Meiosis I Stages: Chromosome Behavior and Cellular Division

Diplotene

The synaptonemal complexes are disassembled, allowing homologous chromosomes to separate. This separation is not total, as they remain attached at points where crossover has occurred. These areas appear as X-shaped structures called chiasmata, which play an important role in the correct alignment of homologous pairs during metaphase I.

Diakinesis

Chromosomes reach their maximum degree of condensation. In each bivalent, the four chromatids (tetrads) are clearly observed, with each pair of sister chromatids joined at their centromeres, while non-sister (homologous) chromatids are held together at the crossover chiasmata.

Metaphase I

Homologous chromosome pairs (bivalents) are directed to the equatorial plane of the spindle, forming a metaphase plate.

• The kinetochores of the homologous pairs are oriented toward opposite poles.
• Spindle microtubules attach to the kinetochores of their respective poles.

The main difference from mitotic metaphase is that in metaphase I, paired homologues are located on the equatorial plate.

Anaphase I

Kinetochore microtubules shorten, pulling the chromosomes toward their respective poles and causing the breakage of the chiasmata that held the homologous chromosomes together. This results in their separation and migration to opposite poles of the cell.

• Homologous chromosomes remain held together by their two sister chromatids at their centromeres.
• After completion of anaphase, each pole of the cell contains one member of each pair, consisting of two sister chromatids.

The main difference from a typical mitotic anaphase is that in anaphase I, the chromosomes do not split at their centromere but migrate intact toward the poles of the cell.

Telophase I

Around each haploid set of homologous chromosomes, a nuclear envelope transiently develops, the nucleolus reappears, spindle fibers depolymerize, and chromosomes decondense slightly to return to a very brief interphase, during which DNA duplication never occurs. Simultaneously, in most cells, cytokinesis occurs, forming two daughter cells with half the original number of chromosomes. Stem cell.