When eukaryotic cells divide, they undergo a complex process with four main stages, including a G2 phase. The cell cycle includes steps such as cell growth, DNA replication and mitosis (a critical topic in cell biology). Because eukaryotic cells have a nucleus that also has to be duplicated, the overall process is more complicated than the binary fission used by prokaryotic cells, which lack a nucleus.
The mitosis phase is the final step in cell division. It results in two new daughter cells, each with a full complement of DNA, a nucleus and organelles. If the cell is to stop dividing, it exits the cell cycle and enters the G0 phase.
If the cell is to divide again, it enters the interphase between two cell divisions. The three parts of the interphase are the G1 phase (or Gap 1 phase) followed by the S phase (or protein and DNA synthesis phase) and finally the G2 phase (or Gap 2 phase) preceding the next mitosis phase.
When Do Cells Enter the Different Phases?
Cell division through mitosis is an asexual form of cell multiplication that is used to produce more of the same kind of cell. Higher animal cells use mitosis to produce new cells include cells that wear out quickly such as skin cells. The process is also used during tissue growth such as in young animals or to repair damage.
In some tissues, once an organism has the required number of cells of a particular type, no new cells are needed, and the existing cells enter the G0 phase where they no longer multiply. This is especially true of highly differentiated cells such as nerve cells. Once the brain or the spinal cord has the right number of cells, the nerve cells don't divide to produce more.
If the cell has to divide again, it enters into the following phases:
The Steps of the Cell Cycle
1. The G1 Gap Phase
This is the gap between cell division and DNA replication. The cell gets ready for its next division in the cell cycle or it exits the cell cycle and enters G0.
2. The S Synthesis Phase
The cell is committed to starting the next cell division and makes copies of its DNA while synthesizing additional proteins required for cell division.
3. The G2 Gap Phase
This is the gap between DNA replication and mitosis. The cell reproduces its organelles and makes sure everything is ready for the split.
Entry into the G2 Phase
After cell growth during the G1 phase and DNA replication during the S phase, the cell is ready to enter the G2 phase. G2 is called a gap phase because no further cell division-specific progress takes place. Instead there are high levels of preparation and checking to make sure everything is in place for a successful mitosis.
Before the G2 phase can start, each chromosome of the cell must have been duplicated, and the proteins required for the extra cell membranes and cell structures must be present. At the beginning of G2, the organelles such as the mitochondria and the lysosomes start multiplying. These organelles have their own DNA and can start dividing independently, but the cell itself has to create extra ribosomes to satisfy the needs of the prospective two daughter cells.
What Happens in the G2 Phase?
The G2 phase has two major functions. First, the cell has to check that everything is ready for mitosis, and it has to correct any deficiencies. If the cell detects major problems that can't be fixed immediately, it may interrupt the cell cycle and stop the process of division. The G2 phase is where the organism makes sure that any new cells are not defective.
Checks that the cell undertakes include verifying that the DNA has been replicated correctly and that there is enough material present for two cells. The strands of DNA have to be complete, without any breaks, and there have to be the correct number of twice the strands of the original cell. If the cell finds a break, the DNA strand is repaired.
The two new cells have to be enclosed by complete membranes, and they each have to receive enough cell material to function properly. During the G2 phase, extra protein is often synthesized, and the organelles multiply until there are enough for two cells. Other cell materials such as lipids for the membrane may also be produced. With all this activity, the cell often grows substantially during G2.
The G2/M Phase Checkpoint
Advanced organisms such as vertebrates have specialized and differentiated cells that coordinate their activity and rely on each other for many functions. As a result, these organisms are very sensitive to cell breakdown and defective cells. To avoid creating cells that don't work properly, many animals have a cell division checkpoint late in the G2 phase. The cell has verified a lot of key factors, and the results are reviewed at the checkpoint.
If the cell found some problems and was able to fix them, it will pass the checkpoint, and cell division will be allowed to proceed. If problems persist, the cell will not divide and will try to fix the problems before continuing the cell division process. Specific assessments carried out at the checkpoint include:
- DNA damage: Specific proteins accumulate at the sites of broken DNA. If these proteins are present, the cell will not divide.
- DNA replication: The cell aborts the division process if not all DNA strands have been completely duplicated.
- Cell condition assessment: Cell proteins, organelles and other structures have to be in place in sufficient quantities.
- Cell stress: If the cell is under stress, cell growth will stop. For example, UV light can stress cells and result in a G2/M phase checkpoint activation, stopping the cell cycle.
Leaving the G2 Phase
Once the G2 checkpoint has been passed, the cell can prepare for mitosis. The first stage of mitosis is the prophase, during which the preparations for the migration of the chromosomes to opposite ends of the cell takes place. As the cell leaves the G2 phase, proteins that promote the mitosis functions are released.
The cell starts the process of division.
Key functions carried out as the cell leaves G2 are initiated by a protein complex called MPF or the mitosis-promoting factor. Once the first mitosis functions are underway, MPF is neutralized. At this point, the spindles for mitosis have started to form, and the nuclear envelope has started to degrade. The duplicated DNA is in the form of chromatin, and it condenses to form the new chromosomes.
While the G2 phase is an important factor in cell growth control for advanced organisms, it is not essential for cell division. Some primitive eukaryotic cells and some cancer cells can go directly from the S phase of DNA replication to mitosis.
The absence of the G2 phase eliminates a checkpoint that can be used to control tissue growth and helps some cancers spread rapidly.
Normal cells in the tissues of advanced animals need the G2 phase and its checkpoint to ensure that all cells of the organism and its tissues grow in a coordinated way. When a cell leaves the G2 phase and has successfully passed the corresponding checkpoint, a successful cell division with two functional daughter cells becomes much more likely.