The Cell cycle

(Chapter 18, pages 609-635)

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. 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 18.2






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 START.

See Figures 18.3 and 18.12






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 18.12






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 18.13





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. See Fig. 18.3





How do cells control the progression of the cell cycle through G1, S, G2 and M phases?
The progress of a cell from one phase to the next of the cell cycle is controlled by special protein kinases that are called Cdks or Cyclin-dependent Kinases. These kinases are activated by the binding of proteins called cyclins, which is why they are called cyclin-dependent kinases. When they are not complexed with cyclins these kinases are inactive. See Fig. 18.4

Different cyclin-cdk complexes trigger different steps in the cell cycle. See Fig. 18.10

Once this is accomplished, the Cdk is inactivated by the destruction of its cyclin partner.
See Fig. 18.11

Is the association of a Cdk with its cyclin partner the only thing that regulates its activity?
Besides the association of the Cdks with cyclins there are also additional 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).

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. Once the DNA has been repaired, the cell is permitted to move into S-phase and begin DNA synthesis.
Fig. 18.16

(If the DNA damage is too extensive for the repair machinery to fix, p53 will, instead, trigger cell death, to prevent the proliferation of a cell with badly damaged DNA.)

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" . These proteins can bind near 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 dissociate and are exported out of the nucleus. No new replication can initiate until the cell has undergone mitosis and the licensing factors bind again in the next G1 phase.





What happens after the DNA has been copied?
The cell moves into G2 phase where yet another checkpoint ensures that all the DNA has been completely replicated before the cell can enter M phase or mitosis. Entry into M phase as well as the various events in mitosis are controlled by M- Cdk.

In many ways, M-Cdk is the most interesting of all the Cdks and it was also the first to be discovered. We will examine, below, the discovery of M-Cdk in the eggs of Xenopus laevis, the African clawed frog.

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.
Fig. 18.7






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. (We now know that MPF is an M-Cdk.)

Investigations revealed that off 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 18.5). 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 does MPF do?
MPF, as we have noted, is an M-Cdk. That is, it is a cyclin-dependent protein kinase. As such, it catalyzes the transfer of a phosphate group from ATP to target proteins. Phosphorylation can activate the target protein.

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, as well as the many events in mitosis.



So far, we have discussed the phases of the cell cycle and the controls that regulate the passage of the cell from one phase to the other. Once the cell has successfully completed S-phase, when its chromosomes are duplicated, and passed through G2, it is ready to enter M-phase, when mitosis occurs. During mitosis, chromosomes condense, the nuclear envelope breaks down, the mitotic spindle is formed. Chromosomes attach to the spindle fibers and the sister chromatids move to opposite ends of the cell. Finally, the cell divides in two by the process known as cytokinesis.

We will now take a look at the stages of mitosis and how they are brought about, largely as a result of the action of a single entity, an M-Cdk (recall that MPF is the signal that sends non-dividing cells into mitosis and that it is an M-Cdk).

What are the main stages of mitosis?
The main stages of mitosis are:
Prophase-prometaphase: Chromosomes condense and centrosomes move to opposite ends of the cell. The mitotic spindle begins to form and the nuclear envelope breaks down.
Metaphase: Chromosomes align themselves along the center of the mitotic spindle
Anaphase: Sister chromatids separate and move to opposite ends of the spindle.
Telophase: Chromosomes decondense and nuclear envelopes are reassembled.
This completes the divisionof the nucleus.

Finally, the cytoplasm must be cleaved in two and two daughter cells created by the process of cytokinesis.

 See Panel 18.1





How are all these changes brought about by a single agent, ?
MPF, as you will remember, is a protein kinase. It exerts it effects by phosphorylating other proteins. We will consider below how each of the major events of mitosis is triggered by the phosphorylation of a key component. Other M-Cdks besides those in Xenopus work also in the same way, so we will, henceforth use the term M-Cdk in our discussions.






What is the role of M-Cdk in chromosome condensation?
Chromosome condensation, as discussed earlier in the term, is a process by which long strands of DNA are wound tightly together with proteins, to make compact structures that we see as the clearly defined chromosomes in a dividing cell. This condensation process is carried out with the help of protein complexes called condensins. Condensins are inactive until they are phosphorylated by M-Cdk. Once they are phosphorylated, they begin the process of condensation.






How is the nuclear envelope breakdown related to M-Cdk activity?
M-Cdk phosphorylates lamins, the proteins that make up the nuclear lamina. Phosphorylation of lamins leads to disassembly of the nuclear lamina, leading to breakdown of the nuclear envelope.






Does M-Cdk affect spindle assembly?
Yes, M-Cdk plays an important role in the formation of the mitotic spindle. As you know, the spindle is composed of microtubules, whose dynamic behavior is altered by M-Cdk. M-Cdk causes the phosphorylation of microtubule associated proteins. This leads to stabilization of the microtubules involved in the formation of the mitotic spindle.






What happens once the spindle is formed?
The spindle microtubules attach to the kinetochores of the chromosomes and begin the process of chromosome movement that ends with the alignment of the chromosomes at the metaphase plate at the center of the spindle.






What triggers the remaining stages of mitosis?
The passage of the cell from metaphase to anaphase is dependent on the breakdown of certain important regulatory proteins. These proteins are tagged for destruction by the addition of ubiquitin to them by an enzyme called E3 ubiquitin ligase, a.k.a. Anaphase promoting complex or APC.






If APC triggers the cell's passage into anaphase, what triggers APC?
All the factors involved in activating APC are not clear, although we know that the APC must be bound to another protein and that phosphorylation of the complex is needed. We also know that the activity of M-Cdk is necessary for the phosphorylation of the complex, but it is not clear whether the APC complex is directly phosphorylated by M-Cdk or by another kinase that is activated by M-Cdk.

Once activated, the anaphase promoting complex degrades specific proteins, whose loss sets the cell on the path to anaphase.






What proteins are degraded by the anaphase promoting complex?
The Anaphase promoting complex is responsible for the degradation of two important proteins:
Securin and the cyclin component of M-Cdk itself.






What is the effect of degrading Securin?
Degradation of securin releases an enzyme called separase. Separase is now free to break down proteins called cohesins that hold sister chromatids together (see Figure 18.29). When cohesins are degraded, the connection between the sister chromatids is broken, and they can then separate and move to opposite poles of the spindle.






What is the effect of degrading the cyclin component of M-Cdk?
As we noted earlier, M-Cdk is made up of a Cdk and a cyclin, both of which must be associated for M-Cdk to be active. If cyclin is destroyed, then the levels of active M-Cdk will fall. As M-Cdk levels fall, the cell begins to return to its interphase state. The inactivation of M-Cdk also triggers the next stage in cell division, which is cytokinesis.
See Figs. 18.5 and 18.11






What happens in cytokinesis?
During cytokinesis in animal cells, a contractile ring of actin and myosin filaments forms beneath the plasma membrane. This ring acts to pinch the cell in two by the contractile action of the actin-myosin filaments.

See Figure 18.33


Copyright © 2011 Indira Rajagopal