Meiosis Lecture Notes

Introduction to Meiosis

• Meiosis is similar to mitosis but aims to produce four genetically diverse gametes (sperm and egg).
• The goal is to create genetic variation for the next generation.

Phases of Meiosis

• Mnemonic: PMAT x 2 (Prophase, Metaphase, Anaphase, Telophase) for two divisions

Major Structures

• Chromosomes: Two of each at the start, one from each parent.
  • Homologous Chromosomes: Pairs with genes from both parents.
  • Sister Chromatids: Duplicated chromosomes from one parent.
• Centrosome: Organizes the spindle for cell division.
  • In animals: includes centrioles and microtubules.
  • In plants: lacks centrioles; nuclear membrane aids division.

Interphase

• Start: One centrosome, two pairs of chromosomes (loose DNA).
• S Phase: DNA duplication leading to visible chromosomes (X shape).

Meiosis I

• Prophase I:
  • Synapsis: Homologous chromosomes pair tightly.
  • Crossing Over: Exchange of chromosome segments for variation.
• Metaphase I:
  • Chromosomes line up at metaphase plate.
  • Independent Orientation: Different arrangements lead to variation.
  • Spindle fibers attach to centromeres.
• Anaphase I: Homologous chromosomes pulled apart.
• Telophase I: New nuclei form, followed by cytokinesis.

Meiosis II

• Prophase II: Chromosomes condense; no crossing over.
• Metaphase II: Chromosomes line up in the middle.
• Anaphase II: Sister chromatids are pulled apart.
• Telophase II & Cytokinesis: Formation of four genetically different cells.

Genetic Variation

• Variation is a result of crossing over and independent orientation.
• Human potential: Over 8 million combinations due to 23 chromosomes.
• Total possible genetic combinations (sperm and egg): 64 trillion.

Gamete Formation

• Sperm Formation: All four cells are used.
• Egg Formation: Only one nucleus is used to retain cellular components.

Review

• Interphase: Start with single chromosome copies.
• Prophase I: Crossing over occurs.
• Metaphase I: Independent orientation.
• Division: Two cells form, then split again in meiosis II.
• End result: Four genetically unique cells with half the original DNA.