Cell Cycle Control and Regulation

Regulation of the Cell Cycle
- The sequential events of the cell cycle are precisely directed by a distinct cell cycle control system.
- This control system is regulated by both internal and external controls.
- The cell cycle "clock" features specific checkpoints where the cell cycle can halt until a go-ahead signal is received, ensuring proper progression.
Experimental Evidence of Cell Cycle Regulators (Figure 12.14)
- Cell Fusion Experiments demonstrated the presence of cytoplasmic chemical signals that regulate the cell cycle.
- Experiment 1: Fusion of $S$ phase and $G_1$ phase cells.
 - When a $G1$ nucleus was fused with an $S$ phase cell, the $G1$ nucleus immediately entered the $S$ phase, and its DNA was synthesized.
 - This indicates that molecules present in the $S$ phase cytoplasm trigger DNA replication in $G_1$ nuclei.
- Experiment 2: Fusion of $G_1$ phase and $M$ phase cells.
 - When a $G1$ nucleus was fused with an $M$ phase cell, the $G1$ nucleus immediately began mitosis without chromosome duplication.
 - This suggests that the $M$ phase cytoplasm contains factors that induce mitosis, even if DNA replication is incomplete or bypassed.
Key Cell Cycle Checkpoints (Figure 12.15)
- The cell cycle has crucial checkpoints that act as internal stop/go signals.
- $G1$ checkpoint: Located at the end of the $G1$ phase, before entry into $S$ phase.
- $G2$ checkpoint: Located at the end of the $G2$ phase, before entry into the $M$ phase.
- M checkpoint: Located during the metaphase of the $M$ phase.
The Cell Cycle Clock: Cyclins and Cyclin-Dependent Kinases (Cdks)
- Cell cycle control relies on two main types of regulatory proteins: cyclins and cyclin-dependent kinases (Cdks).
- Cyclins are named for their concentrations, which fluctuate cyclically throughout the cell cycle.
- Cyclin-Dependent Kinases (Cdks) are kinases that phosphorylate other proteins to activate or inactivate them, thereby regulating cell cycle progression.
  - The activity of a Cdk is entirely dependent on its attachment to a specific cyclin partner.
  - Cdk activity rises and falls in accordance with the fluctuating concentration of its cyclin partner.
MPF (Maturation-Promoting Factor)
- MPF is a specific cyclin-Cdk complex critical for regulating the transition into mitosis.
- It triggers a cell's passage past the $G_2$ checkpoint and into the $M$ phase (mitosis).
- Fluctuation of MPF activity (Figure 12.16a):
 - Cyclin concentration gradually increases during the $S$ and $G_2$ phases.
 - This accumulation leads to the formation of MPF, causing its activity to peak during the $M$ phase.
 - Immediately after the peak, cyclin is rapidly degraded, leading to a sharp drop in MPF activity.
- Molecular Mechanisms of MPF Regulation (Figure 12.16b):
 - Cyclin accumulates during the $G1$ , $S$ , and $G2$ phases.
 - Cyclin combines with Cdk to form the active MPF complex.
 - MPF acts as a kinase, phosphorylating various proteins that initiate mitosis. It can also indirectly activate other kinases.
 - At the end of the $M$ phase (specifically during anaphase), cyclin is degraded by proteasomes.
 - The degradation of cyclin inactivates MPF, leading to the termination of the $M$ phase.
 - The Cdk component is then recycled and can be used again with new cyclin molecules in the next cell cycle.
Internal and External Signals at Checkpoints
- Internal signals often come from cellular surveillance mechanisms monitoring the completeness of cell cycle events (e.g., DNA replication, chromosome alignment).
- External signals originate from outside the cell, such as growth factors or cell-density-dependent inhibition.
- The three most important checkpoints are those in the $G1$ , $G2$ , and $M$ phases.
The Critical $G_1$ Checkpoint (Figure 12.17a)
- For many cells, the $G_1$ checkpoint is considered the most important.
- Go-ahead signal received: If the cell receives a "go-ahead" signal at the $G1$ checkpoint, it will typically proceed to complete the $S$ , $G2$ , and $M$ phases and divide.
- No go-ahead signal received: If the cell does not receive the "go-ahead" signal, it will exit the cell cycle and switch into a non-dividing state called the $G0$ phase. Most mature human cells, like nerve and muscle cells, remain permanently in the $G0$ phase.
Events in Anaphase: Separase and Chromosome Movement
- During anaphase, the sister chromatids separate and move to opposite poles of the cell.
- The proteins called cohesins, which hold sister chromatids together, are cleaved by an enzyme called separase.
- Once separated, the individual chromosomes move along the kinetochore microtubules towards the spindle poles.
- The kinetochore microtubules shorten by depolymerizing (losing tubulin subunits) at their kinetochore ends.
The "Pac-man" Mechanism of Chromosome Movement (Figure 12.9 related content)
- Experimental evidence suggests that motor proteins located on the kinetochores play a crucial role in pulling the chromosomes.
- These motor proteins "walk" the chromosomes along the kinetochore microtubules, using ATP for energy.
- The depolymerization of the microtubules at the kinetochore ends occurs after the motor proteins have passed, essentially "eating" the microtubule from that end.
- This process is famously referred to as the "Pac-man" mechanism, where the kinetochore-associated motor proteins are like Pac-man, consuming the microtubule track.