3.5 Cell Growth and Division

The cell cycle is a crucial process that governs cellular growth and division. It consists of interphase, where cells prepare for division, and mitosis, the actual division of the nucleus. Understanding these stages is key to grasping how cells reproduce and maintain genetic integrity.

Regulation of the cell cycle involves cyclins, cyclin-dependent kinases, and checkpoints that ensure proper progression. Dysregulation can lead to serious consequences like cancer, neurodegenerative disorders, and developmental issues. Mitosis and cytokinesis work together to produce two identical daughter cells.

Cell Cycle and Mitosis

Stages of cell cycle

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• Interphase prepares the cell for division consists of G1, S, and G2 phases
  • G1 phase involves cell growth, normal metabolic functions, and preparation for DNA synthesis (centrosome duplication)
  • S phase is when DNA replication occurs resulting in chromosome duplication
  • G2 phase includes continued cell growth and preparation for mitosis (protein synthesis)
• Mitosis is the division of the nucleus into two genetically identical nuclei has four stages: prophase, metaphase, anaphase, and telophase
  • Prophase condenses chromatin into visible chromosomes, breaks down the nuclear envelope, and forms spindle fibers (centrioles migrate to opposite poles)
  • Metaphase aligns chromosomes at the equatorial plate with kinetochore microtubules attaching to centromeres
  • Anaphase separates sister chromatids and moves them towards opposite poles as spindle fibers shorten and pull chromatids apart
  • Telophase decondenses chromosomes, re-forms the nuclear envelope, and disassembles spindle fibers (nucleolus reappears)
• Cytokinesis divides the cytoplasm to form two genetically identical daughter cells (cleavage furrow in animal cells, cell plate in plant cells)

Regulation of cell cycle

• Cyclins and cyclin-dependent kinases (CDKs) regulate cell cycle progression
  • Cyclins bind to and activate CDKs forming specific cyclin-CDK complexes that regulate each stage of the cell cycle (cyclin D-CDK4/6 for G1, cyclin E-CDK2 for G1/S, cyclin A-CDK2 for S, cyclin B-CDK1 for M)
  • Cyclin levels oscillate throughout the cell cycle while CDK levels remain constant
• Checkpoints ensure proper cell cycle progression and prevent errors
  • G1 checkpoint ensures cell size and nutrient availability are sufficient for division influenced by growth factors and extracellular signals (restriction point)
  • G2/M checkpoint verifies DNA replication is complete and accurate checking for DNA damage before proceeding to mitosis (DNA damage response)
  • Spindle assembly checkpoint (SAC) ensures proper attachment of chromosomes to spindle fibers preventing premature anaphase onset (mad2 and bubr1 proteins)
• Tumor suppressor genes regulate cell cycle and prevent uncontrolled division
  • p53 induces cell cycle arrest or apoptosis in response to DNA damage preventing propagation of mutations (guardian of the genome)
  • Retinoblastoma protein (pRb) regulates G1/S transition by binding and inhibiting E2F transcription factors phosphorylation of pRb by cyclin-CDK complexes releases E2F allowing cell cycle progression

Cell Growth Control and Differentiation

• Contact inhibition prevents cell division when cells come into contact with neighboring cells in a monolayer
• Density-dependent inhibition halts cell division when the cell population reaches a certain density
• Cell differentiation occurs as cells specialize into specific cell types with distinct functions
• Apoptosis is programmed cell death that eliminates damaged or unnecessary cells
• Cell cycle arrest can be induced by various factors to temporarily or permanently stop cell division

Cell cycle dysregulation consequences

• Cancer results from uncontrolled cell division due to mutations in cell cycle regulatory genes
  • Oncogenes like cyclin D and CDK4 promote excessive cell proliferation when overexpressed or constitutively active
  • Tumor suppressor gene inactivation of p53 or pRb removes cell cycle restraints leading to genomic instability and accumulation of mutations (hallmarks of cancer)
• Neurodegenerative disorders involve abnormal cell cycle re-entry in post-mitotic neurons
  • Leads to neuronal cell death and neurodegeneration associated with Alzheimer's disease (amyloid beta) and Parkinson's disease (alpha-synuclein)
• Developmental disorders can arise from abnormalities in cell cycle regulation during embryonic development
  • Can result in birth defects (holoprosencephaly), growth retardation, or embryonic lethality (trisomy 13)
• Aging is associated with cellular senescence, a permanent cell cycle arrest in response to stress or damage
  • Accumulation of senescent cells contributes to age-related tissue dysfunction and chronic inflammation (SASP)

Events of mitosis and cytokinesis

1. Prophase
   • Chromatin condenses into tightly coiled chromosomes visible under a light microscope
   • Centrosomes move to opposite poles of the cell forming the mitotic spindle
   • Nuclear envelope disassembles into vesicles releasing chromosomes into the cytoplasm
2. Prometaphase
   • Kinetochores form on centromeres of chromosomes serving as attachment sites for spindle microtubules
   • Spindle microtubules capture and attach to kinetochores in a search-and-capture mechanism
   • Chromosomes begin to move towards the equatorial plate in a process called congression
3. Metaphase
   • Chromosomes align at the equatorial plate forming the metaphase plate
   • Spindle fibers from opposite poles attach to sister chromatids creating tension
   • Cell cycle checkpoint (SAC) ensures proper chromosome alignment and spindle attachment
4. Anaphase
   • Cohesion proteins are degraded by separase allowing sister chromatids to separate
   • Sister chromatids are pulled towards opposite poles by shortening spindle fibers (anaphase A)
   • Poles of the cell move further apart driven by motor proteins on overlapping microtubules (anaphase B)
5. Telophase
   • Chromosomes arrive at the poles and decondense into loosely coiled chromatin
   • Nuclear envelope re-forms around each set of chromosomes using ER membrane vesicles
   • Spindle fibers disassemble as the cell prepares for cytokinesis
   • Cleavage furrow begins to form at the cell equator signaling the start of cytokinesis
6. Cytokinesis
   • Cleavage furrow deepens and contracts pinching the cell in two (animal cells)
   • Contractile ring of actin and myosin filaments drives furrow ingression
   • Cell membrane and cytoplasm divide forming two genetically identical daughter cells
   • Cell plate forms at the equator and expands outward to divide the cytoplasm (plant cells)