The entire cell cycle is controlled by a checkpoint system. A cell cycle checkpoint regulates critical processes and phase transitions that occur within a cell cycle.
What is the cell cycle checkpoint?
The entire cell cycle is controlled by a control system. A cell cycle checkpoint regulates critical processes and phase transitions that occur within a cell cycle. The sequence of physiological events in cells that have a nucleus is called the cell cycle. This occurs as a cycle that begins after one cell division and initiates the next. It consists of interphase and mitosis. In this process, a mother cell divides into two daughter cells, in which again the interphase begins. The gene activity present there regulates the metabolism of the growing cell, while a nucleolus develops in the cell nucleus. Interphase is the longer of the two and then changes to mitosis. It is again divided into different phases. These are G1 phase, in which the cell grows and chromosome doubling is prepared, S phase, in which chromosomes double, and G2 phase, in which the cell continues to grow and the next mitosis is prepared. This entire cycle is controlled by a molecular control system. Here, cell events are triggered and controlled, which mediate stop and continue signals in the form of checkpoints. A checkpoint thus analyzes critical processes and phase transitions that take place within the cell cycle. These serve to protect the integrity of the genetic material and ensure that the cell does not degenerate. For example, a critical process may be when chromosome segregation occurs in metaphase. Metaphase represents the second phase of cell division, known as mitosis and meiosis. In metaphase, the nucleolus and nuclear envelope regress. During this process, a typical structure is formed, called a monaster. Chromosomes differ significantly from each other during this phase.
Function and task
Cell cycle checkpoints are established in two phases. These are the interphase with G1 and G2 checkpoints, and the mitotic phase. During the first, increased nuclear activity takes place, and associated with this then is an increased risk of DNA damage by carcinogens, such as those induced by UV light. This in turn can lead to malignant tumors. Various toxins, drugs, environmental poisons and toxins can also cause diseases here. In interphase, specialized proteins are built up to counteract such defects, detect them and prevent the cell from changing to another phase at the checkpoint. Cell death is then induced by apoptosis. Figuratively, we can speak of a controlled suicide of the cell, which, in contrast to the death of the cell through e.g. mechanical injury, triggers an inflammatory reaction and does not release cytoplasm. The decision is made at this checkpoint whether the cell will divide or not. Most cells in the human body are in a state that the cell no longer divides. If there is no further signal at this checkpoint, the cell has left the cycle and is no longer dividing. It then switches to the G0 phase. Molecular control mechanisms take place during cell cycle control. In interphase, these are the formation of proteins 53 and 21 and BAX. Protein 53 is instrumental in controlling DNA integrity. It is also referred to as the “guardian” of the genome. In a biological process in which genetic information from a DNA strand is transferred to RNA, the protein acts as a transcription factor that upregulates DNA when damaged and causes the expression of tumor suppressor genes. Also essential to the vertebrate cell cycle is protein 21, a so-called CDK inhibitor, which blocks the cell at phase transitions to allow enzymes for DNA repair sufficient time to, for example, suppress the growth of cancer cells or repair various genetic defects. BAX, in turn, is a protein that acts as a co-factor of protein 53. It monitors the apoptosis of the cell. At the second cell cycle checkpoint, in mitosis phase, chromosome segregation then occurs in metaphase. This is always a critical moment because, for example, incomplete segregation leads to somatic numerical chromosomal aberrations. It is known that the human somatic cell has 46 chromosomes.This condition is called euploidy. When an abnormality forms, chromosomes can multiply. Then we speak of polyploidy. Human life is not possible under these conditions. Inasmuch as the number of chromosomes again does not correspond to the haploid set (n = 23), there is a defective separation of chromosomes or sister chromatids. A disease associated with this is trisomy 21. In the mitosis phase, the correct distribution of chromosomes between mother and daughter cells is ensured. Therefore, mitosis phase is a spindle checkpoint. This entails a spindle control mechanism based on the fact that chromosomes are not separated until there is correct attachment of microtubules to kinetochores. The exact sequence of events during the mitotic phase has not yet been precisely investigated. Physicians assume an interaction of the proteins with the kinetochore and the attached microtubules of the spindle apparatus.
Diseases and disorders
If the cell cycle checkpoints are disturbed, cancer cells may form, for example. The cancer cell is formed by transformation of a normal cell into an abnormal one. In the healthy immune system, a cell is recognized and destroyed. If this does not happen, a tumor forms. If the cell remains in its original place, it is called a benign tumor. This can be eliminated. Cells of a malignant tumor, on the other hand, are capable of damaging other organs and cells, can disrupt metabolism and form metastases. Unlike normal cells, cancer cells can divide infinitely and are therefore also difficult to treat.
