All mitochondria arise from the reproduction of another mitochondria.
When the cell divides, its mitochondria separate into two roughly equivalent groups, which are positioned on either side of the cytoplasm.
At the end of the division each group is in a daughter cell. Later, as cells grow, the mitochondria duplicate and grow, restoring the original number.
Sperm mitochondria penetrate the egg during fertilization and then degenerate, so the mitochondria present in the egg cell originate exclusively from the mother. Ovular mitochondria, which multiply each time the cell reproduces, are the ancestors of all, the mitochondria present in our cells.
Many scientists are convinced that mitochondria are descendants of primitive prokaryote beings that once settled in the cytoplasm of the first eukaryotic cells. Evidence supports this hypothesis, such as the fact that mitochondria have more genetic material resembling that of bacteria than that of the eukaryotic cells in which they are found.
The same is true for protein synthesis machinery: mitochondrial ribosomes are very similar to those of bacteria and quite different from ribosomes in the eukaryotic cell cytoplasm.