Medicine & USMLE


Cell Division
  1. Mitosis
  2. Meiosis


Mitosis is the process by which somatic cells divide and reproduce. Through mitosis, a single somatic cell splits apart into two identical daughter cells. Notably, the genetic material starts in a duplicated diploid form and ends in a diploid but non-duplicated form.

Importantly, mitosis occurs through several successive phases.

The first phase of mitosis is prophase. During prophase, chromosomes condense, the mitotic spindle forms, and the nuclear envelope dissolves.

The second phase is metaphase, where the chromosomes align in a single file at the middle of the cell.

The third phase is anaphase, where sister chromatids split apart and move towards the right and left sides of the cell.

Finally, the fourth and last phase of mitosis is telophase. During telophase, the chromosomes nuclear envelope forms and cytokinesis occurs to end mitosis by splitting apart the two daughter cells!

Key Points

  • Mitosis
    • Asexual cell division by somatic cells 
      • Contrast vs. Meiosis (occurs in germ cells)
      • Mutations not sexually heritable in offspring
        • Heritable to daughter cells and their descendents only
    • Start: 1 diploid cell
    • End: 2 diploid cells
      • Both daughter cells are identical
    • Phases
      • Prophase
        • Chromosomes condense
          • At this point the chromosomes are bivalent, meaning each consists of two sister chromatids
          • A chromosome is defined by a centromere
        • Mitotic spindle forms
          • Made of microtubules from centrosomes, which migrate to opposite poles
          • Spindle fibers attach to chromosomes
        • Nuclear membrane dissolves 
      • Metaphase
        • Chromosomes align at metaphase plate
          • Metaphase plate (equatorial plate) is a plane at midpoint of cell
          • Aligned by spindle apparatus
      • Anaphase
        • Separation of sister chromatids
          • Centromeres split so that each chromatid has its own, separated sisters are pulled by spindle to opposite poles of cell
            • Note: Because a chromosome is defined by a centromere, this is where chromosome number doubles
            • Note that centromere splitting DOES NOT happen in Meiosis I (because that’s homologous pairs)
          • Nondisjunction can occur
            • Either sister chromatids or homologous chromosomes fail to separate
      • Telophase
        • Nuclear envelope reforms
        • Cytokinesis
          • Cell splits into two daughter cells, cytoplasm and organelles are separated between daughter cells
        • Chromosomes decondense
        • Spindle disappears