Meiosis is the process by which a nucleus divides by two divisions into four nuclei, each containing half the number of chromosomes of the mother cell. The resulting nuclei are haplois (n). Meiosis is necessary for sexual reproduction in eukaryotes; without it fertilisation would double the chromosome number ever generation.
- In prophase 1, homologous chromosomes (homologues), each consisting of two genetically identical sister chromatids held together by their centromere, form pairs in the process of synapsis. Each pair is called a bivalent. Crossing over occurs between the homologous chromosomes of a bivalent, creating genetic variation. The chromosomes condence and become more visible, as in mitosis. At the end of prophase 1, the nuclear envelope breaks down. Centrioles, if present, duplicate and move to opposite poles. A spindle of microtubules forms.
- In metaphase 1, bivalents line up independently on the cell's equator and each homologue attaches to spindle microtubules by it's undivided centromere. The orientation of the maternal and…