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The cell. 8. Cell cycle

G2 PHASE

The end of the S phase of the cell cycle leads to the G2 phase, which in turns leads to the M phase. During the G2 phase, there must be an increasing amount of molecules needed for M phase. The G2 phase has been traditionally regarded as a transitional phase between the S and M phases. However, during the G2 phase, it is checked if any errors occurred during DNA replication and if the DNA has been completely replicated. If something goes wrong during the S phase, the M phase does not start until the errors are repaired or the DNA is completely replicated. It is of great importance to detect errors before the M phase starts. Otherwise they will be inherited by daughter cells.

The round shape of cells during the M phase is obtained after the loss of cell adhesion, which begins to happen during the G2 phase. In addition, during the G2 phase, cells increase in size, and the two centrosomes duplicated during the S phase move to opposite perinuclear places to start polymerizing the mitotic spindle. The Golgi apparatus stacks are disorganized, and cisterns are free in the cytoplasm. The connection between Golgi stacks disappears too. It is interesting that the disorganization of the Golgi apparatus is required to enter the M phase. The Golgi cistern stacks split into two populations when centrosomes separate during the G2 phase, and each population is dragged by one centrosome. The Golgi cisterns seem to harbor molecules involved in the proper maturation of centrosomes before the formation of the mitotic spindle. Regarding the endoplasmic reticulum, the cisterns become tubules, the nuclear envelope is disorganized during prophase (M phase), and its membranes become part of these tubules. Endosomes and lysosomes are grouped around centrosomes at the end of the G2 phase, a process that continues during the mitotic prophase. It is suggested that lysosomes are membrane reservoirs because they are transported to the division groove that separates the two daughter cells during cytokinesis. Mitochondria are divided into small segments and scattered through the cytoplasm before cell division.

Centrosome and cell cycle
Centrosome cycle

The moment where G2 phase finishes and M phase starts is not clearly established, and some authors suggest that it is actually in the middle of the mitotic pro-phase. Anyway, the end of the G2 phase is mediated by the type I cyclin dependent kinase (CDK1) and by B1 cyclin. B1 cyclin is synthesized during the late S phase. This CDK1/B1 complex, helped by some kinases and phosphatases, triggers the starting of the M phase, so it is a checkpoint that controls the advance of the cell cycle.

Until recently, it was thought that the G1 phase was the only phase for restricting the progress of the cell cycle because it is where the response to mitogens takes place. However, now we know that there is a time window in the G2 phase that determines decisions that are taken during the G1 phase. For example, there is a dephosphorylation process in the G2 phase needed in the G1 phase for sensing mitogens and making a decision. Without this dephosphorylation, the cell proliferates even without mitogens during the G1 phase. If there is no dephosphorylation during the G2 phase, the cell is committed to proliferating, and the G1 phase is quite short. Mitogens during the G2 phase inhibit the dephosphorylation process.

S phase
Mitosis
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