CELL CYCLE CHECK POINTS & REGULATION

 

CELL CYCLE CHECKPOINTS

 

Overview: Cell cycle checkpoints are critical control mechanisms that ensure the proper progression of the cell cycle. These checkpoints prevent cells from proceeding to the next phase until certain conditions are met, thereby safeguarding the integrity of the cell's genetic material and ensuring that cell division occurs accurately.

Main Phases of the Cell Cycle:

1. G1 Phase (Gap 1):
• Cell grows and prepares for DNA replication.
2. S Phase (Synthesis):
• DNA replication occurs.
3. G2 Phase (Gap 2):
• Cell prepares for mitosis.
4. M Phase (Mitosis):
• Cell divides into two daughter cells.
5. G0 Phase:
• A resting phase where cells may exit the cell cycle temporarily or permanently.

Key Cell Cycle Checkpoints:

1. G1 Checkpoint (Restriction Point):
• Location: Occurs at the end of the G1 phase before entry into the S phase.
• Function: This checkpoint assesses whether the cell has sufficient resources (nutrients, energy, and growth factors) and whether the DNA is undamaged. If conditions are favorable, the cell proceeds to DNA replication. If not, the cell may enter the G0 phase or undergo apoptosis (programmed cell death).
• Key Proteins: Cyclin D, Cyclin-dependent kinases (CDKs), Retinoblastoma protein (Rb), p53.
• Outcome: If DNA is damaged, p53 can activate repair mechanisms or induce apoptosis to prevent the proliferation of damaged cells.
2. S Checkpoint:
• Location: Occurs during the S phase.
• Function: This checkpoint monitors the progress of DNA replication, ensuring that the entire genome is accurately duplicated. It checks for incomplete replication or DNA damage and halts the cell cycle if errors are detected.
• Key Proteins: ATM/ATR kinases, Chk1/Chk2, BRCA1/BRCA2.
• Outcome: If DNA replication errors are found, the checkpoint activates repair mechanisms, ensuring that DNA replication is complete and accurate before proceeding to the G2 phase.
3. G2 Checkpoint:
• Location: Occurs at the end of the G2 phase before the cell enters mitosis (M phase).
• Function: This checkpoint ensures that DNA replication is complete and that the DNA is undamaged. It also checks for the correct replication of centrosomes, which are critical for spindle formation during mitosis. If all conditions are met, the cell proceeds to mitosis.
• Key Proteins: Cyclin B, CDK1 (Cdc2), p53, Wee1 kinase, Cdc25 phosphatase.
• Outcome: If errors are detected, the cell cycle is halted to allow for DNA repair or to trigger apoptosis.
4. M Checkpoint (Spindle Assembly Checkpoint):
• Location: Occurs during metaphase in the M phase.
• Function: This checkpoint ensures that all chromosomes are properly attached to the mitotic spindle and are correctly aligned at the metaphase plate before the cell proceeds to anaphase. Proper spindle attachment is crucial for the accurate segregation of chromosomes to daughter cells.
• Key Proteins: Mad2, BubR1, APC/C (Anaphase Promoting Complex/Cyclosome), Cdc20.
• Outcome: If chromosomes are not properly attached to the spindle, the checkpoint prevents the onset of anaphase, allowing time for proper attachment. If errors are corrected, the checkpoint is lifted, and the cell proceeds to complete mitosis.

Significance of Checkpoints:

• Prevention of Genomic Instability: Cell cycle checkpoints are essential for preventing the accumulation of genetic mutations, which could lead to cancer and other diseases.
• Maintenance of Genetic Integrity: By ensuring that DNA is accurately replicated and evenly distributed to daughter cells, checkpoints maintain the genetic integrity of the organism.
• Apoptosis: If damage is irreparable, checkpoints can initiate apoptosis, preventing damaged cells from proliferating.

Disruption of Checkpoints in Disease:

• Cancer: Mutations in checkpoint proteins, such as p53, can lead to uncontrolled cell division and tumor development. Many cancer cells have defects in one or more cell cycle checkpoints, allowing them to proliferate despite DNA damage or incomplete replication.
• Genetic Disorders: Defects in proteins involved in DNA repair and checkpoint regulation can lead to a range of genetic disorders, including predisposition to cancer and other diseases.

Conclusion:

Cell cycle checkpoints are vital for ensuring that each phase of the cell cycle is completed accurately and safely. By halting the cell cycle in response to DNA damage, incomplete replication, or improper spindle attachment, these checkpoints prevent the propagation of errors that could lead to cell death or disease. Understanding these checkpoints is crucial in the study of cancer biology and the development of therapies that target defective cell cycle regulation.

 

CELL CYCLE REGULATION

 

Overview: Cell cycle regulation involves a complex network of proteins and signaling pathways that ensure cells progress through the different phases of the cell cycle in a controlled and orderly manner. This regulation is crucial for normal growth, development, and maintenance of tissues, and its disruption can lead to diseases such as cancer.

Key Regulators of the Cell Cycle:

1.      Cyclins:

o    Cyclins are proteins that regulate the progression of cells through the cell cycle by activating cyclin-dependent kinases (CDKs). Cyclins are synthesized and degraded in a cyclical manner, with different cyclins being active at different phases of the cell cycle.

o    Types of Cyclins:

§  Cyclin D: Active in G1 phase, helps the cell transition from G1 to S phase.

§  Cyclin E: Active in late G1 phase and early S phase, crucial for the initiation of DNA replication.

§  Cyclin A: Active in S phase and G2 phase, involved in DNA replication and preparation for mitosis.

§  Cyclin B: Active in G2 phase and early M phase, essential for the progression of mitosis.

2.      Cyclin-Dependent Kinases (CDKs):

o    CDKs are enzymes that, when activated by binding to cyclins, phosphorylate target proteins to drive the cell cycle forward. The activity of CDKs is tightly regulated by cyclins, phosphorylation, and CDK inhibitors.

o    CDK Examples:

§  CDK4/6: Works with Cyclin D during the G1 phase.

§  CDK2: Pairs with Cyclin E in G1/S transition and with Cyclin A in S phase.

§  CDK1: Also known as Cdc2, pairs with Cyclin B to drive the cell into mitosis.

3.      CDK Inhibitors (CKIs):

o    CKIs are proteins that inhibit the activity of CDKs, thereby blocking the cell cycle progression. CKIs ensure that cells do not proceed to the next phase of the cell cycle if conditions are not favorable.

o    Types of CKIs:

§  INK4 Family (e.g., p16): Specifically inhibits CDK4/6, preventing the cell from transitioning from G1 to S phase.

§  CIP/KIP Family (e.g., p21, p27): Inhibits a broader range of CDKs, including CDK2 and CDK1, blocking progression from G1 to S phase or from G2 to M phase.

4.      Retinoblastoma Protein (Rb):

o    Rb is a tumor suppressor protein that controls the G1 checkpoint. In its hypophosphorylated state, Rb binds to the E2F family of transcription factors, preventing them from activating genes required for S phase. When Rb is phosphorylated by Cyclin D-CDK4/6 complexes, it releases E2F, allowing the cell to progress to the S phase.

5.      p53:

o    p53 is another tumor suppressor protein that plays a crucial role in responding to DNA damage. It can activate the transcription of p21, a CKI, which inhibits CDKs and halts the cell cycle at the G1 or G2 checkpoints, allowing time for DNA repair. If the damage is too severe, p53 can trigger apoptosis.

6.      Anaphase-Promoting Complex/Cyclosome (APC/C):

o    APC/C is a ubiquitin ligase that regulates the transition from metaphase to anaphase during mitosis by targeting specific proteins for degradation. This degradation ensures the proper separation of sister chromatids and the completion of mitosis.

Regulation Through Checkpoints:

1.      G1 Checkpoint (Restriction Point):

o    The G1 checkpoint checks for DNA damage and ensures that the cell has the necessary resources for DNA replication. If the cell passes this checkpoint, it is committed to completing the cell cycle.

2.      G2 Checkpoint:

o    The G2 checkpoint ensures that DNA replication is complete and that any damage has been repaired before the cell enters mitosis.

3.      M Checkpoint (Spindle Assembly Checkpoint):

o    The M checkpoint ensures that all chromosomes are properly attached to the spindle apparatus before anaphase begins. This prevents the unequal distribution of chromosomes to daughter cells.

Role of External Signals:

·         Growth Factors: External signals such as growth factors bind to cell surface receptors and activate signaling pathways (e.g., MAPK/ERK pathway) that lead to the expression of cyclins and other proteins involved in cell cycle progression.

·         Cellular Stress: Conditions such as DNA damage, nutrient deprivation, or oxidative stress activate pathways that halt the cell cycle. For example, DNA damage activates ATM/ATR kinases, which stabilize p53 and enhance the expression of p21, leading to cell cycle arrest.

Dysregulation and Disease:

·         Cancer: Dysregulation of cell cycle control is a hallmark of cancer. Mutations in genes encoding proteins like p53, Rb, Cyclins, CDKs, or CKIs can lead to uncontrolled cell division. For example, loss of p53 function removes a critical checkpoint, allowing cells with damaged DNA to proliferate.

·         Genetic Disorders: Mutations in genes regulating the cell cycle can lead to various genetic disorders, often characterized by developmental abnormalities or increased susceptibility to cancer.

Conclusion:

Cell cycle regulation is a highly coordinated process involving a network of cyclins, CDKs, CKIs, and other regulatory proteins. This system ensures that cells divide only when conditions are favorable and that any errors in DNA replication or chromosome segregation are corrected. Disruption of this regulation can lead to uncontrolled cell growth and cancer, highlighting the importance of understanding cell cycle control in the development of therapies for various diseases.