Freethought & Rationalism Archive

Hastur · 06-04-2002, 06:30 AM

Wait, wait, wait...we get polar bodies after fertilization?

Me be confuzzled...

scigirl · 06-04-2002, 07:49 AM

That's correct, Hastur.

lpetrich · 06-04-2002, 09:36 AM

Quote:

Originally posted by scigirl:
<strong>That's correct, Hastur.</strong>

I checked, and that's not the case. It's in the diagram earlier in the posting -- polar bodies are small "extra" egg cells that are discarded.

Hastur · 06-04-2002, 10:44 AM

Okay, that was my understanding. Meiosis gives us the 3 polar bodies and one 'live' egg.

scigirl · 06-04-2002, 10:50 AM

We are both correct. One of the polar bodies is formed after ovulation, the other one is formed after fertilization. In most species, the first cell division (i.e. meiosis I) produces one polar body and one "good" cell, and the polar body does NOT undergo further division. Then after fertilization, the "good" cell divides again (meiosis II), creating one more polar body.

Whew, this stuff is complicated! Hope that clears things up (Nick P should be forced to read this thread now!)

<a href="http://www.ultranet.com/~jkimball/BiologyPages/S/Sexual_Reproduction.html" target="_blank">http://www.ultranet.com/~jkimball/BiologyPages/S/Sexual_Reproduction.html</a>

Quote:

Oogenesis
Egg formation takes place in the ovaries.

In contrast to males, the initial steps in egg production occur prior to birth. By the time the fetus is 25 weeks old, all the oogonia that she will ever possess have been formed by mitosis. Hundreds of these diploid cells develop into primary oocytes, begin the first steps of the first meiotic division (meiosis I) and then stop.
No further development occurs until years later when the girl becomes sexually mature. Then the oocytes recommence their development, usually one at a time and once a month.

The primary oocyte grows much larger and completes the meiosis I, forming a large secondary oocyte and a small polar body that receives little more than one set of chromosomes. Which chromosomes end up in the egg and which in the polar body is entirely a matter of chance.

In humans (and most vertebrates), the first polar body does not go on to meiosis II, but the secondary oocyte does proceed as far as metaphase of meiosis II and then stops.

Only if fertilization occurs will meiosis II ever be completed. Entry of the sperm restarts the cell cycle.

scigirl

[ June 04, 2002: Message edited by: scigirl ]</p>

scigirl · 06-04-2002, 11:22 AM

While we are on the subject of meiosis. . .

This unusual way of "holding" eggs in place is thought to be the reason for most cases of trisomy, i.e. Down's syndrome. When the pairs of chromosomes line up and divide in metaphase, sometimes they won't divide right, and you get non-dysjunction. I.e. one cell gets none, and one cell gets both. If the cell with both copies of chromosome 21 gets fertilized, then it gets even one more copy from the sperm (hence trisomy) and in this case, Down's syndrome. Nondysjunction can occur in sperm cells as well, but I think the current theory is that most trisomies are from the oocyte (because of the way eggs develop).

This probably happens quite frequently, but since most trisomies are deadly to the fetus, the woman has a spontaneous miscarriage--often she won't even know she is pregnant.

Another thing about meiosis that is very important to us evolutionists: this is where recombination occurs. When the pairs of chromosomes are lined up during meiosis 1, oftentimes they will 'swap arms,' hence mixing the mom and dad's DNA. Sometimes they will swap unevenly, and genes will become duplicated in one chromosome (and lost in the other). Duplication of genes is a great source for evolution, because it gives the organism two sources for mutation. As long as one copy of the gene stays ok, the other is "allowed" to mutate. Our hemoglobin genes were created in this fashion.

scigirl

Morat · 06-04-2002, 12:47 PM

Did anyone actually point this out to our Linux guru in the thread? Because I'm curious to see what he'd say.

RufusAtticus · 06-04-2002, 02:05 PM

He dropped the conversation like a Hindu leper.

Quetzal · 06-04-2002, 10:20 PM

Just a quibble scigirl: crossing over actually occurs during the prophase of meiosis I. The second big genetic shuffling occurs during metaphase/anaphase of meiosis I - when the homologous chromosomes randomly migrate to the two poles (even though both poles get one of each chromosome, which SPECIFIC chromosome goes to which pole is random). Both crossing over and randomization occur during gametogenesis in both egg and sperm. Finally, the last bit of shuffling occurs during actual fertilization since there's no way to predict which particular sperm fertilizes the egg (which is itself a random selection from the pool of available eggs).

So there are three different processes in meiosis that generate some of the variability upon which selection operates. The cretinists always seem to miss this part with their utter fixation on mutation. Mutation just generates the novel genes. Normal recombination and reshuffling during meiosis provides a whole LOT of variety - and cross-over can even provide novel genes IIRC.

[edited for clarity]

[ June 04, 2002: Message edited by: Morpho ]</p>

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06-04-2002, 06:30 AM	#11
Hastur Regular Member Join Date: Jan 2002 Location: Carcosa Posts: 238	Wait, wait, wait...we get polar bodies after fertilization? Me be confuzzled...

06-04-2002, 07:49 AM	#12
scigirl Veteran Member Join Date: Jul 2001 Location: Seattle Posts: 4,261	That's correct, Hastur.

06-04-2002, 10:44 AM	#14
Hastur Regular Member Join Date: Jan 2002 Location: Carcosa Posts: 238	Okay, that was my understanding. Meiosis gives us the 3 polar bodies and one 'live' egg.

06-04-2002, 11:22 AM	#16
scigirl Veteran Member Join Date: Jul 2001 Location: Seattle Posts: 4,261	While we are on the subject of meiosis. . . This unusual way of "holding" eggs in place is thought to be the reason for most cases of trisomy, i.e. Down's syndrome. When the pairs of chromosomes line up and divide in metaphase, sometimes they won't divide right, and you get non-dysjunction. I.e. one cell gets none, and one cell gets both. If the cell with both copies of chromosome 21 gets fertilized, then it gets even one more copy from the sperm (hence trisomy) and in this case, Down's syndrome. Nondysjunction can occur in sperm cells as well, but I think the current theory is that most trisomies are from the oocyte (because of the way eggs develop). This probably happens quite frequently, but since most trisomies are deadly to the fetus, the woman has a spontaneous miscarriage--often she won't even know she is pregnant. Another thing about meiosis that is very important to us evolutionists: this is where recombination occurs. When the pairs of chromosomes are lined up during meiosis 1, oftentimes they will 'swap arms,' hence mixing the mom and dad's DNA. Sometimes they will swap unevenly, and genes will become duplicated in one chromosome (and lost in the other). Duplication of genes is a great source for evolution, because it gives the organism two sources for mutation. As long as one copy of the gene stays ok, the other is "allowed" to mutate. Our hemoglobin genes were created in this fashion. scigirl

06-04-2002, 12:47 PM	#17
Morat Senior Member Join Date: Jun 2000 Location: Houston, Texas Posts: 932	Did anyone actually point this out to our Linux guru in the thread? Because I'm curious to see what he'd say.

06-04-2002, 02:05 PM	#18
RufusAtticus Veteran Member Join Date: Nov 2001 Location: NCSU Posts: 5,853	He dropped the conversation like a Hindu leper.

06-04-2002, 10:20 PM	#19
Quetzal Senior Member Join Date: Nov 2001 Location: Ecuador Posts: 738	Just a quibble scigirl: crossing over actually occurs during the prophase of meiosis I. The second big genetic shuffling occurs during metaphase/anaphase of meiosis I - when the homologous chromosomes randomly migrate to the two poles (even though both poles get one of each chromosome, which SPECIFIC chromosome goes to which pole is random). Both crossing over and randomization occur during gametogenesis in both egg and sperm. Finally, the last bit of shuffling occurs during actual fertilization since there's no way to predict which particular sperm fertilizes the egg (which is itself a random selection from the pool of available eggs). So there are three different processes in meiosis that generate some of the variability upon which selection operates. The cretinists always seem to miss this part with their utter fixation on mutation. Mutation just generates the novel genes. Normal recombination and reshuffling during meiosis provides a whole LOT of variety - and cross-over can even provide novel genes IIRC. [edited for clarity] [ June 04, 2002: Message edited by: Morpho ]</p>

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