Trisomy 13 is caused predominantly (89% of the time) by a non-disjunction during maternal meiosis which results in the ovum (egg) inheriting two copies of the 13th chromosome from the mother.[1]
Maternal meiosis is shown in Figure 2.1.[2]
Maternal meiosis involves the division of cells in order to eventually form an ovum.
This division is quite similar to that of a mitotic cell division, the cell division occurs in all somatic (body) cells. Both processes undergo the same stages of Prophase, Metaphase, Anaphase, and Telophase, however a meiotic division undergoes part of this process twice. A meiotic division undergoes meiosis I and meiosis II in order to more either four sperm cells or one ovum.[3]
The difference between mitosis and meiosis however is the separation during anaphase I. During this stage the homologous chromosomes separate, forming two new daughter cells. These cells in maternal meiosis are called the Secondary Oocyte and the First Polar Body. The secondary oocyte continues on to the next meiotic division, whilst the first polar body degenerates. The major difference between mitosis and meiosis is evident here as the replicated chromatid or sister chromatid is still connected.[3]
Meiosis II is where these cells separate again to form an Ootid and a Second Polar Body. The second polar body degenerates just as the first polar body did, whilst the ootid goes on to become an ovum or egg.[3]
What is important in this stage is that the chromosomes separate, resulting in a haploid amount of chromosome within the ovum.
Maternal meiosis is shown in Figure 2.1.[2]
Maternal meiosis involves the division of cells in order to eventually form an ovum.
This division is quite similar to that of a mitotic cell division, the cell division occurs in all somatic (body) cells. Both processes undergo the same stages of Prophase, Metaphase, Anaphase, and Telophase, however a meiotic division undergoes part of this process twice. A meiotic division undergoes meiosis I and meiosis II in order to more either four sperm cells or one ovum.[3]
The difference between mitosis and meiosis however is the separation during anaphase I. During this stage the homologous chromosomes separate, forming two new daughter cells. These cells in maternal meiosis are called the Secondary Oocyte and the First Polar Body. The secondary oocyte continues on to the next meiotic division, whilst the first polar body degenerates. The major difference between mitosis and meiosis is evident here as the replicated chromatid or sister chromatid is still connected.[3]
Meiosis II is where these cells separate again to form an Ootid and a Second Polar Body. The second polar body degenerates just as the first polar body did, whilst the ootid goes on to become an ovum or egg.[3]
What is important in this stage is that the chromosomes separate, resulting in a haploid amount of chromosome within the ovum.
Non-disjunction during this process refers to the failure of a cell to separate equally. Resulting in an additional 13th chromosome to be present within either the Secondary Oocyte and/or the Ootid, depending upon the stage at which the error occurred.
Below is a Youtube video which details exactly what occurs during a non-disjunctional meiotic division[4].
Below is a Youtube video which details exactly what occurs during a non-disjunctional meiotic division[4].
REFERENCE LIST:
[1] Bugge, M., Collins, A., Hertz, J., Eiberg, H., Lundsteen, C., Brandt, C., Bak, M., Hansen, C., deLozier, C., Lespinasse, J., Tranebjaerg, L., Hahnemann, J., Rasmussen, K., Bruun-Petersen, G., Duprez, L., Tommerup, N. and Petersen, M. (2007). Non-disjunction of chromosome 13. Human Molecular Genetics, [online] 16(16), pp.2004-2010. Available at: http://hmg.oxfordjournals.org/content/16/16/2004.full [Accessed 11 Aug. 2014].
[2] Facweb.furman.edu, (n.d.). Hereditary, Gene Regulation, and Development. [online] Available at: http://facweb.furman.edu/~wworthen/bio111/meiosis.htm [Accessed 11 Aug. 2014].
[3] University of Illinois at Chicago, (2004). Cell Division, Mitosis, and Meisosis. [online] Available at: http://www.uic.edu/classes/bios/bios100/lecturesf04am/lect16.htm [Accessed 11 Aug. 2014].
[4] YouTube, (2013). Visualizing Nondisjunction. [video] Available at: https://www.youtube.com/watch?v=nQ3Rjh661X8 [Accessed 11 Aug. 2014].
[1] Bugge, M., Collins, A., Hertz, J., Eiberg, H., Lundsteen, C., Brandt, C., Bak, M., Hansen, C., deLozier, C., Lespinasse, J., Tranebjaerg, L., Hahnemann, J., Rasmussen, K., Bruun-Petersen, G., Duprez, L., Tommerup, N. and Petersen, M. (2007). Non-disjunction of chromosome 13. Human Molecular Genetics, [online] 16(16), pp.2004-2010. Available at: http://hmg.oxfordjournals.org/content/16/16/2004.full [Accessed 11 Aug. 2014].
[2] Facweb.furman.edu, (n.d.). Hereditary, Gene Regulation, and Development. [online] Available at: http://facweb.furman.edu/~wworthen/bio111/meiosis.htm [Accessed 11 Aug. 2014].
[3] University of Illinois at Chicago, (2004). Cell Division, Mitosis, and Meisosis. [online] Available at: http://www.uic.edu/classes/bios/bios100/lecturesf04am/lect16.htm [Accessed 11 Aug. 2014].
[4] YouTube, (2013). Visualizing Nondisjunction. [video] Available at: https://www.youtube.com/watch?v=nQ3Rjh661X8 [Accessed 11 Aug. 2014].