11.5:
Nondisjunction
Although human meiosis is regulated, errors, like nondisjunction, arise that yield aneuploid cells with absent or extra chromosomes.
Many nondisjunction incidents happen during meiosis I. Although the mechanisms remain unclear, this may result from mutations affecting how homologous chromosomes are initially joined, such as those disrupting the synaptonemal complex. Defects in the spindle apparatus can also be a contributing factor.
During anaphase I, homologous sets normally separate, or disjoin, and are dragged towards opposite poles. However, rarely, a chromosome pair fails to detach, and both are pulled to one end.
When meiosis ends, this type of nondisjunction may yield two cells that possess an additional chromosome and two that lack this structure.
Similarly, during meiosis II, sister chromatids may remain affixed during anaphase II. This may also result from spindle problems, or from cohesion defects, like how the centromere attaches chromatids.
As division finishes, such errors in meiosis II can produce two normal haploid cells. However, one cell with an extra and one with a lost chromosome are also generated.
11.5:
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.
When an abnormal gamete fuses with a normal gamete, the resulting zygote has an abnormal number of chromosomes and is called aneuploid. An individual with one too few chromosomes has monosomy (45; 2n-1), while trisomy is the presence of one too many chromosomes for a total of 47 (2n+1). Down Syndrome is one well-studied trisomy, where individuals have three copies of chromosome 21. Aneuploid zygotes account for around 70% of spontaneous abortions during gestation.
Nondisjunction is more common in sex chromosomes than autosomes. Individuals can have a variety of sex chromosome combinations, including one or more additional sex chromosomes (e.g., XXY, XXX, XYY) or the presence of only a single sex chromosome (denoted X0). These individuals tend to have normal lifespans, though with sometimes major physiological and reproductive consequences. Nondisjunction appears to be more common when homologous chromosomes fail to recombine. Interestingly X and Y chromosomes normally undergo less recombination compared to autosomes, perhaps explaining the frequency of nondisjunction in sex chromosomes. Mutations in synaptonemal complex proteins, which attach homologous chromosomes, reduce crossing over but apparently increase nondisjunction. This suggests that proper chromosome recombination is an important step in normal meiosis.
Nondisjunction is more frequent during oogenesis than during spermatogenesis. Postzygotic nondisjunction, a failure of mitotic chromatid separation in the early zygote, causes similar consequences to that of meiotic nondisjunction and accounts for around 2% of Down Syndrome cases. Mitotic nondisjunction is also a hallmark of many human cancers.
Suggested Reading
Jones, Keith T., and Simon I. R. Lane. “Molecular Causes of Aneuploidy in Mammalian Eggs.” Development 140, no. 18 (September 15, 2013): 3719–30. [Source]
Hawley, R. Scott. “Human Meiosis: Model Organisms Address the Maternal Age Effect.” Current Biology 13, no. 8 (April 15, 2003): R305–7. [Source]
Wenzel, Elizabeth S., and Amareshwar T. K. Singh. “Cell-Cycle Checkpoints and Aneuploidy on the Path to Cancer.” In Vivo 32, no. 1 (January 1, 2018): 1–5. [Source]