The Cell Cycle

• The cell cycle is the regulated sequences of events that occur between one cell division and the next
• The length of cell cycles varies depending on environmental conditions, cell type, and the organism
• Mitosis is significant for asexual reproduction, growth, and tissue repair
• The cell cycle consists of 3 main phases: interphase, mitosis, and cytokinesis
• Interphase consists of 3 phases:

  • G₁ phase:
  • Cell grows physically larger and prepares for division
  • Organelles duplicate
  • Cells may enter G₀, undergo differentiation, apoptosis, or senescence
  • S phase:
  • DNA is copied and replicated
  • G₂ phase:
  • Genetic material is organized and condensed, preparing for division
• Mitosis has 4 main stages:

  • Prophase:
  • Chromosomes condense into sister chromatids
  • Centrosomes move to opposite poles
  • Spindle fibers emerge
  • Metaphase:
  • Chromosomes line up at the equator of the spindle
  • Spindle fibers attach to centromeres
  • Anaphase:
  • Sister chromatids separate at the centromere
  • Chromosomes are pulled to opposite poles
  • Telophase:
  • Chromosomes decondense
  • Nuclear envelopes reform
  • Spindle fibers break down
• Cytokinesis divides the whole cell into two genetically identical daughter cells
• The movement between phases is triggered by chemical signals called cyclins
• Regulation of the cell cycle includes cell-cycle checkpoints like G₁/S checkpoint and G₂/M checkpoint
• Meiosis is a form of nuclear division that produces 4 haploid daughter cells for sexual reproduction
• Meiosis involves two divisions: meiosis I and meiosis II
• Meiosis I:

  • Prophase I:
  • DNA condenses into visible chromosomes
  • Homologous pairs align, allowing crossing over
  • Metaphase I:
  • Bivalents line up along the equator
  • Independent assortment occurs
  • Anaphase I:
  • Homologous pairs separate
  • Telophase I:
  • Chromosomes arrive at opposite poles
  • Cytokinesis:
  • Division of cytoplasm occurs
• Meiosis II:

  • No interphase between meiosis I and meiosis II
  • Prophase II:
  • Nuclear envelope breaks down and chromosomes condense
  • Metaphase II:
  • Chromosomes line up along the equator
  • Anaphase II:
  • Chromatids are pulled to opposite poles
  • Telophase II:
  • Nuclear membranes form around each group of chromosomes
  • Cytokinesis:
  • Cytoplasm divides to create four haploid cells
• Meiosis produces genetic variation through crossing over, independent assortment, and the production of haploid gametes