As there are many copies of mitochondrial DNA within each cell, it means that the situation may arise where some copies may contain detrimental changes (mutations) in the code of the DNA whereas others will carry the right code, we call this wild type. This mixture of mitochondrial DNA copies with mutations and wild type copies is called heteroplasmy and maintaining the balance so that the cell has enough wild type copies is important for the cell. Cells will only be affected if the percentage of its mitochondrial DNA copies that carry a mutation exceeds a certain limit, this is called the threshold. In most cells the threshold is between 50-60%, once this level is exceeded the cell will show signs of mitochondrial dysfunction. Heteroplasmy is illustrated below.
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