Cell Cycle

Meiosis is the division in which the number of chromosomes in the daughter cells is reduced to half that of the parent cell. It takes place in diploid cells only, in animals at the time of gamete formation and in plants when spores are produced, and each diploid cell produces four haploid cells, because two consecutive divisions follow a single replication of the DNA. The first division is the reduction division and the second is just like mitosis. What separates the two is not the machinery but what moves: in meiosis I the homologous chromosomes separate while the sister chromatids stay joined, and in meiosis II the sister chromatids separate.

Prophase I is very prolonged, and it is where the five sub-stages live. In leptotene the chromosomes become visible, shorten and thicken, the nucleus enlarges, and the homologues begin to move closer together. In zygotene, the first essential phenomenon of meiosis begins, the pairing of homologous chromosomes, called synapsis. The pairing is highly specific and exact but has no definite starting point, and each paired but not fused complex is a bivalent, also called a tetrad, which means two homologous chromosomes carrying four chromatids between them. In pachytene the pairing is completed, each bivalent has four chromatids wrapped around each other, and non-sister chromatids exchange segments through chiasmata, the process called crossing over; pachytene can last days, weeks or even years, while leptotene and zygotene last only hours. In diplotene the paired chromosomes repel each other and begin to separate, but they remain united at their points of interchange, and each bivalent has at least one such point. In diakinesis the condensation reaches its maximum, the separation is completed, and the nucleoli disappear.

The rest of meiosis I is short and decisive. At metaphase I the nuclear membrane disorganises, spindle fibres form, and kinetochore fibres attach to the kinetochores of the homologous chromosomes from each pole to arrange the bivalents at the equator, with the sister chromatids of each chromosome behaving as a unit. At anaphase I the kinetochore fibres contract and the pole fibres elongate, pulling whole chromosomes, each still carrying two chromatids, to their poles, and each pole receives half the total chromosome number. At telophase I a nuclear membrane reorganises around each set, the nucleoli reappear, two nuclei with half the original number of chromosomes are formed, and the cytoplasm divides. Two daughter cells then pass through a short interphase, and in contrast to mitotic interphase there is no replication of chromosomes, which is why the count cannot be silently doubled between the two divisions.