Great question. We used to think that brain cells stopped replicating soon after birth. It makes sense because we learn when brain cells make connections. New cells would be like late-comers shoving their way into an intricate dance. It turns out that there is some brain cell replication in adults, though not a lot.

On the other hand, the DNA in the nucleus of your brain cells and skin cells is also the same. What is different is which parts of the DNA are being used and how much they are being used.

As you say, mitochondria divide on their own schedule. It must be regulated in some way or cells that don’t divide much would end up packed with mitochondria and fast dividers wouldn’t have enough. It turns out that there are probably several proteins that control mitochondria. Mitochondria can fuse (squish together to make one) as well as divide. This may allow them to make repairs, change their shape and function, or cause a cell to die (this is useful if the cell is damaged or taken over by cancer or viruses). It turns out that mitochondria are much more interesting than the passive-looking ovals in your textbook pictures. They’re travelling all over the cell with the help of the cytoskeleton, changing shape, breaking apart, getting together, and probably doing other things we haven’t learned about yet.

If you were designing a way to trigger more or less division of mitochondria, what would your signal be? Oxygen level? Number of other mitochondria around? Amount of ADP?

If you are interested in questions like this, you might want to explore cell and molecular biology.

Thanks for asking. I learned something today.

It was once thought that the cells in the brain don’t replicate in an adult human, but recently it was discovered that a small proportion of these cells continue to replicate. Mitochondria either replicate during cell division or replicate because the cell needs more energy. The requirement for mitochondria replication is that each daughter cell receives at least one mitochondrion. Therefore, a brain cell could divide as long as it has 2 mitochondria that it can partition into the two daughter cells.

There are a number of factors that govern cell death and in fact,mitochondria many times contain the proteins that are important for apoptosis or programmed cell death. There are differences between mitochondria in different cell types, but it’s difficult to know whether the lifetime of the cell is dependent on the mitochondria. It’s likely that in a skin cell, the mitochondria are the first component of the cell to fail so they would probably last longer than the average lifetime of a skin cell. Whether the mitochondria would last as long as a brain cell is unknown. Mitochondria were originally bacteria that were eaten by a eukaryote, but were kept as an organelle instead. As such, they should have somewhat similar properties in all cells.

Mitochondria are organelles in a cell, but are descended from once free-living bacteria. As such, they are capable of reproducing by division just as bacteria do. In order to neither overrun their host cells nor leave their hosts with too few mitochondria, these organelles divide in response to signals sent to them by the host cell. Any cell that is going to divide will signal its mitochondria to divide appropriately so that there are enough of them in the daughter cells.

I don't know the specific behavior of brain versus skin cells, but since mitochondria are so basic organelles common to all animal cells, I would expect that they would behave in a comparable fashion.