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You are watching: The failure of homologous chromosomes to separate properly is called

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 person body is consisted of of trillions of somatic cells through the volume to division into similar daughter cell facilitating organismal growth, repair, and solution to the transforming environment. This procedure is dubbed “mitosis.” In gamete production, a different kind of cell department occurs referred to as “meiosis.” The result of meiosis is the production of four daughter cells, either sperm or egg cells, through reduction division which outcomes in a haploid complement of chromosomes in every gamete. At fertilization, the haploid sperm cell nucleus merges with the haploid egg cell nucleus, which restores the diploid chromosomal complement and also confirms the formation of the zygote. Throughout anaphase of the cell cycle, chromosomes space separated to opposite ends of the cell to develop two daughter cells. Nondisjunction is the fail of the chromosomes to separate, i m sorry produces daughter cells through abnormal numbers of chromosomes. <1><2><3>


Cellular

The genome is encoded through the chemistry sequence the DNA nucleotides within our cells. In periods of cabinet growth, histone proteins roughly DNA are acetylated leading to less interaction between the DNA and also histone protein. This opened up DNA is called euchromatin and also allows transcriptional enzymes access to the DNA. Prior to periods of cabinet division, the histone proteins are deacetylated permitting for the formation of a condensed kind of DNA dubbed heterochromatin. Somatic human being cells save 23 combine chromosomes or 46 complete chromosomes. Forty-six is considered the “diploid” number (2n), while 23 is thought about the “haploid” number (1n) or fifty percent the diploid number. “Aneuploidy” refers to the presence of an abnormal variety of chromosomes. Monosomy (n-1) is a type of aneuploidy defined by absent a solitary chromosome causing 45 complete chromosomes. Trisomy (n+1) is another kind of aneuploidy that has an extra chromosome bring about 47 complete chromosomes. Each type of aneuploidy have the right to be attributed to nondisjunction during mitosis or meiosis. <4><5><6>


Mechanism

There space 2 parts to the cabinet cycle: interphase and also mitosis/meiosis. Interphase have the right to be additional subdivided into growth 1 (G1), synthesis (S), and also growth 2 (G2). During the G phases, the cabinet grows by producing various proteins, and during the S phase, the DNA is replicated so the each chromosome includes 2 the same sister chromatids.

Mitosis includes 4 phases: prophase, metaphase, anaphase, and telophase. In prophase, the atom envelope division down and also chromatin condenses. In metaphase, the chromosomes heat 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 to opposite end of the cell. Finally, in telophase, the nuclear envelopes reappear, the chromosomes unwind into chromatin, and also the cabinet undergoes cytokinesis, i beg your pardon splits the cell right into 2 the same daughter cells.

Meiosis goes v all 4 phases that mitosis twice, through modified mechanisms that ultimately create haploid cells instead of diploid. One modification is in meiosis I. Homologous chromosomes space separated instead of sisters chromatids, developing haploid cells. It is during this procedure where we see crossing over and independent assortment resulting in the increased hereditary diversity the the progeny. Meiosis II progresses the same method as mitosis, yet with the haploid variety of chromosomes, ultimately developing 4 daughter cells every genetically distinctive from the initial cell.

Nondisjunction have the right to occur during anaphase the mitosis, meiosis I, or meiosis II. Throughout anaphase, sister chromatids (or homologous chromosomes because that meiosis I), will separate and move to opposite poles the the cell, traction by microtubules. In nondisjunction, the separation fails to take place causing both sister chromatids or homologous chromosomes come be traction to one pole the the cell.

Mitotic nondisjunction can occur due come the inactivation of one of two people topoisomerase II, condensin, or separase. This will an outcome in 2 aneuploid daughter cells, one through 47 chromosomes (2n+1) and the various other with 45 chromosomes (2n-1).

Nondisjunction in meiosis I occurs when the tetrads fail come separate during anaphase I. At the finish of meiosis I, there will be 2 haploid daughter cells, one v n+1 and the various other with n-1. Both of this daughter cells will certainly then walk on to division once much more in meiosis II, creating 4 daughter cells, 2 through n+1 and also 2 through n-1.

Nondisjunction in meiosis II outcomes from the fail of the sister chromatids to separate during anaphase II. Because meiosis i proceeded there is no error, 2 of the 4 daughter cells will have a normal enhance of 23 chromosomes. The various other 2 daughter cells will certainly be aneuploid, one through n+1 and the other with n-1. 


Testing

In-utero, diagnosis that fetal chromosomal aneuploidy can it is in made by performing cytogenetic evaluation of fetal cells, typically obtained with amniocentesis or chorionic villus sampling. The fetal chromosomal match is analyzed by performing a karyotype test, counting the chromosomes, and analyzing under light microscopy, every while searching for abnormalities in chromosomal number or structure. Numerous prenatal screening tests exist to help administer an age-adjusted hazard of fetal chromosomal aneuploidy through evaluation of miscellaneous markers or cell-free fetal DNA in maternal serum. <7><8>

With in vitro fertilization (IVF), testing can likewise be performed prior to implantation v preimplantation hereditary diagnosis (PGD), polar body diagnosis (PBD), or blastomere biopsy. PGD is a method used to identify normal embryos that will be implanted right into the mother, though technologically demanding and also with added expense compared to prenatal diagnosis. PBD can detect maternally acquired aneuploidies and also is relatively quick to execute when compared to PGD. Lastly, a blastomere biopsy can be obtained before implantation for hereditary analysis. However, blastomere biopsy areas the occurring embryo at greater risk and also therefore is not currently a recommended typical of practice.


Clinical Significance

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

Meiotic nondisjunction is of better clinical significance due to the fact that most aneuploidies space incompatible v life. However, some will an outcome in viable offspring with a spectrum the developmental disorders.

Autosomal Trisomies

Patau syndrome: Trisomy that chromosome 13


Clinical Features: Rocker-bottom feet, microphthalmia (abnormally tiny eyes), microcephaly (abnormally small head), polydactyly, holoprosencephaly, cleft lip and palate, congenital heart disease, and also severe intellectual disability. Life expectancy is seldom much longer than one year.
Clinical Features: Rocker-bottom feet, low set ears, micrognathia (abnormally little jaw), clenched hands v overlapping fingers, congenital love disease, and also severe intellectual disability. Life span is usually less than one year.
Clinical Features: solitary palmar crease, flat facies, influential epicanthal folds, duodenal atresia, congenital heart disease, Hirschsprung disease, intellectual disability. Notably enhanced risk to develop Alzheimer"s disease or leukemia. Life expectations is around 60 years.
Clinical Features: Tall, long extremities, gynecomastia, woman hair distribution, testicular atrophy, developmental delay.
X chromosomes room inactivated as Barr bodies. Therefore, 2 extra Barr bodies space 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.