2. The cell cycle

• The cell cycle is a complex system of coordinated processes that must occur in a specific sequence
• Regulatory proteins and biochemical switches control progression through the cell cycle
• Regulatory proteins act as molecular switches, allowing cells to progress through the cell cycle or pause for repair or preparation for the next phase
• Cyclin-Dependent Kinases (CDKs) are key regulators of the cell cycle
• CDKs' activity is dependent on the presence of cyclin proteins
• There are four major classes of cyclins:

  1. G1/S Cyclins: Crucial for the transition from the G1 phase to the S phase

  2. S-Cyclins: Essential for DNA replication during the S phase

  3. M-Cyclins: Promote events associated with mitosis

  4. G1-Cyclins: Promote passage through the restriction point in late G1 phase
• CDKs are protein kinases that add phosphate groups to proteins, altering their activity
• Protein phosphorylation can activate or deactivate target proteins
• Protein phosphatase removes phosphate groups, reversing phosphorylation effects
• Balance between kinase and phosphatase activity is crucial for cellular processes
• CDK activity is regulated through protein degradation
• Ligases are enzymes responsible for degrading cyclins, leading to decreased activity
• Degradation of cyclins by ligases causes cyclical changes in their activity levels
• Activation of CDKs involves a two-step process:

  1. Cyclin binding causes a conformational change in CDK

  2. Phosphorylation by CDK Activating Kinase (CAK) makes CDK fully active
• Inhibition of CDKs involves:

  1. Addition of an inhibitory phosphate by a kinase

  2. Removal of the inhibitory phosphate by protein phosphatase
• Ubiquitin ligases control the levels of cyclins and CDKs by marking them for destruction in the proteasome
• SCF ligase leads to the destruction of G1/S cyclins and CKI

  • Phosphorylation of CKI is crucial for recognition by SCF ligase
  • Ubiquitination and proteasomal degradation follow, reducing CKI levels
• APC ligase leads to the destruction of securin and M-cyclin

  • APC requires specific activating subunits like Cdc20
  • APC targets M-cyclin for destruction, essential for exiting mitosis
• SCF keeps G1/S cyclins low by degrading them, allowing cell progression into the S phase

  • SCF also marks CKIs for destruction, preventing inhibition of S CDKs
• APC is active during Anaphase, Interphase, and Mitosis, targeting specific cell cycle proteins for degradation
• Activation of M-Cdk triggers mitosis:

  • M-Cdk binds to CDK1 in late G2 phase
  • Wee1 phosphorylates M-Cdk, placing it in an inactive state
  • Cdc25 removes the inhibitory phosphate, activating M-Cdk and triggering entry into mitosis
• Active M-CDK inhibits Wee1, preventing further inhibition of M-CDK
• Basal Activation by SCdk contributes to the activation of Cdc25 during M phase
• SCdk provides a basal level of Cdc25 activation even before the critical buildup of M-CDK, priming the cell for entry into mitosis
• DNA Replication Checkpoint (End of G2 Phase) occurs before entering mitosis to ensure accurate and complete DNA replication
• The Spindle Attachment Checkpoint in metaphase monitors correct attachment of chromosomes to spindle fibers to prevent unequal distribution of genetic material
• DNA Damage Checkpoint (End of G1 Phase) halts progression into S phase if DNA damage is detected, allowing time for repair mechanisms to fix lesions
• Activation of APC involves CDC20 and MCDK, leading to the degradation of Securin and the release of Separase
• Separase cleaves cohesion complexes, allowing sister chromatids to separate during chromatid separation
• Kinetochore is a structure at the centromere where microtubules attach during cell division
• Mad2 protein binds to unattached kinetochores, inhibiting the CDC20-APC complex to prevent premature anaphase
• DNA damage triggers a signaling cascade, leading to the activation of p53 and subsequent inhibition of CDKs by p21
• Total cell mass is determined by cell growth, while total cell number is determined by cell division minus cell death
• Growth factors increase protein synthesis and decrease degradation, favoring cell growth
• Mitogens stimulate cell division and regulate cyclin levels for proper cell cycle progression
• Mitogens are essential for CDK activation and promote entry into the S phase from G1 phase
• The G1 restriction point requires an extracellular signal, typically a mitogen, for cell cycle progression
• Cells in the absence of mitogens enter a non-dividing and terminally differentiated state known as G0
• Mitogens signal cells to re-enter the cell cycle from G0 and progress through G1 phase
• Examples of mitogens include Platelet-Derived Growth Factor (PDGF)