Early during pregnancy, developing oocytes (eggs) in female fetuses are arrested
ID: 94274 • Letter: E
Question
Early during pregnancy, developing oocytes (eggs) in female fetuses are arrested in Prophase I. Resumption of development does not occur until a woman enters puberty, and even then, only a few eggs finish development each month. At the age of 42, how long has a woman's remaining eggs been stalled in Prophase I? During both Anaphase I and Anaphase II of Meiosis, chromosomes are separated by a process called disjunction. What types of errors can occur at this time and what are the consequences? How might your answers to the two questions above be related to the increased incidence of Down syndrome among children born to older women?Explanation / Answer
1. The eggs (oogonia) remain in prophase 1 (meiosis 1) only till the time puberty is reached. During puberty the oestrogen levels surge that trigger the increase in LH and FSH hormone leading to release of oocytes (eggs). Thereon, every month during the ovulation cycle eggs are released. However not all eggs mature at same time. It has been found that certain eggs can remain in the dormant stage upto 50 years. Therefore we can say that the given age of 42, the women’s eggs have remained in prophase for almost the same number of years i.e. 42 years.
2. During Anaphase I and Anaphase II of Meiosis, when homologous chromosomes or sister chromatids do not separate, this type of error is known as nondisjunction. When nondisjunction occurs in meiosis I, out of the four gametes form two are (n+1) and the other two are (n-1).
When the gamete with (n+1) fuses with a normal gamete (n) a trisomic error (2n+1) occurs. Similarly when (n-1) fuses with normal gamete, a monosomic error occurs (2n-1).
When nondisjunction occurs in meiosis II, we have three types of gemetes : 1 (n+1) : 1 (n-1) : 2 (n).
In this case when each of them fuse with a normal gamete we have trisomic , monosomic and normal diploid conditions respectively.
3.Down’s syndrome occurs mostly (90-95% cases) due to nondisjunction of the autosome number 21. At the age of 42, the ova ( eggs) have been dormant for about 42 years. Cells which have been arrested in the meiotic division stage for a longer period are susceptible to damage from environmental factors such as x-ray, chemicals and viruses which interfere with normal cell division. Therefore, the chances of older women giving birth to children with Down’s syndrome increase with age.
Related Questions
drjack9650@gmail.com
Navigate
Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.