Cell Cycle Checkpoints and DNA Replication Initiation

Cell Cycle Checkpoints

Introduction to Cell Cycle Checkpoints

Cell cycle checkpoints are surveillance mechanisms that monitor and regulate the progression of the cell cycle. They ensure that cells only proceed to the next phase when conditions are optimal and the genetic material is intact. These checkpoints are crucial for maintaining genomic stability and preventing the propagation of damaged DNA.

• G1-S Checkpoint: Checks for sufficient growth and ensures all DNA damage is repaired before DNA synthesis begins.

• G2-M Checkpoint: Ensures that DNA replication is complete and checks for DNA damage before mitosis.

• Metaphase (Spindle) Checkpoint: Verifies that all chromosomes are properly attached to the spindle apparatus before anaphase.

Key Point: If damage is detected and cannot be repaired, the cell may undergo apoptosis (programmed cell death) to prevent the propagation of mutations.

Example: The G1 checkpoint prevents cells with damaged DNA from entering S phase, thereby avoiding the replication of mutations.

Checkpoint Genes: Rb and p53

Role of Rb and p53 in Cell Cycle Regulation

The Rb (Retinoblastoma) and p53 genes are critical regulators of the cell cycle. They function as tumor suppressors, preventing uncontrolled cell division and tumorigenesis.

• Checkpoint Genes: Rb and p53 are both considered tumor suppressor genes.

• Mutations: Mutations in Rb and p53 are among the most common genetic alterations in human cancers. Often, both genes are found to be mutated in the same tumor.

• Tumor Suppressors: Genes that inhibit cell cycle progression. Loss of function leads to increased risk of tumor formation.

• Gain-of-Function Mutations: Mutant alleles that activate the cell cycle without proper regulation, such as constitutively active Ras.

Example: Loss of Rb function leads to retinoblastoma, a childhood eye cancer, while p53 mutations are found in over 50% of human cancers.

Checkpoints: p53 Pathway

p53 Response to DNA Damage

The p53 protein is a transcription factor that responds to DNA damage by halting the cell cycle and initiating DNA repair or apoptosis. It is often referred to as the "guardian of the genome."

• DNA Damage Detection: DNA damage leads to stabilization and activation of p53.

• Cell Fate Decisions:

  • If damage is within repair capacity: p53 induces cell cycle arrest, allowing time for DNA repair. If repair is successful, the cell reenters the cycle.

  • If damage is irreparable: p53 triggers apoptosis to eliminate the damaged cell.

• Downstream Targets: p53 regulates the expression of genes involved in cell cycle arrest (e.g., p21), DNA repair, and apoptosis.

Example: In response to UV-induced DNA damage, p53 activates the transcription of p21, which inhibits cyclin-dependent kinases (CDKs) and halts the cell cycle.

p53 and CDK Regulation

Mechanism of Cell Cycle Arrest by p53

p53 mediates cell cycle arrest primarily through the regulation of cyclin-dependent kinases (CDKs) and their inhibitors.

• CDK Inhibitors (CKIs): Proteins such as p21 and p27 are upregulated by p53 and inhibit CDK activity, preventing progression through the cell cycle.

• Phosphorylation Events: Kinases (e.g., CAK) and phosphatases (e.g., Cdc25) regulate the activation state of CDKs. DNA damage leads to the inhibition of Cdc25 by checkpoint kinases (Chk1/Chk2), further enforcing cell cycle arrest.

• Feedback Loops: The interplay between cyclins, CDKs, and their inhibitors ensures tight control over cell cycle transitions.

Example: Upon DNA damage, p53-induced p21 binds to and inhibits the Cyclin-CDK complex, blocking the transition from G1 to S phase.

Table: Major Cell Cycle Checkpoints and Their Functions

Checkpoint	Location in Cell Cycle	Main Function	Key Regulators
G1 Checkpoint	End of G1 phase	Checks for cell size, nutrients, growth factors, DNA damage	p53, Rb
G2 Checkpoint	End of G2 phase	Checks for DNA replication completion, DNA damage	p53, Cyclin B/CDK1
Metaphase (Spindle) Checkpoint	Metaphase of Mitosis	Ensures all chromosomes are attached to spindle fibers	Mad2, BubR1

Additional info: The notes also reference the importance of methylation and licensing factors in DNA replication initiation, which are covered in more detail in subsequent sections of the original material.