The body is made up of trillions of somatic cells with the capacity to divide into identical daughter cells facilitating organismal growth, repair, and response to the changing environment. This process is called “mitosis.” In the gametes, a different form of cell division occurs called “meiosis.” The outcome of meiosis is the creation of daughter cells, either sperm or egg cells, through reduction division which results in a haploid complement of chromosomes so that on joining with another sex cell at fertilization a new diploid chromosomal complement is restored in the fertilized egg.
Genomic diversity and genetic variation is produced through the process of meiosis due to chromosomal recombination and independent assortment. Each daughter cell created is genetically half-identical to that of its parent cell yet distinctly different from its parent cell and other daughter cells.
The genome is encoded by the chemical sequence of DNA nucleotides within our cells. If stretched from end to end, the DNA in one cell would span about 3 meters. In order to fit into each cell, the DNA is condensed by proteins to create “chromatin,” a complex of DNA and proteins. Somatic human cells contain 23 paired chromosomes or 46 total chromosomes. 46 is considered the “diploid” number (2n), while 23 is considered the “haploid” number (1n), or half the diploid number.
Meiosis is important for creating genomic diversity in a species. It accomplishes this primarily through 2 processes: independent assortment and crossing over (recombination).
There are 2 parts to the cell cycle: interphase and mitosis/meiosis. Interphase can be further subdivided into Growth 1 (G1), Synthesis (S), and Growth 2 (G2). During the G phases, the cell grows by producing various proteins, and during the S phase, the DNA is replicated so that each chromosome contains two identical sister chromatids (c). Mitosis contains 4 phases: prophase, metaphase, anaphase, and telophase.
The cell can now enter Interphase where it grows and replicates its DNA in preparation for division, yet again.
Meiosis goes through all 5 phases of the cell cycle twice, with modified mechanisms that ultimately create haploid cells instead of diploid. In sperm cells, the male gametes, meiosis proceeds in the following manner:
There is a brief pause between each round of meiosis providing time for the cell to replenish proteins; however, there is no S phase.
In egg cells, the female gametes, meiosis follows the same general phases with only a slight variation. During telophase I, the cytoplasm divides unequally, creating a larger daughter cell and a smaller polar body. The polar body and the daughter cell both then enter meiosis II. In telophase II, the cytoplasm of the daughter cell again divides unequally and creates a daughter cell and another polar body. In addition, the polar body from meiosis I divides and forms 2 smaller polar bodies. After meiosis is completed, there is one daughter cell (1n, 1c) and 3 polar bodies (1n 1c). The polar bodies disintegrate as they do not have enough cytoplasm and proteins to survive as gametes.
Clinically, errors in meiosis can create many life-threatening outcomes. The most common error of meiosis is nondisjunction, when chromatids fail to separate during either anaphase I or II, creating imbalances in the number of chromosomes in each daughter cell. Most imbalances are incompatible with life, but some will result in viable offspring with a spectrum of developmental disorders. These medical conditions include Down syndrome, Patau syndrome, Edwards syndrome, Klinefelter syndrome, Turner syndrome, Triple X syndrome, and XYY syndrome.
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