Cell Cycle/Checkpoint


Cell Cycle


The cell cycle is the cycle by which a dividing eukaryotic cell grows from the end of one mitosis to the end of the next (specifically, there is a cell cycle with or without mitosis as long as there is DNA replication. Germ cells have no cell cycle). The length of the cell cycle reflects the state of the cell, which is a cycle of cell material accumulation and cell division. Cancerous cells and embryonic cells at certain stages often have abnormal cycles of division.



Cell life cycle


Cell Cycle Division


Generally speaking, the cell cycle can be divided into the interphase (phase I) and the division phase (phase M), with the interphase being the phase of material preparation and accumulation and the division phase being the implementation process of cell proliferation. The whole cycle is denoted as phase I →M.


The interphase (I) can be divided into the early phase (G1, GAP 1), the late phase (G2, GAP2) and the early phase (S, synthesis) of DNA synthesis. the main task during this period is to complete DNA replication in chromatin and the synthesis of related proteins. After a subdividing phase I, the whole cell cycle can be expressed as: G1 →S →G2 →M phase.


Cells entering the G1 phase may appear in three situations, among which they do not continue to proliferate temporarily, such as bone marrow stem cells and cancer cells in an unfavorable state, but under certain stimuli, these cells can continue to grow and divide, so some people call this non-proliferative state of G1 cells as G0 cells. To distinguish proliferating G1 cells.


The division phase is usually divided into five stages: Prophase, Prometaphase, Metaphase, Anaphase and Telophase, during which the cell material is evenly distributed and two new cells are formed.







G Phase 0

G 0

The stage in which a cell leaves the cycle and stops dividing.


G Phase 1

G 1

Cell growth in the G1 phase. The G1 checkpoint control mechanism ensures that everything is ready for DNA synthesis.



DNA replication occurs at this stage.

G Phase 2

G 2

During the gap between DNA synthesis and mitosis, cells will continue to grow. The G2 checkpoint control mechanism ensures that everything is ready to enter the M (mitosis) stage and divide.

Cell Division



Cell growth stops at this stage and cellular energy is focused on orderly dividing into two daughter cells. The Metaphase Checkpoint ensures that cells can complete cell division.


After cell division, each daughter cell begins the interphase of a new cycle. Although the stages of interphase are usually morphologically indistinguishable, each stage of the cell cycle has a distinct set of specialized biochemical processes that prepare for the initiation of cell division.



Cell cycle division: G1 phase →S phase →G2 phase →M phase, and G0 phase.


Regulation of the Eukaryotic Cell Cycle


The cell cycle is regulated by different cyclins. According to the current cognition, just like the progress of G1 phase →S phase →G2 phase →M phase of the cell cycle, cyclin D (a large number of expressions in the G1 phase) is gradually replaced by cyclin E(A), and the subsequent change is E→A→B.


Role of Cyclin and CDK


Two key types of regulatory molecules, cyclins and cyclin-dependent kinases (CDKs), determine the progression of cells through the cell cycle. The 2001 Nobel Prize in Physiology and Medicine went to Leland H. Hartwell, R. Timothy (Tim) Hunt and Paul M. Nurse, In recognition of their years of research and discovery of key factors and regulatory mechanisms of the cell cycle, promoting the world's understanding of cell cycle, and then opening the research direction of the generation of cancer and the regulation of abnormal cell cycle. Many of the gene-encoding cyclins and CDK are conserved in all eukaryotes, but generally more complex organisms have more elaborate cell-cycle control systems that contain more individual components. Many related genes were first identified by studying yeast, especially Saccharomyces cerevisiae. The genetic nomenclature in yeast follows many of these gene CDC (for "cell division cycle") with an identification number, CDC25 or CDC20.


Cell Cycle Checkpoint


Cell cycle checkpoints are used by cells to monitor and regulate the progression of the cell cycle. Whether cyclin or checkpoint function is abnormal, the cells often cannot smoothly carry out the division process in the M phase and go to apoptosis, only a very few cells can escape this disaster, and finally evolve into cancer cells.

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