Cell Cycle

• Interphase is when the cell grows and the organelles double prior to the actual splitting of the nucleus
• 93% of a cell's life is spent in interphase
• Interphase has three parts:

  • Growth 1 (G1)
  • Synthesis (S)
  • Growth 2 (G2)
• G1 is when organelles double because each new cell needs a complete set of organelles
• S is when DNA is replicated because each cell needs a complete and identical set of DNA
• G2 is when proteins needed for Mitosis are produced
• Mitosis is the process by which the cell nucleus divides into two identical cell nuclei
• In some human cells, interphase lasts 15.3 hours, while mitosis lasts only 0.7 hours
• Mitosis occurs in a series of steps:

  • Prophase
  • Metaphase
  • Anaphase
  • Telophase
  • Cytokinesis
• Chromosomes must duplicate and separate during Mitosis
• Chromosomes are structures of tightly packaged DNA, tangled up into chromatin
• Prophase:

  • Chromosomes double to form short thick rods called chromatids
  • Chromatids pair up and line up in the center of the nucleus
  • A centromere connects the two halves of the doubled chromatids
  • Spindle fibers begin to form
  • Centrioles move to opposite sides of the cell
  • The nuclear membrane breaks down
• Metaphase:

  • Centromeres of the chromatid pairs line up in the middle of the cell
  • Metaphase plate is the location where the centromeres line up in the center of the cell
  • Each chromatid attaches to spindle fibers
• Anaphase:

  • Spindle fibers pull the chromatids apart, separating each one from its duplicate
  • Chromatids move to opposite sides of the cell, resulting in two identical sets of chromosomes
• Telophase:

  • Chromosomes reach opposite sides of the cell
  • Spindle fibers break up
  • The nuclear membrane begins to reform
  • A furrow develops between the two sets of chromosomes
• Cytokinesis:

  • The two identical cells completely divide and the cell membrane is completely formed
• Meiosis is the process of cell division in which gametes are formed and the number of chromosomes is halved for sexual reproduction and zygote formation
• Haploid cells have one of each kind of chromosome and are called gametes, which are either sperm or eggs
• Homologous chromosomes are paired chromosomes with genes for the same trait arranged in the same order
• Meiosis occurs in stages:

  • Interphase: chromosomes replicate
  • Prophase I: homologous chromosomes form tetrads and crossing over occurs
  • Metaphase I: homologous chromosomes line up together in pairs
  • Anaphase I: homologous chromosomes separate and move to opposite ends of the cell
  • Telophase I: spindle fibers break down, chromosomes uncoil, and cytoplasm divides
  • Meiosis II: similar to mitosis but chromosomes do not duplicate in interphase II
• Meiosis II:

  • Prophase II: chromosomes begin to line up in the middle of the cell
  • Metaphase II: chromosomes line up on the metaphase plate
  • Anaphase II: centromeres split, sister chromatids separate and move to opposite sides of the cell
  • Telophase II: nuclei reform, spindle fibers disappear, cytoplasm divides into two, reducing the number of chromosomes in each daughter cell by half