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Nondisjunction

Nondisjunction

During meiosis, chromosomes occasionally separate improperly. This occurs due to failure of homologous chromosome separation during meiosis I or failed sister chromatid separation during meiosis II. In some species, notably plants, nondisjunction can result in an organism with an entire additional set of chromosomes, which is called polyploidy. In humans, nondisjunction can occur during male or female gametogenesis and the resulting gametes possess one too many or one too few chromosomes.

Nondisjunction

Nondisjunction

Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate correctly and move to the opposite poles of the cells. This produces daughter cells with abnormal chromosome numbers. Nondisjunction is common during anaphase I or anaphase II of meiosis. Mutations in synaptonemal complex proteins that attach homologous chromosomes increase the chances of nondisjunction in anaphase I of meiosis I. In contrast, mutations in topoisomerases and condensins that hold...

Meiosis I

Meiosis I

Meiosis is a carefully orchestrated set of cell divisions, the goal of which—in humans—is to produce haploid sperm or eggs, each containing half the number of chromosomes present in somatic cells elsewhere in the body. Meiosis I is the first such division, and involves several key steps, among them: condensation of replicated chromosomes in diploid cells; the pairing of homologous chromosomes and their exchange of information; and finally, the separation of homologous chromosomes by...

Meiosis I

Meiosis I

Meiosis is the division of a diploid cell into haploid cells forming sperm and eggs in animals through differentiation. Meiosis I is the first stage of meiosis, where the genetic recombination of homologous chromosomes and the reduction of the ploidy level by half occurs.
Prophase I is the most extended and complex step of meiosis I characterized by synapsis, chromosome pairing, and recombination of the homologous chromosomes. This process is facilitated by a proteinaceous structure called the...

Gene Conversion

Gene Conversion

Other than maintaining genome stability via DNA repair, homologous recombination plays an important role in diversifying the genome. In fact, the recombination of sequences forms the molecular basis of genomic evolution. Random and non-random permutations of genomic sequences create a library of new amalgamated sequences. These newly formed genomes can determine the fitness and survival of cells. In bacteria, homologous and non-homologous types of recombination lead to the evolution of new...

Meiosis II

Meiosis II

Meiosis II entails cell division and segregation of the sister chromatids, resulting in the production of four unique haploid gametes. The steps for meiosis II are similar to mitosis, except that meiosis II occurs in haploid cells, whereas mitosis occurs in diploid cells.
The timing and cell division patterns of meiosis differ between males and females. In male meiosis, the centrosomes are part of the formation of the meiotic spindle. However, in oocytes, including that of humans, Drosophila,...

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Related Experiment Video

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Preparation of Meiotic Chromosome Spreads from Mouse Oocytes for Assessment of Synapsis and Recombination

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Published on: July 18, 2025

Nested Inversion Polymorphisms Predispose Chromosome 22q11.2 to Meiotic Rearrangements.

Wolfram Demaerel¹, Matthew S Hestand¹, Elfi Vergaelen¹

¹Department of Human Genetics, Katholieke Universiteit Leuven, Leuven, Belgium.

American Journal of Human Genetics

|October 3, 2017

Summary

Chromosomal inversions near low-copy repeats (LCRs) predispose individuals to 22q11.2 deletion syndrome (22q11.2DS) by increasing meiotic rearrangements. These inversions are key risk factors for transmitting genomic disorders.

Keywords:

22q11.2 deletion syndrome 22q11.2DS DiGeorge syndrome Genomic disorder VCFS fiber-FISH inversion polymorphism low-copy repeats microdeletion segmental duplications

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Published on: February 23, 2011

Area of Science:

Genetics
Genomic Medicine
Molecular Biology

Background:

Genomic disorders often arise from non-allelic homologous recombination (NAHR) between low-copy repeats (LCRs).
Inversion polymorphisms within LCRs are suspected to predispose chromosomes to NAHR, but have not been identified for 22q11.2 deletion syndrome (22q11.2DS).

Purpose of the Study:

To investigate the role of inversion polymorphisms in the etiology of 22q11.2DS.
To determine if inversions predispose chromosome 22 to meiotic rearrangements.

Main Methods:

Fiber-FISH (Fluorescence In Situ Hybridization) was employed to analyze chromosomal structures.
Parental transmission of de novo 22q11.2 rearrangements was studied.

Main Results:

Inversions of LCR22B-D or LCR22C-D were identified in parents transmitting 22q11.2DS rearrangements.
These inversions were found to be nested or flanking the rearranged regions, not coinciding with deletion/duplication sizes.
The presence of inversions increases the risk of meiotic rearrangements and transmission of these events.

Conclusions:

Inversions within LCRs are a significant risk factor for 22q11.2DS.
Inversions may be a prerequisite for NAHR-mediated genomic disorders beyond 22q11.2DS.