During oogenesis, the secondary oocytes are arrested during what phase of meiosis?
a) Prophase I
b) Metaphase II
c) Prophase II
d) Metaphase I
B is correct. Metaphase II. During the process of oogenesis, secondary oocytes are formed from primary oocytes during meiosis I. The secondary oocyte, as opposed to the polar body, is capable of continuing meiosis II to form the ovum. However, as soon as the secondary oocyte begins meiosis II, it is halted at metaphase II until fertilization occurs. If fertilization does not occur, the secondary oocytes will be shed.
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This MCAT post covers oogenesis, the process by which egg cells or ovum divide into a mature ovum, capable of further development after fertilization. In humans, this process begins during gestation, before the female is born, with stem cells called oogonia. Each oogonium (singular form of oogonia) is capable of undergoing mitosis, forming two identical daughter cells: another oogonium and a primary oocyte.
After the second oogonium and the primary oocyte are formed, the second oogonium will continue undergoing mitotic division, forming new daughter cells in order to replenish the stem cell population continuously. The primary oocyte, however, will undergo meiosis and will eventually form the ovum. It is important to note that the process of forming an ovum does not happen all at once. When the primary oocyte starts meiosis I, it will not complete it right away. Instead, it will halt the process during prophase I. In this way, when the female is born, she will have several primary oocytes that have halted development at prophase I of meiosis I.
Also, the process of the female forming a primary oocyte and oogonium, and the oogonium renewing itself only happens during gestation. Thus, once the female is born, the number of primary oocytes she has will not increase. In fact, the number decreases as the female grows due to the natural death cycle of the cells. Only when the female enters puberty will the primary oocytes finish meiosis I. In other words, the primary oocytes will complete meiosis I beginning at menarche or the first menstrual cycle. During every subsequent menstrual cycle once every month or about 28 days, more primary oocytes will complete meiosis I.
The primary oocyte is diploid throughout meiosis I, and when the homologous chromosomes eventually separate, two haploid cells are formed. Note, however, when the primary oocyte undergoes meiosis I, two identical daughter cells are not formed. Instead, because of an unequal division of the cytoplasm, one substantially smaller cell is formed called the polar body.
The polar body is essentially a nucleus with DNA and will be discarded. The other cell that is formed is the secondary oocyte. The secondary oocyte will retain a majority of the cytoplasm and will proceed to meiosis II. However, once it does so, it will halt once again during metaphase II. This same process will repeat every menstrual cycle. The primary oocytes will complete meiosis I to form secondary oocytes, and the secondary oocytes will begin meiosis II and halt development during metaphase II.
In order for the secondary oocytes to complete meiosis II, fertilization must occur. If fertilization does not happen, the secondary oocytes will simply be shed. If a sperm fertilizes the secondary oocyte, then meiosis II will finish, forming another polar body and a daughter cell. Like the previous polar body, this one consists of a nucleus with DNA and minimal cytoplasm and will be discarded. In the remaining daughter cell, now a mature ovum, genetic material from the sperm fuses with that of the ovum, thus forming a new diploid cell known as a zygote.
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