Overview

Your body needs to do two things: grow/repair itself, and reproduce. It uses two different tools for this.

  • Mitosis: The photocopier. It makes exact clones. Used for skin, blood, bone.
  • Meiosis: The shuffler. It mixes genes to make something new. Used for sperm and eggs.

Core Idea

The core idea is Ploidy (the number of chromosome sets).

  • Mitosis: Maintains ploidy (Diploid -> Diploid). 2n -> 2n.
  • Meiosis: Reduces ploidy (Diploid -> Haploid). 2n -> 1n. This is crucial because when sperm (1n) meets egg (1n), they make a baby (2n). If meiosis didn’t halve the number, the baby would have 4n, then 8n, and explode.

Formal Definition

  • Mitosis: Asexual reproduction. One division. Result: 2 genetically identical daughter cells.
  • Meiosis: Sexual reproduction. Two divisions. Result: 4 genetically unique daughter cells (gametes).

Intuition

  • Mitosis: You have a document. You copy it. Now you have two identical documents.
  • Meiosis: You have two decks of cards (Mom’s deck and Dad’s deck). You shuffle them together, then deal out four hands of 23 cards each. Every hand is unique.

Examples

  • Healing a cut: Mitosis. Your skin cells divide to fill the gap. They must be identical to the old skin cells.
  • Making a baby: Meiosis. This is why you don’t look exactly like your brother. You both came from the same parents, but you got a different “shuffle” of their genes.
  • Cancer: Mitosis gone wrong. Uncontrolled cell division. The “stop” button is broken.

Common Misconceptions

  • They are the same: They look similar under a microscope (chromosomes line up and pull apart), but the outcome is opposite (Cloning vs. Variation).
  • Meiosis happens everywhere: No, only in the gonads (testes/ovaries). Every other cell in your body does mitosis.
  • Crossing Over: The magic moment in Meiosis I where Mom’s chromosomes and Dad’s chromosomes hug and swap pieces of DNA. This creates infinite genetic variety.
  • Interphase: The boring part before division where the cell copies its DNA. Both processes start here.

Applications

  • IVF: Doctors look at cells dividing (mitosis) in a petri dish to choose the healthiest embryo.
  • Down Syndrome: Caused by an error in Meiosis (Nondisjunction). A chromosome fails to separate, so the egg gets two copies instead of one.

Criticism / Limitations

  • Cost of Sex: Meiosis is expensive. You throw away half your genes. Why do it? Because the genetic variation helps the species survive plagues and changes. Mitosis is efficient, but risky (if one dies, all die).

Further Reading

  • Alberts, Bruce. Molecular Biology of the Cell. (The standard textbook).
  • Ridley, Matt. The Red Queen: Sex and the Evolution of Human Nature. 1993. (Why sex/meiosis exists).