Meiosis is a part of the life cycle of every organism that reproduces sexually. In meiosis four haploid daughter cells are produced, each with half the number of chromosomes of the parent cell. Halving the chromosome number is essential, since when two gametes fuse at fertilisation the number is doubled.
Meiosis involves two divisions of the nucleus, known as meiosis I and meiosis II. As in mitosis, chromosomes replicate to form chromatids during interphase. Then, early in meiosis I, homologous chromosomes pair up. By the end of meiosis I, the homologous chromosomes have separated again, but the chromatids they consist of do not separate until meiosis II. Thus meiosis consists of two nuclear divisions but only one replication of the chromosomes.
The process of meiosis
Prophase I (early)
During interphase the chromosomes replicate into chromatids held together by a centromere. Now the chromosomes condense (shorten and thicken) and become visible.
Prophase I (mid)
Homologous chromosomes pair up (becoming bivalents) as they continue to shorten and thicken. Centrioles duplicate.
Prophase I (late)
Homologous chromosomes repel each other. Chromosomes can now be seen to consist of chromatids. Sites where chromatids have broken and rejoined, causing crossing over, are visible as chiasmata.
Nuclear membrane breaks down. Spindle forms. Bivalents line up at the equator, attached by centromeres.
Homologous chromosomes separate. Whole chromosomes are pulled towards opposite poles of the spindle, centromere first (dragging along the chromatids).
Nuclear membrane re-forms around the daughter nuclei. The chromosome number has been halved. The chromosomes start to decondense.
The chromosomes condense and the centrioles duplicate.
The nuclear membrane breaks down and the spindle forms. The chromosomes attach by their...