Which Of The Following Molecules Triggers The Cell’s Passage Past The G2 Checkpoint Into Mitosis?

**What Triggers the Cell’s Passage Past the G2 Checkpoint Into Mitosis?**

The cell cycle is a complex process that regulates the growth and division of cells. It consists of several checkpoints, where the cell checks for DNA damage and determines if it’s ready to proceed to the next phase. One of these checkpoints is the G2 checkpoint, which ensures that the DNA is properly replicated before entering mitosis. The transition from the G2 phase to mitosis is crucial for cell division, and the trigger for this passage lies in the levels of certain molecules within the cell. So, which of the following molecules triggers the cell’s passage past the G2 checkpoint into mitosis?

The answer lies in the presence of cyclin-dependent kinases (CDKs) and cyclins. These molecules play a central role in regulating the cell cycle and are responsible for the transition from one phase to another. CDKs are enzymes that phosphorylate proteins, and cyclins are regulatory subunits that activate CDKs. Together, CDKs and cyclins form active complexes that phosphorylate target proteins involved in cell cycle progression.

**CDK Activation and the G2 Checkpoint**

At the G2 checkpoint, the cell assesses whether the DNA has been properly replicated and checks for any DNA damage. If the DNA is undamaged and replicated correctly, it triggers the activation of CDK1, also known as cyclin-dependent kinase 1 or Cdc2. CDK1 is regulated by its partner cyclin B, forming the CDK1-cyclin B complex. This complex is necessary for the initiation of mitosis.

The activation of CDK1-cyclin B is tightly controlled by several factors. The levels of cyclin B increase during the G2 phase, reaching a peak just before entering mitosis. The rise in cyclin B levels triggers the activation of CDK1, allowing it to phosphorylate target proteins and initiate mitosis.

**Role of CDK1-Cyclin B in Mitosis**

CDK1-cyclin B complex plays a crucial role in several events during mitosis. Once activated, CDK1 phosphorylates target proteins involved in the breakdown of the nuclear envelope, assembly of the mitotic spindle, and condensation of chromosomes. These phosphorylation events coordinate the sequential events of mitosis, ensuring the proper segregation of duplicated chromosomes into two daughter cells.

**Regulation of CDK1-Cyclin B Complex**

The activation of CDK1-cyclin B complex is tightly regulated by several mechanisms. One of the key regulatory mechanisms is the phosphorylation of CDK1 at specific residues. In the inactive state, CDK1 is phosphorylated at two inhibitory sites, which prevent its activation. These inhibitory phosphorylation sites are removed by a dual-specificity protein phosphatase called Cdc25, allowing CDK1 to become active.

Another regulatory mechanism involves the degradation of cyclin B. During mitosis, an enzyme called the Anaphase-Promoting Complex/Cyclosome (APC/C) targets cyclin B for degradation. This degradation ensures that CDK1 is inactivated, allowing the cell to exit mitosis and proceed to the next phase of the cell cycle.

**Frequently Asked Questions**

Q: Are there any other molecules involved in the transition from G2 to mitosis?

Yes, other molecules play a role in the transition from G2 to mitosis. Along with CDK1 and cyclin B, various other cyclins and CDKs are involved in different stages of the cell cycle. For example, CDK2 and cyclin E are involved in the transition from G1 to S phase, where DNA replication occurs.

Q: What happens if the cell fails to pass the G2 checkpoint?

If the cell fails to pass the G2 checkpoint, it will not proceed to mitosis. Instead, the cell may undergo repair mechanisms if there is DNA damage or enter a state of cell cycle arrest until the issues are resolved. This checkpoint ensures that the cell does not divide with damaged or incompletely replicated DNA.

Q: Are there any diseases associated with dysregulation of the G2 checkpoint?

Yes, dysregulation of the G2 checkpoint can lead to various diseases, including cancer. If the checkpoint fails to detect and repair DNA damage, the cell may enter mitosis with abnormal chromosomes, leading to genomic instability and potential oncogenic transformation. Therefore, a properly functioning G2 checkpoint is crucial for maintaining genomic integrity.

**Final Thoughts**

The G2 checkpoint serves as a critical barrier in the cell cycle, ensuring the proper replication and integrity of DNA before entering mitosis. The transition from the G2 phase to mitosis is triggered by the activation of CDK1-cyclin B complex, which phosphorylates target proteins involved in various events during mitosis. Understanding the molecular mechanisms that govern this transition is essential for deciphering the intricacies of cell cycle control and its implications in health and disease. By studying the molecules that trigger the cell’s passage past the G2 checkpoint, we gain valuable insights into the fundamental processes that govern cell division and proliferation.

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