Have you ever wondered why children do not look exactly like their parents? Or why some siblings look so different from each other? The answer lies in the process of meiosis, which generates the gametes for reproduction. Meiosis is the process by which haploid cells are produced from a diploid cell. For this to occur, the chromosomes must be correctly sorted and distributed in a manner to create genetically unique cells with half the number of chromosomes as the original cell. Meiosis occurs in special cells called germ cells within the gonads of males and females. For the successful reduction in the number of chromosomes to occur in the new haploid daughter cells, two rounds of divisions are necessary. These divisions are termed meiosis 1 and meiosis 2. Although the processes of meiosis and meiosis look similar, there are a few major differences. One obvious difference is that meiosis results in two diploid daughter cells, and meiosis results in four haploid cells. As in meiosis, meiosis begins after a cell has successfully completed the G1, S and G2 stages of interphase. During the S phase of interphase, the DNA is replicated, producing two copies of each chromosome called sister chromatids. The paired centrioles in the cytoplasm duplicate and begin extending micro tubules that will form the myotic spindle. Sister chromatids remain attached at the centromere and condense as the cell enters prophase 1 of meiosis. Up to this point, the cell looks similar to meiosis, but two events occur in meiosis that do not occur in meiosis and lead to genetic diversity. The first event occurs during prophase 1 when homologous pairs of sister chromatids lie side by side in a process called synapsis, forming a tetrad or bivalent. The homologous chromosomes share similar but not necessarily identical genes. Once this structure is formed, the second event called crossing over can occur. During crossing over, a physical exchange between chromosome segments of non-sister chromatids occurs, increasing genetic diversity. Pro phase 1 concludes with the fragmentation of the nuclear envelope as the duplicated centriole pairs move to opposite poles of the cell. As they move, the centrioles extend spindle fibers forming the myotic spindle. In prometaphase 1 with the paired centrioles in place, the myotic spindle is fully formed. The sister chromatids attach to the spindle fibers by their kineticors. Here another key difference between mitosis and meiosis occurs. Because of synapsis and crossing over in meiosis, homologous chromosomes remain aligned so that a pair of sister chromatids is attached to only one pole by the kineticor microtubules. In mitosis, each sister chromatid is attached to a spindle fiber. The fibers originate from opposite poles. During metaphase 1, by valence randomly align along the metaphase plate. Due to independent assortment, this alignment is random and adds to genetic diversity. In anaphase 1, the homologous chromosomes separate and move toward opposite poles. Myosis 1 ends with telephase 1 when the chromosomes decondence and the nuclear envelope reforms. Cytocinesis separates the cytoplasmic material and the two daughter cells are separated by a cleavage furrow. Since the final product of myosis 1 is two haploid cells, myosis 2 begins without the chromosomes going through another round of DNA replication. Centrioles again duplicate and begin moving to opposite poles of each cell. In pro phase 2, the sister chromatids condense while the spindle starts to form as the nuclear envelope disappears. In pro metaphase 2, the sister chromatids attach to the spindle by kineticor microtubules with sister chromatids attached to opposite poles. The spindle aligns the sister chromatids along the metaphase plate during metaphase 2. During anaphase 2, sister chromatids separate and individual chromosomes move toward the poles. The entire process ends with telephase 2 as the chromosomes decondence and the nuclear envelope reforms. Cytocinesis occurs and cleavage furrows separate the two daughter cells into four haploid daughter cells. The haploid daughter cells will specialize into gametes either sperm or egg. These fuse in fertilization to form a zygote which will grow into a child. The child receives half its chromosomes from its mother and half from its father. Because men and women produce millions of gametes and the selection of gametes in fertilization is random, this contributes to genetic diversity. This explains why a child isn't identical to either parent. But why aren't siblings identical? The diversity comes from several sources. In pro phase 1, non-sister chromatids can exchange DNA through crossing over, increasing the genetic diversity of individual chromatids. In addition to crossing over, in metaphase 1, the pairs of homologous chromosomes align independently along the metaphase plate and sort independently into the daughter cells. This process, called independent assortment, produces four genetically distinct haploid gametes. As the total number of chromosomes increases in an organism, the number of genetically distinct gametes increases by 2 to the nth power. This means an organism with n equals 3 can produce 8 unique gametes. For humans where n equals 23, there are 2 to the 23rd power unique gametes formed, or over a million different possible combinations. The combination of independent assortment, crossing over, and the random pairing of gametes during sexual reproduction increases genetic diversity, and explains why a child will not look exactly like his or her parents or siblings.