​MEIOSIS II AND THE OUTCOMES OF MEIOSIS 

During meiosis II, the chromosomes condense and become attached to a new spindle apparatus  (prophase II).  They then move to positions in the  equatorial plane of the cell  (metaphase II),  and their centromeres split to allow the constituent sister chromatids to move to opposite poles  (anaphase II),  a phenomenon called  chromatid disjunction. 
During  telophase II,  the separated chromatids—now called chromosomes—gather at the poles and daughter nuclei form around them. Each daughter nucleus contains  a  haploid set of chromosomes. Mechanistically, meiosis II is therefore much like mitosis. However, its products are haploid, and unlike the products of mitosis, the cells that emerge from meiosis II are not genetically identical. Pair of homologous chromosomes Homologue 1 Centromeres One chromatid Chiasma Two chiasmata Homologue 2 Synapsis and crossing over Tetrad Recombinant chromatids
 One reason these cells differ is that homologous chromosomes pair and disjoin from each other during meiosis I. Within each pair of chromosomes, one homologue was inherited from the organism’s mother, and the other was inherited from its father. During meiosis I, the maternally and paternally inherited homologues come together and synapse. They are positioned on the meiotic spindle and become oriented randomly with respect to the spindle’s poles. Then they disjoin. For each pair of chromosomes, half the daughter cells produced by the first meiotic division receive the maternally inherited homologue, and the other half receive the paternally inherited homologue. Thus, from the end of the first meiotic division, the products of meiosis are destined to be different. These differences are compounded by the number of chromosome pairs that disjoin during meiosis I. Each of the pairs disjoins independently. Thus, if there are 23 pairs of chromosomes, as there are in humans, meiosis I can produce 223  chromosomally different daughter cells—that is, more than 8 million possibilities. To test your understanding of this concept go to Solve It: How Many Chromosome Combinations in Sperm?

Advertisements