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StatPearls . Treasure Island (FL): StatPearls Publishing; 2021 Jan-.


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Introduction

The body is comprised of trillions of somatic cells v the volume to division into the same daughter cell facilitating organismal growth, repair, and an answer to the changing environment. This procedure is referred to as “mitosis.” In the gametes, a different type of cell division occurs dubbed “meiosis.” The result of meiosis is the production of daughter cells, either sperm or egg cells, with reduction department which results in a haploid complement of chromosomes so that on joining with another sex cabinet at fertilization a new diploid chromosomal complement is revived in the fertilized egg.<1><2><3>

Genomic diversity and also genetic variation is developed through the procedure of meiosis because of chromosomal recombination and independent assortment. Each daughter cell produced is gene half-identical to the of its parent cell yet distinctly various from its parent cell and also other daughter cells.<4><5>


Cellular

The genome is encoded by the chemical sequence the DNA nucleotides in ~ our cells. If extended from end to end, the DNA in one cell would certainly span about 3 meters. In order come fit right into each cell, the DNA is condensed by protein to create “chromatin,” a complex of DNA and also proteins. Somatic human cells save 23 combine chromosomes or 46 complete chromosomes. 46 is thought about the “diploid” number (2n), while 23 is thought about the “haploid” number (1n), or half the diploid number.<6><7>


Function

Meiosis is vital for developing genomic diversity in a species. It perfect this primarily through 2 processes: independent assortment and also crossing end (recombination).


The law of elevation assortment says that the random orientation that homologous chromosome pairs throughout metaphase I allow for the manufacturing of gametes with numerous different assortments the homologous chromosomes. For example, tetrads comprise chromosomes 1A/1B and also 2A/2B can produce 2 different variations in daughter cells: 1A2A, 1A2B, 1B2A, or 1B2B. V 46 cells in the human body, around 8 million different variations have the right to be produced.
Crossing over refers to a phenomenon that takes place during prophase I. As soon as homologous chromosomes come together to type tetrads, the arms of the chromatids deserve to swap at random, developing many much more possibilities for genetic variation of the gametes.

Mechanism

There space 2 parts to the cell cycle: interphase and mitosis/meiosis. Interphase have the right to be more subdivided into expansion 1 (G1), synthesis (S), and also Growth 2 (G2). Throughout the G phases, the cabinet grows by developing various proteins, and also during the S phase, the DNA is replicated so that each chromosome includes two identical sister chromatids (c). Mitosis has 4 phases: prophase, metaphase, anaphase, and also telophase. 

Mitosis


Metaphase: The chromosomes line up along the metaphase plate. Microtubules originating from the centrosomes in ~ the 2 the opposite poles the the cell connect to the kinetochores of every chromosome.
Telophase: The chromosomes gather at the poles the the cell, and the cabinet divides via cytokinesis creating 2 daughter cells. The atom envelope reappears, the spindle device disappears and the chromosomes de-condense earlier into chromatin.

The cell can now get in Interphase where it grows and replicates the DNA in preparation for division, yet again.

Meiosis goes with all 5 phases of the cell cycle twice, with modified mechanisms that ultimately produce haploid cells instead of diploid. In sperm cells, the masculine gametes, meiosis proceeds in the complying with manner:

Meiosis I


Prophase I: The nuclear envelope breaks down. The dyed condenses right into chromosomes. Homologous chromosomes comprise the two chromatids come together to kind tetrads, involvement at their centromeres (2n 4c). This is when “crossing over” occurs, which creates hereditary variation.
Metaphase I: The tetrads line up along the metaphase plate. Microtubules originating indigenous the centrosomes at the 2 the opposite poles that the cell connect to the kinetochores of each chromosome.
Telophase I: The chromosomes gather at the poles the the cell, and also the cell divides via cytokinesis creating 2 daughter cell (1n 2c). The nuclear envelope reappears, the spindle apparatus disappears and also the chromosomes de-condense back into chromatin.

Interkinesis/Interphase II 

There is a short pause between each round of meiosis providing time because that the cabinet to replenish proteins; however, over there is no S phase.

Meiosis II


Prophase II: In each of the daughter cells, a brand-new spindle device forms, the nuclear envelope division down, and the chromatin condenses right into chromosomes again.
Metaphase II: The chromosomes line up follow me the metaphase plate. Microtubules originating indigenous the centrosomes at the 2 the opposite poles of the cell attach to the kinetochores of each chromosome.
Anaphase II: sister chromatids separate and are pulled by the microtubules come opposite poles of the cell.
Telophase II: The chromosomes gather at the 2 poles that the cell and also the cell divides via cytokinesis creating 2 daughter cells (1n 1c) from each of the 2 cells from meiosis I. The nuclear envelope reappears, the spindle apparatus disappears and also the chromosomes de-condense back into chromatin.

In egg cells, the female gametes, meiosis complies with the same basic phases with just a slight variation. Throughout telophase I, the cytoplasm divides unequally, producing a bigger daughter cell and also a smaller sized polar body. The polar body and also the daughter cabinet both then get in meiosis II. In telophase II, the cytoplasm the the daughter cabinet again divides unequally and also creates a daughter cell and another polar body. In addition, the polar human body from meiosis i divides and forms 2 smaller sized polar bodies. After meiosis is completed, there is one daughter cabinet (1n, 1c) and 3 polar body (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 have the right to create many life-threatening outcomes. The most usual error of meiosis is nondisjunction, once chromatids fail to separate during either anaphase i or II, developing imbalances in the variety of chromosomes in each daughter cell. Most imbalances are incompatible v life, yet some will result in viable offspring v a spectrum of developmental disorders. This medical problems include under syndrome, Patau syndrome, Edwards syndrome, Klinefelter syndrome, Turner syndrome, Triple X syndrome, and XYY syndrome.

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