Meiosis Step by Step

1.  Watch these videos

1.a. Meiosis Walkthrough for AP Bio

1.b. Meiosis! (Music Vide0)

2.  Read this summary

Overview of Meiosis

  • Meiosis reduces chromosome number from diploid to haploid. As it does, processes like independent assortment and crossing over create genetically unique gametes, which leads to variation between parents and offspring, and among siblings.
  • Meiosis includes two rounds of cell division—Meiosis I and Meiosis II—each with distinct steps contributing to genetic variation.

Meiosis I

  1. Interphase:
    • DNA is replicated, resulting in chromosomes made of two sister chromatids.
    • Prepares the cell for the first round of division.
  2. Prophase I:
    • Homologous chromosomes (one maternal, one paternal) pair up in a process called synapsis.
    • Crossing Over: Chromosomes exchange segments of DNA at points called chiasmata (plural of chiasma), creating recombinant chromosomes with unique genetic sequences.
  3. Metaphase I:
    • Homologous pairs align at the cell equator.
    • Independent Assortment: The orientation of each pair is random and independent, setting the stage for the generation of chromosomally unique daughter cells during the end of Meiosis 1.
    • The number of chromosomal arrangements in gametes can be calculated as , where n is the number of homologous pairs.
  4. Anaphase I:
    • Homologous chromosomes are separated and pulled to opposite poles of the cell.
  5. Telophase I and Cytokinesis:
    • Nuclear membranes reform around the separated chromosomes.
    • The cell divides, resulting in two haploid cells, each containing one chromosome from each homologous pair. Each chromosome is still doubled, consisting of two sister chromatids.

Meiosis II

  1. Prophase II:
    • Chromosomes condense again.
    • Unlike Prophase I, the daughter cells are now haploid, containing only one set of chromosomes. However, each chromosome is doubled, and consists of two sister chromatids.
  2. Metaphase II:
    • Chromosomes (still consisting of sister chromatids) align at the cell equator.
  3. Anaphase II:
    • Sister chromatids are separated and pulled to opposite poles of the cell.
  4. Telophase II and Cytokinesis:
    • Nuclear membranes reform around the separated chromatids.
    • The cells divide, resulting in four unique haploid gametes.

Key Transitions and Results

  • Diploid to Haploid: Meiosis reduces the chromosome number by half, ensuring that gametes contain one set of chromosomes.
  • Genetic Uniqueness: Each gamete has a unique genetic makeup due to crossing over and independent assortment.
  • Final Result: Four haploid gametes, each with single chromosomes, ready for fertilization.

3. Complete these flashcards

[qdeck qrecord_id=”sciencemusicvideosMeister1961-Meiosis, Step by Step, APBVP”]
[h] Meiosis, Step by Step

[q] What happens during interphase in meiosis?
[a] During interphase, DNA is replicated, creating chromosomes made of two sister chromatids. This prepares the cell for the first division in meiosis.

[q] What are the key events of prophase I in meiosis?
[a]

  1. Homologous chromosomes pair up through synapsis, forming tetrads.
  2. Crossing over occurs, where segments of DNA are exchanged between homologous chromosomes, creating recombinant chromosomes.

[q] How does crossing over contribute to genetic variation?
[a] Crossing over exchanges segments of DNA between homologous chromosomes, resulting in recombinant chromosomes with unique genetic sequences.

[q] What is independent assortment, and when does it occur?
[a] Independent assortment is the random alignment of homologous pairs at the equator during metaphase I. Each pair’s orientation is independent of the others, generating genetic diversity.

[q] How can you calculate the number of possible chromosomal arrangements in gametes due to independent assortment?
[a]The number of arrangements is 2n, where n is the number of homologous pairs. For example:

  • 2 pairs: 22 = 4
  • 23 pairs (humans): 223 = 8,388,608

[q] What happens during anaphase I in meiosis?
[a] Homologous chromosomes are separated and pulled to opposite poles of the cell by spindle fibers.

[q] What occurs during telophase I and cytokinesis in meiosis?
[a]

  1. Nuclear membranes reform around the separated chromosomes.
  2. The cell divides into two haploid cells, each containing one chromosome from each homologous pair. The chromosomes are still doubled, consisting of two sister chromatids.

[q] How does meiosis I reduce chromosome number?
[a] Meiosis I separates homologous chromosomes, reducing the chromosome number from diploid to haploid.

[q] What happens during prophase II in meiosis?
[a] Chromosomes condense again in the haploid cells formed after meiosis I, preparing for the second division.

[q] What are the key events of metaphase II in meiosis?
[a] Chromosomes (each consisting of sister chromatids) align at the cell equator.

[q] What occurs during anaphase II in meiosis?
[a] Sister chromatids are separated and pulled to opposite poles of the cell.

[q] What happens during telophase II and cytokinesis in meiosis?
[a]

  1. Nuclear membranes reform around the separated chromatids.
  2. The cells divide, resulting in four haploid gametes.

[q] What are the final results of meiosis?
[a]

  1. Four haploid gametes, each with single chromosomes.
  2. Gametes are genetically unique due to crossing over and independent assortment.

[/qdeck]

 

4. Tackle this quiz

[qwiz random= “true” use_dataset=”Phases of Meiosis” dataset_intro=”false” qrecord_id=”sciencemusicvideosMeister1961-Phases of Meiosis, APBVP”]

[h]Phases of Meiosis

[i]

[/qwiz]

 

5. What’s Next?

Continue to the next tutorial, Sex Determination