Question

When does the second meiotic division take place in the egg?

Answer 1

Hint: Oogenesis is the process by which a mature female gamete (ovum) is formed. The oogonia or female gamete mother cells undergo meiosis to produce an ovum. This meiotic process is stopped one time after the formation of secondary oocytes after completion of meiosis-I, i.e., in the meiosis-II process.
The process of oogenesis is initiated during embryonic development itself. During this stage, a couple of million oogonia are formed within each fetal ovary. This process then comes to a temporary halt after child birth and is again resumed during the onset of puberty.

Complete answer:
The primary oocyte within the tertiary follicle grows in size and completes its first meiotic division at puberty. This is an unequal division and leads to the formation of the secondary oocyte and a polar body.
This secondary oocyte when enters the second meiotic division, is arrested at prophase-II of meiosis-II at the diplotene substage. This condition remains until and unless the ovum is fertilized by a sperm in the fallopian tube.
During fertilization, when the sperm enters into the ovum, then the meiosis-II is resumed. The entry of sperm into the ovum induces completion of the meiotic division of the secondary oocyte. Entry of sperm causes breakdown of metaphase promoting factor (MPF) and turns on anaphase promoting complex (APC). This results in completion of meiosis-II. The second meiotic division is also unequal and results in the formation of a second polar body and a haploid ovum. Soon the haploid nucleus of the sperm and that of ovum fuse together to form a zygote.

Note:
Sometimes, the chromosomes may not get separated completely during meiosis-II and this results in genetic disorders. Also, some students have a thinking that the twins are always born because the 2nd polar body released during meiosis-II also gets fertilized. But that is absolutely not the case. The twins are born in 90% of the cases because more than one ovum reaches maturity in the menstrual cycle and get fertilized. These twins are fraternal and not maternal.
In the remaining 10% cases, the zygote breaks into two parts just prior to implantation. As the cells are totipotent, so they form two fetuses and twins are born.