interpretation requires more than identifying a shared maternal lineage or reading a single genome-size statistic.

A separate mitochondrial genome is not a second complete instruction set for making a human. It is a small genetic compartment within a cell that also depends heavily on its nucleus. The source account links mitochondrial DNA to energy production while emphasizing this division of genetic labor. Location matters, but a DNA molecule's location does not by itself reveal how many cellular functions it can support independently.