• I'm going to assume that you don't know the answer to the question, but do know about the cell cyce in general. I apologize if that is not the case... There are five main cell cycle phases G1, S, G2, M, and G0. We'll start with G1... In G1 the cell is just growing, and doing all the normal cell things. After being checked a few times, etc, etc, it enters S phase, where the DNA is replicated. After that, and a few more checkpoints, it moves to G2 phase, where it keeps growing and doing all the cell processes normally. Finally, there is another checkpoint and then the cell jumps into M phase, or mitosis - prophase, metaphase, anaphase, telophase and finally cytokinesis - the cell splits! Making two new ones both starting at G1. G0 is a special phase for specialized cells, as you are talking about. Cells enter G0 after prompting from hormones and environmental factors, and before entering the S phase. Think of it as an alternate route for the cell to follow other than the cell cycle circle. This is that terminal phase you are talking about. Once in G0, the cell ceases to divide. Its rarely terminal though. A vast majority of your cells are in G0, but can easily slip into G1 when they need to replicated. Think of it like this: your liver isn't constantly growing from replication - the cells are in G0. When one cell dies, another slips back into the cell cycle to replicate and then they both go into G0. (This is extremely generalized, its much more complicated than that.) Like you said, there are cells that won't ever divide again. They lack the ability to go back into G1. Neurons and blood cells are just two. For both of these, basically, they were divided specially from a stem cell. One of the half kept most of everything that is needed to replicate again, and the other went of and did its thing (carried blood, caused brain function). When they die, they are dead. Blood cells themselves do not replicated, but are continually made from adult stem cells in bone marrow. Neurons, on the other hand, were made from embryonic stem cells and once dead are gone forever.

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