Meiosis: Reduction Division

Overview

• Meiosis, also known as reduction division, is a form of cell division
• It produces gametes (sex cells):
  • Sperm cells in males
  • Egg cells in females
• Meiosis consists of two stages of cell division: Meiosis I and Meiosis II

Meiosis I

• Prophase I:
  • Begins with a diploid cell
  • Chromatin contains two uncoiled sets of chromosomes, one from each parent
  • DNA replicates and condenses into X-shaped chromosomes
  • Synapsis: Homologous chromosomes pair up forming tetrads (4 chromatids)
  • Crossing Over: Exchange of alleles between chromatids leads to genetic variation
  • Nuclear membrane disintegrates
  • Centrioles move to cell poles, spindle fibers form
• Metaphase I:
  • Homologous chromosomes line up at the equator
  • Attach to spindle fibers from opposite poles
• Anaphase I:
  • Spindle fibers separate homologous chromosomes to opposite poles
• Telophase I:
  • One chromosome from each homologous pair at separate poles
  • Chromosomes consist of sister chromatids
  • Nuclear membrane reforms
  • Cytokinesis occurs, ending with two genetically different haploid cells

Meiosis II

• No DNA replication before beginning
• Prophase II:
  • Nuclear membrane disappears
  • Spindle fibers form from paired centrioles
• Metaphase II:
  • Chromosomes line up at equator and attach to spindle fibers
• Anaphase II:
  • Sister chromatids separate, move to opposite poles
  • Separated chromatids are now individual chromosomes
• Telophase II:
  • Nuclear membranes reform
  • Cytokinesis occurs
  • Results in four genetically different haploid daughter cells

Key Points

• Meiosis begins with a diploid cell and produces haploid gametes
• Gametes are genetically different
• Meiosis I involves separation of homologous chromosomes
• Meiosis II involves separation of sister chromatids
• Crossing over in Prophase I contributes to genetic diversity
• All gametes produced are haploid

Important Processes

• Synapsis: Pairing of homologous chromosomes
• Tetrad Formation: Each homologous chromosome pair forms a tetrad
• Crossing Over: Exchange of alleles increases genetic variation

This summary outlines the key stages and processes of meiosis, highlighting important concepts such as synapsis, crossing over, and the production of genetically diverse haploid gametes.