36.6:
Nondisjunction
During meiosis I and II, errors like nondisjunction cause chromosomes to fail to separate, producing aneuploid daughter cells with absent or extra chromosomes.
Nondisjunction occurs at anaphase I or II of meiosis.
During anaphase I, homologous chromosomes separate and get dragged towards opposite poles. If a chromosome pair fails to detach, both are pulled to one end.
Nondisjunction may result from mutations in the proteins of the synaptonemal complex that holds the chromosome together, asymmetric spindle fibers, or misorientation of chromosomes.
As a result, four haploid daughter cells are formed, two with an additional chromosome and two lacking a chromosome.
Similarly, during anaphase II of meiosis II, sister chromatids may remain affixed and move to the same pole.
Such nondisjunction arises from defects in topoisomerase II or condensins that are essential for chromosome separation.
These errors yield four haploid daughter cells, two with a standard set of chromosomes and two aneuploids.
36.6:
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 sister chromatids together promote nondisjunction during anaphase II of meiosis II.
Nondisjunction of chromosomes in germ cells results in gametes possessing additional or fewer chromosomes than normal. Nondisjunction is more frequent during oogenesis than during spermatogenesis. When a gamete with abnormal chromosomes fertilizes a gamete with a normal chromosome number, the resulting zygote has an abnormal number of chromosomes or aneuploidy. Such aneuploid zygotes can have fewer chromosomes than normal, leading to monosomy (45; 2n-1), or more chromosomes than normal, leading to trisomy (47; 2n+1). Some females lack one of the X chromosomes, a typical case of monosomy (45, X), and develop Turner Syndrome. In other instances, individuals who develop Down Syndrome have trisomy with three copies of chromosome 21.
Suggested Reading
- Jones, K. T., & Lane, S. I. (2013). Molecular causes of aneuploidy in mammalian eggs. Development, 140(18), 3719-3730.
- Hawley, R. S. (2003). Human meiosis: model organisms address the maternal age effect. Current Biology, 13(8), R305-R307.
- Wenzel, E. S., & Singh, A. T. (2018). Cell-cycle checkpoints and aneuploidy on the path to cancer. In Vivo, 32(1), 1-5.