Introduction

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.[1][2][3]

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.[4][5]

Cellular

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.[6][7]

Function

Meiosis is important for creating genomic diversity in a species. It accomplishes this primarily through 2 processes: independent assortment and crossing over (recombination).

1. The law of independent assortment states that the random orientation of homologous chromosome pairs during metaphase I allow for the production of gametes with many different assortments of homologous chromosomes. For example, tetrads containing chromosomes 1A/1B and 2A/2B can create 2 different variations in daughter cells: 1A2A, 1A2B, 1B2A, or 1B2B. With 46 cells in the human body, about 8 million different variations can be produced.
2. Crossing over refers to a phenomenon that takes place during prophase I. When homologous chromosomes come together to form tetrads, the arms of the chromatids can swap at random, creating many more possibilities for genetic variation of the gametes.

Mechanism

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. 

Mitosis

• Prophase: The nuclear envelope breaks down. The chromatin condenses into chromosomes.
• Metaphase: The chromosomes line up along the metaphase plate. Microtubules originating from the centrosomes at the 2 opposite poles of the cell attach to the kinetochores of each chromosome.
• Anaphase: Chromatids separate and are pulled by microtubules to opposite ends of the cell.
• Telophase: The chromosomes gather at the poles of the cell, and the cell divides via cytokinesis forming 2 daughter cells. The nuclear envelope reappears, the spindle apparatus disappears and the chromosomes de-condense back into chromatin.

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:

Meiosis I

• Prophase I: The nuclear envelope breaks down. The chromatin condenses into chromosomes. Homologous chromosomes containing the two chromatids come together to form tetrads, joining at their centromeres (2n 4c). This is when “crossing over” occurs, which creates genetic variation.
• Metaphase I: The tetrads line up along the metaphase plate. Microtubules originating from the centrosomes at the 2 opposite poles of the cell attach to the kinetochores of each chromosome.
• Anaphase I: Homologous chromosomes are separated by the microtubules to opposite poles of the cell.
• Telophase I: The chromosomes gather at the poles of the cell, and the cell divides via cytokinesis forming 2 daughter cells (1n 2c). The nuclear envelope reappears, the spindle apparatus disappears and the chromosomes de-condense back into chromatin.

Interkinesis/Interphase II 

There is a brief pause between each round of meiosis providing time for the cell to replenish proteins; however, there is no S phase.

Meiosis II

• Prophase II: In each of the daughter cells, a new spindle apparatus forms, the nuclear envelope breaks down, and the chromatin condenses into chromosomes again.
• Metaphase II: The chromosomes line up along the metaphase plate. Microtubules originating from the centrosomes at the 2 opposite poles of the cell attach to the kinetochores of each chromosome.
• Anaphase II: Sister chromatids separate and are pulled by the microtubules to opposite poles of the cell.
• Telophase II: The chromosomes gather at the 2 poles of the cell and the cell divides via cytokinesis forming 2 daughter cells (1n 1c) from each of the two cells from meiosis I. The nuclear envelope reappears, the spindle apparatus disappears and the chromosomes de-condense back into chromatin.

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.

Clinical Significance

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.