Cell cycle & Meiosis

[pgoyal]

Okay players,

I am having trouble finding this on google & SDN (i must be using the wrong search terms or something). Or I could be wayyyyy off on this topic.

When exactly/what stage of cell cycle does Meiosis occur.

I know it goes G1-->S-->G2-->M(mitosis) but when does meiosis occur. But before we start throwing around PMATs and **** please read the following and tell me if I am wrong.

I understand that meiosis only occurs in germ cells. Further I realize that for females this happens before birth and for males this takes place for life (spermatagonium --> spermatids).

So when during the cell cycle does this meiosis take place? when/how are spermatagoniums formed continuously for life? via mitosis of Type(Ad) cells and then spontaneous differentiation of these cells leads to the meiosis pathway and subsequent formation of spermatids? if so, then for germ cells the cell cycle would be G1-->S-->G2-->M(mitosis)-->M(meiosis)-->G0?

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[rfvbnmju]

Okay players,

I am having trouble finding this on google & SDN (i must be using the wrong search terms or something). Or I could be wayyyyy off on this topic.

When exactly/what stage of cell cycle does Meiosis occur.

I know it goes G1-->S-->G2-->M(mitosis) but when does meiosis occur. But before we start throwing around PMATs and **** please read the following and tell me if I am wrong.

The first cell division in meiosis is the actual meiosis where the chromosomal number reduction occurs due to homologous chromosomes lining up.

The second cell division in meiosis (Metaphase II, Anaphase II etc..) is just like regular mitosis where sister chromatids are separated.

I understand that meiosis only occurs in germ cells. Further I realize that for females this happens before birth and for males this takes place for life (spermatagonium --> spermatids).

For females, by the time birth occurs, the eggs called primary oocyte are frozen in Prophase I. They start the meiosis I but are halted. This is right before the chromosomal reduction of meiosis occurs.
When puberty hits, during each ovarian cycle, the primary oocyte undergoes meiosis I to become the secondary oocyte. This is due to FSH if I recall correctly.
The secondary oocyte is then frozen at Metaphase II. Only when the egg is fertilized does the secondary oocyte go through Anaphase II, Telephase II etc..

I just remember Prophase I and Metaphase II as the two check points where division stops until something else starts it again.

So when during the cell cycle does this meiosis take place? when/how are spermatagoniums formed continuously for life? via mitosis of Type(Ad) cells and then spontaneous differentiation of these cells leads to the meiosis pathway and subsequent formation of spermatids? if so, then for germ cells the cell cycle would be G1-->S-->G2-->M(mitosis)-->M(meiosis)-->G0?

Spermagonium are formed by seminiferus tubes. They undergo mitosis to give you your primary spermacyte. These primary spermacyte goes through meiosis and give you your 4 spermatid.

Hope that helps.

 

[pgoyal]

so if a question asked to draw the cell cycle of a female/male gamete that stops dividing would you draw it as:

a. G1-->S-->G2-->Meosis-->Cytokenesis-->G0 OR
b. G1-->S-->G2-->Mitosis-->Meosis-->Cytokenesis-->G0

 

[pgoyal]

so if a question asked to draw the cell cycle of a female/male gamete that stops dividing would you draw it as:

a. G1-->S-->G2-->Meosis-->Cytokenesis-->G0 OR
b. G1-->S-->G2-->Mitosis-->Meosis-->Cytokenesis-->G0

 

[sazerac]

It would be drawn as G1-->meiosis or maybe G1-->S-->meiosis, depending on the somewhat arbitrary human definition of when meiosis actually begins.

I get what you are asking, and the fact that you are thinking about it suggests that you understand the concept well enough for the MCAT (and for medical school for that matter).

If you are asking as idle speculation, I will try to humor you. Meiosis is one way. There is nothing cyclical about it, so asking when it happens in the "cell cycle" doesn't make much sense. One thing is for sure, G2 implies that the cell is going to do mitosis and not meiosis, so there won't be any G2 in the meiotic path. The cyclins and other cellular signals (perhaps the true definition of where we are in the "cell cycle") are totally different.

 

[pgoyal]

ah I see. unfortunately as an engineering major bio is my weakness...but thanks for the words of encouragement

so to clarify it is the sperm stem cells (aka spermatagonium?) that undergo the cell cycle and thus mitosis, however once this stem cell decides to differentiate into a spermatocyte it goes to G1-->S/Meosis (essentially no longer part of the traditional "cell cycle" definition")

Same thing for neurons? Do neural stem cells undergo canonical cell cycle and once they start differentiating they exit into G0?

 

[rfvbnmju]

Yes what you said sounds right. The spermagonium will have to undergo mitosis to ensure there are more stem cells. Other wise you'd run our off stem cells. Some of these stem cells, receiving the appropriate signal will undergo meiosis to make the spermatid.

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[quechua]

so if a question asked to draw the cell cycle of a female/male gamete that stops dividing would you draw it as:

a. G1-->S-->G2-->Meosis-->Cytokenesis-->G0 OR
b. G1-->S-->G2-->Mitosis-->Meosis-->Cytokenesis-->G0

Technically it would be the same as mitosis. So the answer is A.

A female gamete is arrested IN meiosis I at birth. (I think it is arrested at metaphase I actually.) consider like a really long meiosis. It won't undergo cytokinesis if meiosis isn't complete... Because cytokinesis is the physical cell division itself.

Also remember that the cell does need to get more cytoplasm, duplicate its spindle apparatus, duplicate its DNA to 4N before it begins meiosis I.

 

[pgoyal]

Also remember that the cell does need to get more cytoplasm, duplicate its spindle apparatus, duplicate its DNA to 4N before it begins meiosis I.

followup questions:

1. does the spindle apparatus duplicate in S phase also like DNA? (partly lazy to google, partly offering you a chance to show off your knowledge ;-)
2. I believe duplication of DNA during S phase is still labeled as 2N, not 4N, right? i dont think the cell ever gets to 4N - barring some mutational effect, please confirm.

 

[quechua]

followup questions:

1. does the spindle apparatus duplicate in S phase also like DNA? (partly lazy to google, partly offering you a chance to show off your knowledge ;-)
2. I believe duplication of DNA during S phase is still labeled as 2N, not 4N, right? i dont think the cell ever gets to 4N - barring some mutational effect, please confirm.

1. No. Technically the spindle apparatus does not form until prophase... But growth of the cell and protein synthesis, many of which that will construct the spindle apparatus, are formed during G1. Organelles will replicate during G2.

2. Yes, that was a mistake on my part. It's only 2N. Sorry about that.