G2/MI transition of meiotic cell cycle

Definition

Target type: biologicalprocess

The cell cycle process in which a cell progresses from meiotic G2 phase to M phase of meiosis I. [PMID:15084480]

The G2/M transition in the meiotic cell cycle represents a critical checkpoint that ensures proper chromosome duplication and alignment before the cell commits to division. This transition is orchestrated by a complex interplay of regulatory proteins, particularly cyclin-dependent kinases (CDKs) and their regulatory subunits, cyclins.

**1. Activation of CDK1:**
The key player in the G2/M transition is CDK1, also known as the maturation promoting factor (MPF). It remains inactive throughout G2 phase but becomes activated in response to rising levels of cyclin B. The binding of cyclin B to CDK1 forms the active MPF complex.

**2. Phosphorylation of Key Substrates:**
Activated MPF initiates a cascade of phosphorylation events targeting specific proteins involved in chromosome condensation, nuclear envelope breakdown, spindle assembly, and activation of other downstream signaling pathways.

**3. Chromosome Condensation:**
MPF-mediated phosphorylation of condensin proteins, a complex of proteins responsible for packaging DNA into compact chromosomes, triggers the condensation of replicated chromosomes. This allows for efficient segregation during meiosis.

**4. Nuclear Envelope Breakdown (NEBD):**
MPF also phosphorylates lamins, proteins that make up the nuclear lamina, causing the nuclear envelope to fragment. This allows the chromosomes to attach to the mitotic spindle.

**5. Spindle Assembly:**
MPF phosphorylation of various spindle components, such as microtubule-associated proteins, promotes microtubule polymerization and spindle formation. The spindle apparatus, composed of microtubules, will eventually pull sister chromatids apart during meiosis.

**6. Checkpoint Regulation:**
The G2/M transition is tightly regulated by a checkpoint mechanism that ensures proper chromosome duplication and alignment. If errors are detected, the checkpoint machinery halts the cell cycle and activates repair pathways.

**7. Meiosis-Specific Features:**
Meiosis is characterized by two rounds of division, producing four haploid gametes from a single diploid cell. During the first meiotic division (Meiosis I), homologous chromosomes pair up and exchange genetic material through a process called crossing over. This genetic recombination increases genetic diversity. MPF activity plays a key role in regulating this complex process.

**8. Entry into Meiosis I:**
The G2/M transition initiates the first meiotic division, leading to the separation of homologous chromosomes. The transition into meiosis II, the second division, is regulated by a similar mechanism involving a second wave of MPF activation.

**9. Exit from Meiosis:**
After the completion of meiosis II, MPF activity is downregulated, allowing the cell to exit meiosis and enter a quiescent state until fertilization occurs.'
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