Cell Cycle and Cell Division for AP Biology
Practice problems for these concepts can be found at : Cell Division Review Questions for AP Biology
Cell Division in Prokaryotes
Prokaryotes are simple single-celled organisms without a nucleus. Their genetic material is arranged in a single circular chromosome of DNA, which is anchored to the cell membrane. As in eukaryotes, the genetic material of prokaryotes is duplicated before division. However, instead of entering into a complex cycle for cell division, prokaryotes simply elongate until they are double their original size. At this point, the cell pinches in and separates into two identical daughter cells in a process known as binary fission (Figure 9.1).
The Cell Cycle
Eukaryotic cell reproduction is a bit more complicated. The cell cycle functions as the daily planner of growth and development for the eukaryotic cell. It tells the cell when and in what order it is going to do things, and consists of all the necessary steps required for the reproduction of a cell. It begins after the creation of the cell and concludes with the formation of two daughter cells through cell division. It then begins again for the two daughter cells that have just been formed. There are four main stages to the cell cycle and they occur in the following sequence: phases G1, S, G2, and M (Figure 9.2). Segments G1 and G2 are growth stages, S is the part of the cell cycle during which the DNA is duplicated, and the M phase stands for mitosis, the cell division phase.
Stages of the Cell Cycle
G1 phase. The first growth phase of the cell cycle. The cell prepares itself for the synthesis stage of the cycle, making sure that it has all the necessary raw materials for DNA synthesis.
S phase. The DNA is copied so that each daughter cell has a complete set of chromosomes at the conclusion of the cell cycle.
G2 phase. Second growth phase of the cycle. The cell prepares itself for mitosis and/or meiosis, making sure that it has the raw materials necessary for the physical separation and formation of daughter cells.
M phase, mitosis. This is the stage during which the cell separates into two new cells.
The first three stages of the cycle (G1, S, and G2) make up the portion of the cell cycle known as interphase. A cell spends approximately 90 percent of its cycle in this phase. The other 10 percent is spent in the final stage, mitosis.
The amount of time that a cell requires to complete a cycle varies by cell type. Some cells complete a full cycle in hours, while others can take days to finish. The rapidity with which cells replicate also varies. Skin cells are continually zipping along through the cell cycle, whereas nerve cells do not replicate—once they are damaged, they are lost for good. This is one reason why the death of nerve cells is such a problem—these cells cannot be repaired or regenerated through mitotic replication.
Control of Cell Division
Control of the cell cycle is important to normal cell growth. There are various ways in which the cell controls the process of cell division:
- Checkpoints. There are checkpoints throughout the cell cycle where the cell verifies that there are enough nutrients and raw materials to progress to the next stage of the cycle. The G1 checkpoint, for example, makes sure that the cell has enough raw materials to progress to and successfully complete the S phase.
- Density-dependent inhibition. When a certain density of cells is reached, growth of the cells will slow or stop because there are not enough raw materials for the growth and survival of more cells. Cells that are halted by this inhibition enter a quiescent phase of the cell cycle known as G0. Cancer cells can lose this inhibition and grow out of control.
- Growth factors. Some cells will not divide if certain factors are absent. Growth factors, as their name indicates, assist in the growth of structures.
- Cyclins and protein kinases. Cyclin is a protein that accumulates during G1, S, and G2 of the cell cycle. A protein kinase is a protein that controls other proteins through the addition of phosphate groups. Cyclin-dependent kinase (CDK) is present at all times throughout the cell cycle and binds with cyclin to form a complex known as MPF (maturation or mitosis promoting factor). Early in the cell cycle, because the cyclin concentration is low, the concentration of MPF is also low. As the concentration of cyclin reaches a certain threshold level, enough MPF is formed to push the cell into mitosis. As mitosis proceeds, the level of cyclin declines, decreasing the amount of MPF present and pulling the cell out of mitosis.