Meiosis I (Meiotic First Division)

Prophase I - It is the most remarkable stage of meiosis. In it occurs the pairing of homologous chromosomes and can happen a phenomenon known as crossing-over (also called exchange).

Since prophase I is long, there is a sequence of events that, for study purposes, can be divided into the following steps:

  • Chromosomal spiraling begins. It is the phase of leptotene (leptós = thin), in which the chromosomal filaments are thin, barely visible, and already each made up of two chromids.

  • The attraction and pairing of homologous chromosomes begins; is a point-by-point pairing known as a synapse (the prefix sin comes from the Greek and means union). This is the phase of zygote (zygós = pair).

  • The spiraling has progressed: now, the two chromatids of each paired counterpart are clearly visible; as there are then four chromatids, the set forms a bivalent tetrad or pair. This is the phase of pachytene (pakhús = thick).

  • Occasional breaks in the chromatids occur and an exchange of pieces between homologous chromatids, a phenomenon known as crossing over (or barter). Then the counterparts move away and some regions are still in contact with each other. These regions are known as chiasmas (which corresponds to the letter “x” in Greek). The chiasmas represent the regions in which the pieces were exchanged. This phase of prophase I is the diplomat (diplomats = double).

  • The pairs of chromatids tighten a little more and the chiasmas seem to “slip” to the ends; the spiraling of chromosomes increases. is the last phase of prophase I, known as diakinesis (day = through; kinesis = movement).

While these events take place, the centrioles, which have come duplicated from interphase, migrate to opposite poles and organize the division spindle; the nucleoli disappear; the library breaks down after the end of prophase I, foreshadowing the occurrence of metaphase I.

Metaphase I - paired homologous chromosomes are arranged in the median region of the cell; each chromosome is attached to single pole fibers.

Anaphase I - the shortening of the spindle fibers separates the homologous chromosomes, which are conducted to opposite poles of the cell, there is no separation of sister chromatids. When chromosomes reach the poles, their de-spiralization occurs, although not obligatory, even because the second stage of meiosis follows. Sometimes not even the library reconstitutes itself.

Telophase I - At the end of this phase, cytokinesis occurs, separating the two haploid daughter cells. A short interval follows the interkinesis, which proceeds to prophase II.