Phases of Meiosis

Jul 24, 2024

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Phases of Meiosis

Overview

  • Meiosis (Reduction Division): Cell division producing gametes (sex cells: sperm and egg).
  • Stages: Meiosis I and Meiosis II, each with 4 phases.

Meiosis I

Prophase I

  • Starts with a diploid cell (two sets of chromosomes, one from each parent).
  • DNA Replication: Chromatin (uncoiled spread out chromosomes) replicates and condenses into X-shaped chromosomes.
  • Synapsis: Homologous chromosomes pair up and bind, forming tetrads (group of 4 sister chromatids).
  • Genes & Alleles: Different versions of the same gene on each chromatid.
  • Crossing Over: Exchange of chromosome segments (alleles) between homologous chromosomes, leading to genetic variety.
  • Nuclear membrane disappears, centrioles move to poles, spindle fibers form.

Metaphase I

  • Homologous chromosomes line up at the equator and attach to spindle fibers.

Anaphase I

  • Homologous chromosomes in tetrads separate to opposite poles.
  • Chromosomes consist of sister chromatids (no longer identical due to crossing over).

Telophase I

  • Chromosomes at opposite poles; nuclear membrane reforms.
  • Spindle fibers disappear, cytokinesis occurs.
  • Ends with 2 genetically different haploid daughter cells (chromosomes as paired sister chromatids).

Meiosis II

Prophase II

  • No DNA replication prior.
  • Nuclear membrane disappears, spindle fibers form from paired centrioles.

Metaphase II

  • Chromosomes line up at the equator and attach to spindle fibers.

Anaphase II

  • Sister chromatids separate and move to opposite poles (now called chromosomes).

Telophase II

  • Spindle fibers disappear, nuclear membranes reform, cytokinesis occurs.
  • Ends with 4 genetically different haploid daughter cells (each with one set of chromosomes).

Key Points

  • Starts: With a diploid cell.
  • Only Produces: Gametes (sperm or eggs).
  • Produces: Genetically different haploid cells.
  • Meiosis I: Homologous chromosomes separate, resulting in two haploid cells with paired sister chromatids.
  • Meiosis II: Sister chromatids separate, resulting in four haploid gametes.
  • Prophase I Details: Synapsis (pairing of homologous chromosomes), tetrad formation, and crossing over lead to genetic diversity.
  • Gametes: Resulting from meiosis are haploid.