Meiosis and its phases

  Created April 2020, Offline version here
  Videos by Paul Anderson, also on his website Bozeman Science.

       are formed by a process called meiosis,which is a specialized type of cell division that generates four cells. Meiosis involves two separate cell division events, in meiosis I and meiosis II. The process begins with interphase, during which the DNA is before the cell enters meiosis. During interphase, the DNA is in a conformation, and it compacts to form chromosome structures at the beginning of meiosis. The first stage of meiosis I is called . Here, the chromosomes are condensed, and undergo an important process called . The homologous chromosomes wrap around one another to genetic material. Homologous chromosomes carry the same genes, but one is inherited from the father, and one is inherited from the mother, so they might contain different versions of these genes. Thus, by crossing over, we have generated new of paternal and maternal DNA, creating a whole new chromosome. The next stage of meiosis I is . In this stage, the homologous chromosomes at the centre of the cell, and the attaches to the centromere of each chromosome. The next stage is , during which homologous chromosomes are separated, each being pulled to one side of the cell. The final stage of meiosis I is , during which two nuclei are reformed. The cytoplasm is finally split during , forming two new cells. Each of these cells now enters meiosis II. Meiosis II beings with prophase II, during which there is more crossing over. The chromosomes make their way to the centre of the cell during , and the spindle attaches to the centromere of each chromosome. The are separated during anaphase II. The sister chromatids (now referred to as individual chromosomes) then during telophase II, and the nuclei reform. Finally, a second cytokinesis event occurs to produce 4 haploid cells. These cells are genetically . Genetic diversity during meiosis and reproduction is created by three factors. The first is the process of crossing over, which creates new chromosomes distinct from the maternal and paternal chromosomes. The second mechanism of generating genetic diversity is assortment of chromosomes into gametes. This occurs during metaphase I and II, where chromosomes randomly align at either side of the plate. Finally, the process of random fertilization (the fact that any egg can be fertilized by any ) gives huge potential for variation in the population.