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1.6.2.1. Meiosis I. First meiotic division

Prophase I

The main events of prophase I are:

  • In the same way as in prophase of mitosis, in prophase I of meiosis the following occurs:
    • Disappearance of the nuclear envelope.
    • Chromatin condensation to form chromosomes.
      • Formation of the achromatic spindle, a bundle of filaments that go from end to end of the cell:
        • In animal cells, the centrioles, already duplicated, migrate to each pole of the cell and, between them, the achromatic spindle is formed.
        • Plant cells do not have centrioles, although the achromatic spindle is also formed.
  • Other events other than those of prophase I also occur:
    • Chromosomes are grouped in pairs, such that chromosomes that are the same (homologous chromosomes) join chromatid by chromatid.
    • The crosslinking or crossoverof homologous chromosomes. With genetic recombination, the chromatids on both homologous chromosomes exchange DNA segments, allowing new recombined chromatids to appear. These will give rise to individuals different from the previous ones, since the daughter cells are genetically different from the progenitor.
    • After recombination, each pair of homologous chromosomes binds to an achromatic spindle fiber (not one as in mitosis). Afterwards, each pair moves towards the central part of the cell.

Profase I de la meiosis

Metaphase I

During metaphase I, homologous chromosomes, joined in pairs (bivalent), are arranged in the equatorial plane or plate of the cell.

Anaphase I

In anaphase I, the homologous chromosomes are separated, dragged by the fibers of the achromatic spindle, towards each pole of the cell. Unlike what happened in mitosis, in anaphase I it is whole chromosomes, not chromatids, that separate.

At the end of anaphase I, there is a set of chromosomes at each opposite pole of the cell, one from each pair of homologous chromosomes, so the number of chromosomes has been reduced by half. At this time, there are two groups of n chromosomes each with two chromatids.

Telophase I

In telophase I, the chromosomes (with two chromatids) have reached the respective poles.

nuclear envelope begins to form at each pole, and the cell divides into two cells with n chromosomes each.

This division, in which the number of chromosomes has been halved, is also called a reductional division.

Cytokinesis or division of the cytoplasm begins.

Cytokinesis I

With cytokinesis, the cytoplasm is divided into two parts, one for each daughter cell, with half the chromosomes of a mother cell.