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The mitochondria are responsible for the cellular energy production but they also play a big role in several other cellular processes including apoptosis or cell cycle control. They are distributed throughout the cytosol of the cell and are enclosed by not only one but two membranes. The characteristic folds of the inner membrane are called cristae and are the scene of chemical reactions inside the organelle. Unlike all other animal organelles (besides the nucleus) the mitochondria possess a small genome on their own, consisting of 37 genes.
Mitochondrial stainings are often easy to recognize as they have a long, thread like pattern. They are spread throughout the cell, starting close to the nucleus and stretching all the way out to the edges of the cell. Depending on the cell type, the mitochondria can also be stained as shorter threads that are separated from each other.
Read more about the proteome of the mitochondria.