a.) One cell regulatory mechanism are checkpoints in the cell cycle at the end of G1, S, and during mitosis. These checkpoints make sure that the cell has correctly gone through each stage of the cell cycle up to that point. Thus, if something is not correct, the cell is able to tell, and will stop the cell from continuing throughout the cycle and will prevent the cell from replicating.
b.)One reason a cell would be given the signal to stop replicating is before anaphase in mitosis. The spindles may not all properly be attached to the chromosomes, thus, when the chromosomes are pulled to opposite poles, there will be an unequal amount of chromosomes in each cell. This can cause many issues for a cell and can stop it from properly growing and performing its function. Thus, at the checkpoint, the cell would be given the signal to stop.
c.) Endocrine signalling is used for long-distance signals. In this type of signaling, a hormone is secreted from the cell. The hormone travels through the bloodstream, thus it can reach virtually anywhere cell type in the body.