Nuclear DNA and mitochondrial DNA differ fundamentally in location, structure, quantity, and inheritance patterns. Nuclear DNA resides in the cell nucleus and contains the vast majority of human genes—approximately 20,000 protein-coding genes organized into 23 pairs of chromosomes. It is linear in structure, inherited from both parents (biparental inheritance), and tightly packaged with proteins called histones. Mitochondrial DNA, by contrast, exists in the mitochondria (cellular powerhouses) and is much smaller—only about 16,500 base pairs encoding 37 genes, primarily involved in energy production. It is circular in structure, similar to bacterial DNA, and is inherited almost exclusively from the mother (maternal inheritance) since sperm contribute negligible mitochondria to the embryo. Nuclear DNA undergoes recombination during sexual reproduction, creating genetic diversity, while mitochondrial DNA replicates independently and passes down largely unchanged through maternal lineages. Cells contain only one nucleus but hundreds to thousands of mitochondria, meaning there are far more copies of mitochondrial DNA than nuclear DNA per cell. The evolutionary origin of this difference traces to endosymbiotic theory: mitochondria descended from ancient bacteria engulfed by ancestral eukaryotic cells, retaining their own genetic material and semi-autonomous replication machinery. These distinctions make mitochondrial DNA particularly useful for tracing maternal ancestry and studying human migration patterns, while nuclear DNA provides the comprehensive genetic blueprint for building and maintaining organisms.