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Understanding Meiosis and Its Stages

Mar 27, 2025

Meiosis Explained by Amoeba Sisters

Introduction

  • Meiosis is the process that allows siblings with the same parents to look different.
  • Different from mitosis, which creates identical body cells (e.g., skin cells).
  • Meiosis contributes to genetic variety by creating gametes (sperm and egg cells).

Chromosome Basics

  • Humans have 46 chromosomes in most body cells.
  • Sperm and egg cells (gametes) have 23 chromosomes.
  • When a sperm cell (23 chromosomes) and an egg cell (23 chromosomes) combine, they form a fertilized egg with 46 chromosomes.

Meiosis: A Reduction Division

  • Starts with a cell having 46 chromosomes; ends with gametes having 23 chromosomes.
  • The process begins after the interphase stage, where the cell grows, replicates its DNA, and carries out cell processes.
  • Chromosomes duplicate during interphase but remain counted by centromeres.
  • After replication, you still have 46 chromosomes, but now 92 chromatids.

Stages of Meiosis

Meiosis I

  1. Prophase I

    • Chromosomes condense, line up with homologous pairs.
    • Crossing over occurs: homologous chromosomes exchange genetic information, leading to recombinant chromosomes.

  2. Metaphase I

    • Chromosomes line up in the middle of the cell in pairs.

  3. Anaphase I

    • Chromosomes are pulled away by spindle fibers.

  4. Telophase I

    • Two newly formed nuclei.
    • Cytokinesis splits the cytoplasm, creating two new cells.

Meiosis II

  1. Prophase II

    • Similar to prophase I but no homologous pairs, no crossing over.

  2. Metaphase II

    • Chromosomes line up in single file in the middle.

  3. Anaphase II

    • Chromatids are pulled away to opposite sides.

  4. Telophase II

    • Nuclei reform, cells divide into four cells through cytokinesis.

Outcome of Meiosis

  • In males: produces four sperm cells, each genetically unique.
  • In females: produces egg cells, each genetically unique.
  • Ensures genetic variation, explaining why siblings can differ in appearance.

Importance of Meiosis Research

  • Nondisjunction occurs when chromosomes don't separate correctly during meiosis.
  • Can lead to genetic disorders, a key area of ongoing scientific research.

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