Phases of Meiosis to Know for AP Biology

Meiosis is a crucial process for sexual reproduction, reducing chromosome numbers and increasing genetic diversity. It consists of two main divisions, each with distinct phases that ensure proper chromosome separation and genetic variation through mechanisms like crossing over and independent assortment.

1. Prophase I

   • Chromosomes condense and become visible.
   • Homologous chromosomes pair up in a process called synapsis.
   • Crossing over occurs, allowing genetic material exchange between homologs.
   • The nuclear envelope begins to break down.
   • Spindle fibers start to form.

2. Metaphase I

   • Homologous chromosome pairs align at the metaphase plate.
   • Spindle fibers attach to the centromeres of each homolog.
   • Orientation of each pair is random, contributing to genetic diversity.

3. Anaphase I

   • Homologous chromosomes are pulled apart to opposite poles of the cell.
   • Sister chromatids remain attached at their centromeres.
   • This separation reduces the chromosome number by half.

4. Telophase I

   • Chromosomes reach the poles and begin to de-condense.
   • The nuclear envelope may reform around each set of chromosomes.
   • Cytokinesis occurs, resulting in two haploid daughter cells.

5. Prophase II

   • Chromosomes condense again if they had de-condensed.
   • The nuclear envelope breaks down if it reformed.
   • Spindle fibers form and attach to the chromosomes.

6. Metaphase II

   • Chromosomes align at the metaphase plate.
   • Spindle fibers attach to the centromeres of sister chromatids.
   • Each chromosome is positioned independently of others.

7. Anaphase II

   • Sister chromatids are pulled apart to opposite poles.
   • Each chromatid is now considered an individual chromosome.
   • This ensures that each daughter cell will receive an equal number of chromosomes.

8. Telophase II

   • Chromosomes reach the poles and begin to de-condense.
   • The nuclear envelope reforms around each set of chromosomes.
   • Cytokinesis occurs, resulting in four genetically diverse haploid cells.

9. Interkinesis

   • A short resting phase between meiosis I and II.
   • Chromosomes may de-condense slightly, but no DNA replication occurs.
   • Cells prepare for the second meiotic division.

10. Crossing over

    • Exchange of genetic material between non-sister chromatids during prophase I.
    • Increases genetic variation in gametes.
    • Occurs at chiasmata, where homologous chromosomes are closely aligned.

11. Synapsis

    • The pairing of homologous chromosomes during prophase I.
    • Facilitates crossing over and genetic recombination.
    • Essential for proper segregation of chromosomes.

12. Homologous chromosome pairing

    • Homologous chromosomes align closely during prophase I.
    • Ensures accurate separation during meiosis.
    • Allows for genetic diversity through recombination.

13. Chiasmata formation

    • Physical points where crossing over occurs between homologous chromosomes.
    • Visible under a microscope as X-shaped structures.
    • Critical for genetic diversity and proper chromosome segregation.

14. Independent assortment

    • The random orientation of homologous chromosome pairs during metaphase I.
    • Results in a mix of maternal and paternal chromosomes in gametes.
    • Contributes to genetic variation in offspring.

15. Reduction division

    • The process of reducing the chromosome number by half during meiosis I.
    • Produces haploid cells from diploid parent cells.
    • Essential for maintaining chromosome number across generations.