The Cell cycle

(Chapter 16, pages 650-661)

Having examined some of the ways in which cells communicate with each other, we start now to look at that most fundamental and mysterious process, cell division. All cells must divide at some point in their lives. In multicellular organisms, cell divisions are needed to produce a new individual from a single cell, the fertilized egg. The young organism must grow, also by cell division, and even in the adult, cells must divide to replace worn out cells. Different cell types divide at different rates. e.g., skin, hair, etc. divide frequently, to replace sloughed off cells, while certain other cells such as liver cells might not divide, unless they had to, for example, in response to injury. While cell division is vital to the growth and development of an organism and to the maintenance of its tissues, it must be tightly controlled. Loss of control of cell division leads to the inappropriate proliferation of cells that we call cancer.

What are the different phases of the cell cycle?
Cells spend a good deal of their lives in a state we call Interphase, during which they grow, and synthesize DNA, i.e., copy their chromosomes. They then undergo mitosis.

Interphase may be divided into three stages:
G1 - Growth phase 1 or gap 1
S - Synthesis, for DNA Synthesis
G2 - growth phase 2 or gap 2

Mitosis (or M phase), starts when G2 ends. Once mitosis is complete, the cell is once again in G1.

See Figure 16.1






What are some factors that affect cell division?
Certain substances stimulate or inhibit cell division- these have mostly been identified by studies with cultured cells.
These include:
Growth factors
Cell density
The last two are important in that uncontrolled cell division can occur if the levels of growth factors are not strictly regulated, or if cells fail to respond to the signals that tell them that cell density has reached a maximum for healthy growth.






At what point in the cell cycle do cells decide to divide or not to divide?
Cells in G1 grow, as we noted earlier, and late in G1 phase, will either commit to dividing again, or "drop out" of the cell cycle. This point in the cell cycle is appropriately called the START, or sometimes, the Restriction point.

See Figures 16.5 and 16.6






What happens to cells that "decide" not to divide?
If the decision is to not divide, the cell goes into a quiescent, or nondividing state, called G0 (G zero). The decision not to divide is often the result of not receiving the appropriate growth factor that signals that the cell should divide.

See Figure 16.6






What happens in cells that "decide" to divide?
If the decision is to divide (e.g., if a growth factor signal reaches cell), then the cell will go into S phase, and DNA synthesis will begin. Once this happens, the cell must go through the cell division process, all the way through to the end. However, each stage of the cell cycle must be successfully completed, before the next stage can be entered, and each step is carefully regulated. The cell has various mechanisms, called checkpoints, that allow it to proceed from one stage to the next only after the previous one has been confirmed to be completed.

See Figure 16.8 





What kinds of checkpoints are there?
The cell has several checkpoints. One, for example, in G1, checks whether the cell has reached a large enough size to divide. Another G1 checkpoint checks whether the cell's DNA is damaged. If it is, then the cell is prevented from proceeding to S phase (when DNA will be replicated) until the damage is fixed. A major G2 checkpoint prevents initiation of mitosis till all the DNA has been completely replicated.






How can the cell ensure that the DNA is replicated only once during a given cycle?
DNA is replicated only once per cycle because of the action of proteins called "licensing factors" or MCM proteins. These proteins can bind to the origins of replication on a chromosome only during G1 phase, and when they bind, they allow DNA replication to initiate. Once replication is started, the licensing factors fall off. No new replication can initiate until the cell has undergone mitosis and the licensing factors bind again in the next G1 phase.

See Figure 16.9






Is there a chemical signal in dividing cells that can induce non-dividing cells to start mitosis?
Scientists found that the cytoplasm of cells undergoing mitosis, if injected into cells at other stages of the cell cycle, could induce the non-dividing cells to start mitosis. This suggested that a chemical signal was present in the cytoplasm of dividing cells that was responsible for "telling" the cells that they should proceed into mitosis. By fractionating the cytoplasm into its components, scientists were able to isolate the signal that triggers mitosis.






What is the nature of this chemical signal?
The signal that sends cells into mitosis was found to be composed of protein, and was named MPF (maturation promoting factor or M-phase promoting factor). Further study revealed that MPF was composed of two proteins, Cdk 1 and cyclin B, which associated with one another to form active MPF.

See Figure 16.13 






What does MPF do?
MPF is a protein kinase, that is, it catalyzes the transfer of a phosphate group from ATP to target proteins. Phosphorylation can activate the target protein, which itself is sometimes a protein kinase. The phosphorylation of the target protein may thus lead to phosphorylation of yet another protein, and so on, setting off a cascade of reactions in the cell.

What effect does this have on the cell?
The phosphorylation of the various target proteins can activate many different pathways in the cell. The activation of these pathways causes the transition of the cell from G2 to M-phase.







Are both Cdk and cyclin required for MPF to work as a protein kinase?
Both proteins are required, and must be associated for the protein kinase activity of MPF. Cdk stands for cyclin-dependent kinase. The Cdk is the part that has the protein kinase activity, but it is inactive unless it is bound to the cyclin. There are many different Cdks but the one that is part of MPF is called Cdk1.






Since MPF sends cells from G2 to M phase, do the concentrations of Cdc2 and cyclin vary during the cell cycle?
Of the two proteins that make up MPF, the concentration of Cdk1, does not vary much during the different stages of the cell cycle. Cyclin, as its name suggests, cycles, or fluctuates, in concentration during the cell cycle (see Figure 16.12). It is the concentration of cyclin that causes the changes in MPF concentrations during the cell cycle. Synthesis of cyclin occurs during interphase, causing a build-up of cyclin. As more cyclin is available to associate with Cdk1, the concentration of active MPF increases.






What happens to MPF once the cell has made the transition to M-phase?
One of the pathways activated by MPF is an enzyme that destroys cyclin. So as mitosis gets under way, the enzyme that breaks down cyclin is activated, and levels of cyclin start to drop. The declining levels of cyclin lead to lowered levels of MPF by the end of mitosis. As MPF levels drop, so does the activity of the enzyme that destroys cyclin. As the cell enters interphase once again, cyclin synthesis begins and the cycle starts over again.






Is MPF the only factor in regulating cell division?
No, there are many different cyclin dependent kinases and cyclins that function together in various combinations to control the passage of a cell through the different phases of the cell cycle (MPF was just the first to be discovered and contains a cyclin known as cyclin B). See Figure 16.15.

Besides the association of the Cdks with cyclins there are also additional factors that regulate the activity of the Cdk-cyclin complexes.






What are the factors that regulate the activity of the Cdk-cyclin complexes?
Cyclin dependent kinases have sites that can be phosphorylated. Phosphorylation on one of these sites activates the Cdk-cyclin complex, while phosphorylation on the other two inactivates it. In order for the complex to be active, therefore, it must be phosphorylated on the first site, but dephosphorylated on the other two.
Another mechanism that controls the activities of the Cdk-cyclin complexes is binding of inhibitory proteins called Cdk Inhibitors (CKI).

See Figure 16.16






What do Cdk Inhibitors do?
These proteins inhibit Cdk activity by binding to them. A good example of the action of a CdK inhibitor is seen in the mechanism of the G1 checkpoint that arrests the cell cycle if the cell's DNA is damaged.






How does this process work?
If a cell's DNA is damaged, the level of a protein called p53 increases. p53 is a transcriptional activator that can turn up the expression of the gene for a protein called p21. p21 is a Cdk Inhibitor and can prevent the cell from moving into S-phase till the DNA damage is fixed.

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Copyright © 2008 Indira Rajagopal