The cell cycle refers to the whole process that a cell undergoes from the completion of one division to the end of the next division, and is divided into two stages: the interphase and the division.
(a) Interval
The interval is further divided into three phases, namely, the pre-DNA synthesis phase (G1 phase), the DNA synthesis phase (S phase), and the late DNA synthesis phase (G2 phase).
1. G1 (first gap) A period from mitosis to DNA replication, also known as pre-synthesis, which mainly synthesizes RNA and ribosomes. This period is characterized by active metabolism of substances, rapid synthesis of RNA and protein, and a significant increase in cell volume. The main significance of this period is to prepare the material and energy for DNA replication in the next phase S. After the cells enter the G1 phase, they will not continue to proliferate in the next phase without exception. At this time, there may be three different prospective cells: 1 proliferating cells: this cell can enter the S phase from the G1 phase in time, and Maintain a strong ability to split. For example, digestive tract epithelial cells and bone marrow cells; 2 temporarily non-proliferating cells or resting cells: such cells do not immediately transfer to the S phase after entering the G1 phase, and when needed, such as injury, surgery, etc., enter the S phase to continue to proliferate. For example, hepatocytes and renal tubular epithelial cells, etc.; 3 non-proliferating cells: After entering the G1 phase, such cells lose their ability to divide, remain in the G1 phase for life, and finally pass through differentiation, aging, and death. For example, highly differentiated nerve cells, muscle cells, and mature red blood cells.
2. S phase (synthesis) is the period of DNA synthesis. In this period, in addition to synthetic DNA, histones are also synthesized. The enzymes required for DNA replication are synthesized during this period.
3. The G2 phase (second gap) is the late stage of DNA synthesis and is the preparation period for mitosis. During this period, DNA synthesis was terminated, and a large amount of RNA and proteins were synthesized, including tubulin and pro-maturation factors.
(2) Splitting period
M phase: cell division.
Cell division: pre-, mid-, late, and terminal.
The mitosis of cells needs to be anterior, middle, posterior, and terminal, and is a continuous process of division, which is divided into two daughter cells by one mother cell. It usually takes 1 to 2 hours.
1. The prophase chromatin filament is highly helical and gradually forms a chromosome. The chromosome is short and thick and strongly basophilic. The two centrosomes move in opposite directions to form two poles in the cell; then the microtubules are synthesized starting from the central granule with the body as a starting point to form a spindle. As the nucleolus phase spirals with the chromatin, the nucleolus gradually disappears. The nuclear envelope begins to collapse into a discrete vesicular endoplasmic reticulum.
2. The metaphase cells become spherical, and the nucleolus and nuclear envelope have completely disappeared. The chromosomes are all moved to the equatorial plane of the cell, and the microtubules emanating from the poles of the spindle are attached to the centromere of each chromosome. From the metaphase cells, a complete chromosome group can be isolated, a total of 46, of which 44 are autosomes and 2 are sex chromosomes. The male karyotype is 44+XY and the female is 44+XX. The isolated chromosomes are short, thick or hairpin-like, and are composed of two chromatids linked by narrow centromere.
3. In the anaphase, due to the activity of the spindle microtubules, the centromere is longitudinally split, and the two chromatids of each chromosome are separated and moved in opposite directions, approaching the respective centrosomes, and the chromatids are divided into two groups. At the same time, the cell wave is elongated, and due to the activity of the microfilament bundle under the cell membrane of the equator, the part is narrowed and the cell is dumbbell-shaped.
4. At the end of the telophase, the chromatids gradually de-screwed, and the chromatin filaments and nucleoli reappeared. The endoplasmic reticulum vesicles were combined into a nuclear envelope; the cell equator was narrowed and deepened, and finally split into two diploid daughter cells.
Stage G0: A period in which cells that temporarily leave the cell cycle, stop cell division, and perform certain biological functions.
They can be divided into three categories according to their ability to divide in the body: 1 periodic cells, such as hematopoietic stem cells, epidermal and gastrointestinal mucosal epithelial stem cells. Such cells always maintain an active dividing ability and continuously enter the cell cycle; 2 terminally differentiated cells, such as mature red blood cells, nerve cells and other highly differentiated cells, which lose their ability to divide, also known as end cells; 3 temporarily do not proliferate cell population (G0 phase cells), such as liver cells, renal tubular epithelial cells, cardiomyocytes, thyroid follicular epithelial cells. They are differentiated cells that perform specific functions, and are usually in the G0 phase, so they are also called G0 phase cells. Under certain stimuli, these cells re-enter the cell cycle. After partial hepatectomy, the remaining hepatocytes divide rapidly.
Life is a continuous process that passes from one generation to the next, so it is a process of constantly updating and constantly starting from scratch. The life of a cell begins with the division of its mother cell, the formation of its daughter cells, or the death of the cell itself. The formation of daughter cells is usually a sign of the end of a cell division, which refers to the process that occurs from the time when a cell divides to form a daughter cell until the next cell divides to form a daughter cell. In this process, the genetic material of the cell is replicated and equally distributed to the two daughter cells.
Periodic action
At present, scientists have discovered that several types of regulatory factors play an important role in the cell cycle. One type is a cell growth factor that regulates cell division and proliferation. For example, the second type of cell cycle regulator, also known as endogenous regulator, is a self-synthesized protein in cells.
The prelude to the cell cycle regulation mechanism has been opened. Scientists are studying the relationship between cell cycle and oncogenes, tumor suppressor genes, growth factors, and cell proliferation and differentiation from different perspectives. I believe that through hard work, we can finally find the magic to control the cell cycle. "switch".
In the treatment of cancer, we can also use the principle of cell cycle to prescribe the right medicine. If G0 cells are not sensitive to chemotherapy, they often become the root cause of cancer recurrence in the future. Therefore, it can be tried to induce G0 phase cancer cells to enter the proliferative cycle and then kill. This is a question of theoretical and practical significance in the exploration.
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