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I am reading the answer and I am getting confused by the sentence:

At the end of meiosis I females have two daughter cells and meiosis II only occurs if and when fertilization occurs by a sperm cell.

The clause suggests me that meiosis II starts after the sperm has ignited the fertilization process in female: I do not know how that exactly works.

Wikipedia says:

The process of fertilization involves a sperm fusing with an ovum.

So female meiosis II must happen before the sperm fuses with the ovum. Sperm must ignite some process in female that puts female meiosis II going on before sperm can fuse with egg. Is that right? How is that going to happen?

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Does this mean that the sperm fertilises a cell with 46 chromosomes instead of 23? –  user3633 May 23 '13 at 2:16

3 Answers 3

up vote 5 down vote accepted

Primary oocytes are formed prenatally and reain suspended in prophase of meiosis I for years until the onset of puberty. An oocyte completes meiosis I as its follicle matures (during ovulation) resulting in a secondary oocyte and the FIRST polar body. After ovulation, each oocyte continues to metaphase of meiosis II. Meiosis II is completed only if fertilization occurs, resulting in a fertilized mature ovum and the second polar body.

So in short, the egg is stuck in metaphase II until fertilization.

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Do you mean that the egg is stuck in metaphase II already before the sperm comes? - Does the egg remain in metaphase II a couple of days each month? –  Masi Feb 8 '12 at 22:53
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Yes, the secondary oocyte stays in metaphase II throughout ovulation until fertilization. Upon fertilization, the ovum and the third polar body form. If no fertilization occurs, the secondary oocyte (still in meiosis II) is expelled via menstruation. –  jp89 Feb 9 '12 at 1:24
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I am considering know which follicle is at metaphase II here biology.stackexchange.com/questions/1205/… –  Masi Feb 27 '12 at 16:23

Untill puberty the follicles grown from primordial to antral stage (secondary folliccle) and oocytes are arrested in diplotene of profase I of meiosis I, without polar body, with a nucleous called germinative vesicle. After puberty, with gonadotrophis, they can grow more and ovulate. With LH surge they are stimulated to continuing meiosis and they lose the nucleous and became metafase I oocytes (MI) and extrude the first polar body to became secondary oocytes (MII oocytes) just before ovulation. So, the Graaf folicle, last stage of follicular development, has a secondary oocyte arrested in metafase II. Only after ovulation and with fecundation it is possible to escape from MII arrest and extrude the second polar body.

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Meiosis, as you know, have two stages, Meiosis I and II. The oocyte is arrested during metaphase II of MEOISIS II. This arrest is facilitated by a complex called "Cytostatic Factor" (CSF).

After fertilization, the sperm induces a rise in intracellular calcium ion which activates and enzyme, Calmodulin Kinase II. This complex, through a series of phosphorylation and ubiquitination, degrades the CSF comples and in turn, activates APC (Anaphase Promoting Complex). APC will then degrades cyclins, securins and this will promotes the completion of Meiois II.

Sperm must ignite some process in female that puts female meiosis II going on before sperm can fuse with egg.

I think the statement is a bit incorrect. The sperm will ignite (I prefer induce) the above changes and fuse at the same time. Because, what starts all the process above is part of the sperm's cytoplasm that need to be assimilated into the oocyte's cytoplasm.

I hope this clears it all up. Or if you are interested, I can provide the long list of signalling pathways that leads to zygotic development.

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