Cell Cycle and Cell Division

Cell Cycle

The cell cycle is the sequence of events that occur in a cell as it grows, replicates its DNA, and divides into two daughter cells.

Interphase

• Longest stage of the cell cycle
• Cell grows, replicates its DNA, and prepares for cell division
• Three stages:
  1. G1 phase (Gap 1): Cell grows, increases in size, and prepares for DNA replication
  2. S phase (Synthesis): DNA replication occurs, resulting in two identical copies of DNA
  3. G2 phase (Gap 2): Cell prepares for cell division, organelles and proteins are synthesized

Mitosis

• Process of cell division that results in two daughter cells with the same number of chromosomes as the parent cell
• Four stages:
  1. Prophase: Chromatin condenses into visible chromosomes, nuclear envelope breaks down
  2. Metaphase: Chromosomes line up at the center of the cell, attached to the spindle fibers
  3. Anaphase: Sister chromatids separate, moving to opposite poles of the cell
  4. Telophase: Nuclear envelope reforms, chromosomes uncoil, and nuclei reassemble

Cytokinesis

• Process of cytoplasmic division, resulting in two separate cells
• Occurs after mitosis, and can occur through two mechanisms:
  • Cleavage: In animal cells, the cell membrane pinches inward, dividing the cytoplasm
  • Cell plate formation: In plant cells, a cell plate forms in the center of the cell, dividing the cytoplasm

Meiosis

• Special type of cell division that occurs in reproductive cells (gametes)
• Results in four daughter cells with half the number of chromosomes as the parent cell
• Two successive cell divisions (meiosis I and meiosis II) occur, with a reduction in chromosome number after each division
• Meiosis I:
  • Prophase I: Homologous chromosomes pair up, exchange genetic material (crossing over)
  • Metaphase I: Paired chromosomes line up at the center of the cell
  • Anaphase I: Homologous chromosomes separate, moving to opposite poles
• Meiosis II:
  • Prophase II: Chromosomes condense, nuclear envelope breaks down
  • Metaphase II: Chromosomes line up at the center of the cell
  • Anaphase II: Sister chromatids separate, moving to opposite poles