Meiosis

Meiosis

Meiosis, also called reduction division, is a type of cell division that produces gametes—sex cells such as sperm and eggs. It begins with a diploid cell and occurs in two successive divisions: meiosis I and meiosis II. The result is four genetically different haploid cells.

Meiosis I

• Prophase I
  • The cell’s DNA replicates and condenses into chromosomes, each made of two sister chromatids.
  • Homologous chromosomes—one inherited from each parent—pair up in a process called synapsis.
  • Each paired set, containing four chromatids, is called a tetrad.
  • During crossing over, chromatids from homologous chromosomes exchange segments containing alleles. This recombination creates new gene combinations and contributes to genetic variation among gametes and offspring.
  • The nuclear membrane disappears, centrioles move to opposite ends of the cell, and spindle fibers form.
• Metaphase I
  • Homologous chromosome pairs line up at the cell’s equator and attach to spindle fibers from opposite poles.
• Anaphase I
  • Homologous chromosomes separate and move to opposite poles. Sister chromatids remain joined.
• Telophase I and cytokinesis
  • Nuclear membranes reform and the cell divides.
  • Meiosis I produces two genetically different haploid cells. Each chromosome still consists of paired sister chromatids, which may no longer be identical because of crossing over.

Meiosis II

DNA does not replicate before meiosis II.

• Prophase II: Nuclear membranes disappear and spindle fibers form.
• Metaphase II: Chromosomes line up at each cell’s equator and attach to spindle fibers from both poles.
• Anaphase II: Sister chromatids separate and move to opposite poles. Once separated, they are considered individual chromosomes.
• Telophase II and cytokinesis: Nuclear membranes reform and both cells divide.

Meiosis II ends with four genetically different haploid cells, each containing one set of chromosomes.

Key Takeaways

• Meiosis starts with a diploid cell and produces haploid gametes.
• In meiosis I, homologous chromosomes separate.
• In meiosis II, sister chromatids separate.
• Crossing over in prophase I creates genetic variation.
• The final four gametes are genetically different from one another.

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