Meiosis Step by Step

1.  Watch this video

2.  Read this summary

Overview of Meiosis

  • Purpose: Meiosis reduces chromosome number from diploid to haploid. As it does, it creates genetic variation between parents and offspring, and among siblings.
  • Key Phases: The process 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, 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, generating genetic diversity.
    • 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.

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 chromatides.
  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]
[h] Meiosis: The Whole Shebang

[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.
  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 2n2^n, where nn is the number of homologous pairs. For example:

  • 2 pairs: 22=42^2 = 4
  • 23 pairs (humans): 223=8,388,6082^{23} = 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.

[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. Complete this quiz

[qwiz random= “true” use_dataset=”Phases of Meiosis”]

[h]Quiz: Phases of Meiosis

[i]

[/qwiz]

5. What’s Next?

Continue to the next tutorial, Sex Determination