Meiosis: Detailed Stages of Cell Division

The Stages of Meiosis

Overview of Meiosis

  • Initial State (Interphase):
    • Before meiosis begins, the chromosomes in a diploid parent cell are duplicated during interphase.
    • This results in a pair of duplicated homologous chromosomes, where each chromosome consists of two sister chromatids.
    • The sister chromatids remain closely associated along their lengths, a phenomenon known as sister chromatid cohesion.
  • Purpose and Outcome:
    • Meiosis is a two-division process that sorts the duplicated chromatids into four haploid daughter cells.
    • This process effectively reduces the chromosome number by half and ensures genetic diversity.

Meiosis I: Separation of Homologous Chromosomes

  • Meiosis I is the first meiotic division, during which homologous chromosomes are separated.
  • It consists of four main phases:
    • Prophase I
    • Metaphase I
    • Anaphase I
    • Telophase I and Cytokinesis
Prophase I
  • Pairing of Homologs: In early Prophase I, each chromosome finds and pairs with its homologous chromosome.
  • Crossing Over: During this pairing, genetic material is exchanged between non-sister chromatids of homologous chromosomes. This process is called crossing over.
  • Chiasmata: The X-shaped regions that become visible after homologous chromosomes begin to separate are called chiasmata. These are the visible manifestations of previous crossovers.
Metaphase I
  • Alignment at the Metaphase Plate: Pairs of homologous chromosomes, still associated after crossing over, line up at the metaphase plate – an imaginary plane equidistant from the two spindle poles.
  • Orientation: Each homologous pair is oriented such that one chromosome faces one pole and its homolog faces the opposite pole.
  • Microtubule Attachment: Microtubules from one spindle pole attach to the kinetochore of one chromosome from each homologous pair. Conversely, microtubules from the other pole attach to the kinetochore of the other chromosome in that pair.
Anaphase I
  • Separation of Homologs: The pairs of homologous chromosomes separate from each other.
  • Movement to Poles: One chromosome of each homologous pair moves towards one pole, guided by the spindle apparatus, while its homolog moves towards the opposite pole.
  • Sister Chromatid Integrity: Crucially, sister chromatids remain attached at their centromere and move together as a single unit towards the pole. This means each pole receives a haploid set of chromosomes, but each chromosome is still duplicated (consisting of two sister chromatids).
Telophase I and Cytokinesis
  • Haploid Cells with Duplicated Chromosomes: At the beginning of Telophase I, each half of the cell now contains a haploid set of chromosomes. Importantly, each chromosome still consists of two sister chromatids.
  • Cytokinesis: This process typically occurs simultaneously with Telophase I, leading to the formation of two haploid daughter cells.
    • In animal cells, a cleavage furrow forms to divide the cytoplasm.
    • In plant cells, a cell plate forms to create a new cell wall between the daughter cells.
  • No Interphase/Replication: After Meiosis I, there is no chromosome replication (interphase II) between the end of Meiosis I and the beginning of Meiosis II, because the chromosomes are already replicated (composed of two sister chromatids).

Meiosis II: Separation of Sister Chromatids

  • Meiosis II is the second meiotic division, which is very similar to mitosis.
  • It also occurs in four phases:
    • Prophase II
    • Metaphase II
    • Anaphase II
    • Telophase II and Cytokinesis
  • This division separates the sister chromatids, resulting in four haploid cells with unduplicated chromosomes.