Question

If an interphase cell is treated with cyanide (a metabolic poison), the cell does not divide by mitosis. However, if cyanide is added right after mitosis has started, the same cell completes mitosis. Why?

Answer 1

During interphase, the cell carries out different metabolic activities in preparation for mitosis. The cell in this phase utilizes the nutrients it obtains in metabolism, carries out protein synthesis and grows, the cell also encodes its DNA. A crucial part of the metabolic process is the formation of energy in the form of ATP that drives the mitotic process. If this process is treated with cyanide, which is a metabolic poison, these crucial metabolic activities will be hampered and energy is not produced since cyanide actively hinders the phosphorylation of ADP to ATP. The result this has is that mitosis will not occur since the materials and energy required for cell division won't be available.
On the other hand, if interphase is carried out succesfully and mitosis begins first before the cell is treated with cyanide, the process of mitosis goes on. This is because the necessary material and energy  for the process of cell division are provided and the cell can divide to the two daughter cells.