Question

1. occasionally cells stop dividing and enter another phase, g₀. if you damage your liver, new liver cells can be produced to replace up to 75% of the liver. however, if you sustain brain damage, your body does not produce new brain cells. explain this observation using what you have learned about the cell cycle.

Explanation:

Liver cells enter the reversible $G_0$ phase, meaning they can re-enter the cell cycle (via growth signals from injury) to divide and regenerate damaged tissue. Most mature brain cells enter a permanent, non-reversible $G_0$ state; they lose the ability to re-activate cell division pathways, so no new cells are produced to replace damaged ones.

Answer:

Liver cells are in a reversible $G_0$ phase, so they can re-enter the cell cycle and divide to regenerate damaged tissue. In contrast, mature brain cells are in a permanent, non-dividing $G_0$ phase and cannot re-initiate cell division, which is why the body cannot produce new brain cells to replace damaged ones.