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


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Genetics, Nondisjunction

Samantha F. Gottlieb; Connor Tupper; Connor C. Kerndt; David H. Tegay.

Author Information

Introduction

The human being body is made up of trillions that somatic cells v the capacity to divide into similar daughter cells facilitating organismal growth, repair, and solution to the an altering environment. This process is dubbed “mitosis.” In gamete production, a different kind of cell department occurs referred to as “meiosis.” The outcome of meiosis is the production of four daughter cells, one of two people sperm or egg cells, v reduction department which outcomes in a haploid complement of chromosomes in every gamete. At fertilization, the haploid sperm cabinet nucleus merges v the haploid egg cell nucleus, which restores the diploid chromosomal complement and confirms the formation of the zygote. Throughout anaphase of the cell cycle, chromosomes room separated to opposite end of the cell to produce two daughter cells. Nondisjunction is the failure of the chromosomes to separate, i beg your pardon produces daughter cells v abnormal number of chromosomes. <1><2><3>


Cellular

The genome is encoded through the chemistry sequence that DNA nucleotides within our cells. In periods of cabinet growth, histone proteins approximately DNA room acetylated resulting in less interaction in between the DNA and also histone protein. This opened up DNA is called euchromatin and also allows transcriptional enzymes accessibility to the DNA. Prior to periods of cell division, the histone proteins are deacetylated allowing for the development of a condensed form of DNA dubbed heterochromatin. Somatic human being cells contain 23 combine chromosomes or 46 total chromosomes. Forty-six is thought about the “diploid” number (2n), when 23 is thought about the “haploid” number (1n) or fifty percent the diploid number. “Aneuploidy” describes the visibility of an abnormal number of chromosomes. Monosomy (n-1) is a kind of aneuploidy characterized by absent a single chromosome resulting in 45 complete chromosomes. Trisomy (n+1) is another type of aneuploidy that has an extra chromosome bring about 47 total chromosomes. Each form of aneuploidy can be attributed to nondisjunction during mitosis or meiosis. <4><5><6>


Mechanism

There are 2 components to the cabinet cycle: interphase and also mitosis/meiosis. Interphase have the right to be further subdivided into growth 1 (G1), synthesis (S), and growth 2 (G2). Throughout the G phases, the cell grows by developing various proteins, and during the S phase, the DNA is replicated so the each chromosome includes 2 identical sister chromatids.

Mitosis contains 4 phases: prophase, metaphase, anaphase, and also telophase. In prophase, the atom envelope breaks down and also chromatin condenses. In metaphase, the chromosomes line up follow me the metaphase plate, and microtubules connect to the kinetochores of every chromosome. In anaphase, the chromatids separate and are pulled by the microtubules come opposite end of the cell. Finally, in telophase, the atom envelopes reappear, the chromosomes unwind right into chromatin, and the cabinet undergoes cytokinesis, i beg your pardon splits the cell into 2 similar daughter cells.

Meiosis goes through all 4 phases that mitosis twice, through modified mechanisms the ultimately develop haploid cells rather of diploid. One modification is in meiosis I. Homologous chromosomes room separated rather of sisters chromatids, producing haploid cells. It is during this process where we see crossing over and independent assortment bring about the increased hereditary diversity that the progeny. Meiosis II progresses the same way as mitosis, however with the haploid number of chromosomes, ultimately developing 4 daughter cells every genetically distinctive from the original cell.

Nondisjunction can occur during anaphase of mitosis, meiosis I, or meiosis II. Throughout anaphase, sisters chromatids (or homologous chromosomes for meiosis I), will certainly separate and move to opposite poles the the cell, pulled by microtubules. In nondisjunction, the separation falls short to occur causing both sister chromatids or homologous chromosomes to be traction to one pole the the cell.

Mitotic nondisjunction have the right to occur due come the inactivation of either topoisomerase II, condensin, or separase. This will result in 2 aneuploid daughter cells, one through 47 chromosomes (2n+1) and also the other with 45 chromosomes (2n-1).

Nondisjunction in meiosis I occurs when the tetrads fail to separate throughout anaphase I. At the end of meiosis I, there will be 2 haploid daughter cells, one v n+1 and also the other with n-1. Both of these daughter cells will certainly then walk on to division once more in meiosis II, producing 4 daughter cells, 2 with n+1 and 2 v n-1.

Nondisjunction in meiosis II outcomes from the fail of the sister chromatids to separate during anaphase II. Because meiosis ns proceeded without error, 2 of the 4 daughter cell will have a normal enhance of 23 chromosomes. The other 2 daughter cells will certainly be aneuploid, one with n+1 and the other with n-1. 


Testing

In-utero, diagnosis that fetal chromosomal aneuploidy can be made by performing cytogenetic analysis of fetal cells, typically obtained v amniocentesis or chorionic villus sampling. The fetal chromosomal match is analyzed by performing a karyotype test, count the chromosomes, and analyzing under irradiate microscopy, all while in search of abnormalities in chromosomal number or structure. Countless prenatal screening tests exist to help carry out an age-adjusted risk of fetal chromosomal aneuploidy through analysis of various markers or cell-free fetal DNA in maternal serum. <7><8>

With in vitro fertilization (IVF), trial and error can additionally be performed prior to implantation with preimplantation hereditary diagnosis (PGD), polar human body diagnosis (PBD), or blastomere biopsy. PGD is a an approach used to determine normal embryos that will certainly be implanted into the mother, despite technologically demanding and with additional expense compared come prenatal diagnosis. PBD can detect maternally acquired aneuploidies and also is fairly quick to carry out when compared to PGD. Lastly, a blastomere biopsy have the right to be obtained before implantation for genetic analysis. However, blastomere biopsy places the arising embryo at higher risk and also therefore is not currently a recommended standard of practice.


Clinical Significance

Mitotic nondisjunction can reason somatic mosaicism, with the chromosome imbalance only reflected in the direct offspring the the original cell whereby the nondisjunction occurred. This can reason some creates of cancer, including retinoblastoma.

Meiotic nondisjunction is of higher clinical significance since most aneuploidies space incompatible with life. However, some will an outcome in viable offspring with a spectrum that developmental disorders.

Autosomal Trisomies

Patau syndrome: Trisomy of chromosome 13


Clinical Features: Rocker-bottom feet, microphthalmia (abnormally little eyes), microcephaly (abnormally little head), polydactyly, holoprosencephaly, slit lip and also palate, congenital heart disease, and severe intellectual disability. Life expectancy is seldom much longer than one year.
Clinical Features: Rocker-bottom feet, low set ears, micrognathia (abnormally tiny jaw), clenched hands with overlapping fingers, congenital heart disease, and severe intellectual disability. Life expectations is typically less than one year.
Clinical Features: solitary palmar crease, flat facies, influential epicanthal folds, duodenal atresia, congenital love disease, Hirschsprung disease, pundit disability. Notably boosted risk to build Alzheimer"s condition or leukemia. Life expectancy is around 60 years.
Clinical Features: Tall, long extremities, gynecomastia, woman hair distribution, testicular atrophy, developmental delay.
X chromosomes space inactivated as Barr bodies. Therefore, 2 extra Barr bodies room seen, though no clinical abnormalities result.

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Clinical Features: Unusually quick stature, shield chest, congenital love disease, webbed neck, horseshoe kidney, ovarian dysgenesis.