Mitosis and DNA Replication

• Mitosis:


  • Maintains chromosome number and genome of cells.
  • Results in two genetically identical daughter cells.
  • Important for growth, repair, and asexual reproduction.
• In humans:

  • Mitosis produces genetically identical somatic cells with 46 chromosomes each.
• The Foundation of Cell Division

  DNA replication is a crucial prerequisite for both mitosis and meiosis. This process ensures that each daughter cell receives a complete and accurate copy of the genetic material.
• Key Points about DNA Replication:

  • Occurs during the S phase of the cell cycle, before mitosis or meiosis begins.
  • Results in each chromosome consisting of two identical DNA molecules called chromatids.
  • Chromatids are held together at the centromere until anaphase.
• The term "sister chromatids" refers to the two identical chromatids of a replicated chromosome.
• Phases of Mitosis


  Mitosis is a continuous process that is typically divided into four main phases for ease of study. Understanding these phases helps us appreciate how mitosis produces two genetically identical daughter cells.
• Overview of Mitotic Phases:

  1. Prophase
  2. Metaphase
  3. Anaphase
  4. Telophase
• Prophase

  • Chromosomes condense and become visible
  • Nuclear envelope begins to break down
  • Centrosomes move to opposite poles of the cell
  • Spindle fibers start to form
• Metaphase


  • Chromosomes align at the cell's equator (metaphase plate)
  • Spindle fibers attach to the kinetochores of chromosomes
• Anaphase

  • Sister chromatids separate and move to opposite poles of the cell
  • Cell elongates as the chromatids are pulled apart
• Telophase

  • Chromosomes decondense
  • Nuclear envelopes reform around each set of chromosomes
  • Spindle fibers disappear
  • Cytokinesis usually begins during this phase
• Outcome of Mitosis

  The result of mitosis is two genetically identical daughter cells, each containing the same number and type of chromosomes as the parent cell.
• Identification of Mitotic Phases

  Being able to identify mitotic phases in diagrams, microscope slides, or micrographs is an essential skill. Here are some key features to look for:

  1. Prophase: Visible, condensed chromosomes; nuclear envelope starting to break down
  2. Metaphase: Chromosomes aligned at the cell equator
  3. Anaphase: Sister chromatids separating and moving to opposite poles; cell elongating
  4. Telophase: Two distinct clusters of chromosomes at opposite poles; nuclear envelopes reforming
• Chromosome condensation in mitosis:

  1. DNA wraps around histones forming nucleosomes
  2. Nucleosomes coil into 30nm fiber
  3. Further coiling creates highly compact mitotic chromosomes
• Chromosome Movement:


  • Utilizes microtubules and microtubule motors
  • Ensures accurate distribution of chromosomes

  In mitosis, the spindle apparatus forms and attaches to kinetochores of sister chromatids.