Multiple copies of the mitochondrial genome can be found in individual mitochondria, with many mitochondria present in a single cell. The precise number of genomes per mitochondrion is unknown and is likely to vary between cell types; the generally accepted range is between 2 to 10 molecules per organelle. The number of mitochondria per cell is dependent on the metabolic demands of that cell type and can vary from 100s to 10,000s. In most individuals at birth, all of the mtDNA molecules are identical (called homoplasmy). However, the presence of mutated mtDNA usually generates a mixture of wild-type and mutated molecules within the same cell. This phenomenon is referred to as heteroplasmy. The percentage level of mutated mtDNA often varies from cell to cell, and from organ to organ within an individual and may change over time.
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