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In humans, sex precursor cells undergo meiosis, a process with two divisions named meiosis one and two, to generate haploid sperm or egg cells.

Prophase one commences meiosis one, where a diploid cell's chromatin, involving one paternal and one maternal genetic set, condenses. This forms typical x-shaped chromosomes, which anchor to the nuclear envelope.

Father and mother inherited copies of the same chromosome are joined by the development of protein threads between them and oriented, so that equivalent genes line up.

Such homologous chromosomes then swap pieces during crossing over and remain affixed at the spot of exchange even as the threads between them dissolve. Outside the nucleus the meiotic spindle apparatus appears, which involves microtubules emanating from centrosomes.

During prometaphase one the nuclear envelope disperses, and protein kinetochores form on centromeres. Microtubules then elongate and connect to these structures, fastening each homologous chromosome in a pair to a different pole.

With metaphase one the pairs position randomly along the cell's midline and anaphase one is marked by the retraction of mictrotubules, which splits homologous chromosomes.

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