The mitochondria are membrane-enclosed organelles housed within the cell's body; they act as the cell's power-plant producing adenosine triphosphate (ATP) for chemical energy. Mitochondria are also responsible for other functions to the cell's growth and also the cell's death so they can be considered the maker and death dealer within the cell so if they malfunction then the cell corrupts.
One way the organelles begins cell ageing is by releasing a byproduct of ATP production which free radicals. Oxidative stress then mutates the mitochondrial DNA and produces abnormalities and ageing effects.
Bringing in the same act system from my idea #1 I can house the mitochondria next to the chromosome factory (nucleus) as two big engines. As the factory and chromosome production begins to fail the mitochondria engines swell and enter a state of overload. This in turn corrodes the whole cell and then the cell replicates into another cancer cell.
Also mitochondria group themselves massively in areas with a lot of aerobic energy consumption like the brain, organs and muscles so I could locate the scene within a muscle strand to get a visually appealing background.