Meiosis

Meiosis involves two nuclear divisions that produce four haploid cells. Meiosis I is the reduction division. It reduces the chromosome number from diploid to haploid and separates the homologous pairs. Meiosis II separates the sister chromatids and the result is four haploid gametes. N is the haploid number. The fist phase of Meiosis I is called interphase. Here DNA synthesis or DNA replication. Each chromosome is now made up of two strands, or chromatids, joined together at the centromere region. Next phase is prophase I. Homologous chromosomes now come together and synapse (crossing over) along their entire length. A tetrad (4 chromatids) is formed. The result of crossing over is called a chiasma. A tetrad is two chiasmata. The next phase is metaphase I. Here the crossed over tetrads line up in the center of the cell. The next phase is anaphase I. Here the homologous chromosomes separate and are "pulled" to opposite sides of the cell. The next phase is telophase I. Here there is a formation of a nuclear envelope and possibly the division of the cytoplasm (cytokinesis). 2 haploid cells have formed and each chromosome is composed of two chromatids. Then Meiosis II happens. The first phase is called interphase II or Interkinesis. This is a period of rest for the cell. Here DNA replication does not happen. Also, the amount of time in rest depends on the type of organism. The next phase is called prophase II. Here replicated centrioles separate and move to opposite sides of the chromosome groups. The next phase is called metaphase II. The chromosomes are centered in the middle of each of the daughter cells. The next phase is anaphase II. Here the centromere regions of the chromatids now appear to be separate. The chromatids are separated and the daughter chromosomes are pulled toward the opposite sides of each of the daughter cells. The final phase is called telophase II. Here the chromosomes are at the opposite sides of the dividing cell. A nuclear envelope forms and the cytoplasm divides.