first aid and ploidy

[xx216xx]

Advertisement - Members don't see this ad

Ok, so i know this isn't that important to know for step but this is driving me crazy. First aid shows that spermato and oogenesis go through a 4N phase. This is completely wrong is it not? Meiosis doesn't go through a tetraploid stage does it? And on top of that when the cell is in meiosis 1 the chromatid number is 92 right, not 46? 46*2=92 Am i the only one that thinks first aid completely screwed these up or am i ******ed? It should be: meiosis I= 46 chroms at 2N with 92 chromatids....meiosis II: 23 chroms at N with 46 chromatids...right?

 

[Sol Rosenberg]

You said it yourself -- after the DNA is duplicated, there are 4N chromatids, so meiosis does go through a 4N phase.

However, I think you are correct about the chromosome vs. chromatid distinction. There are 46 chromosomes, but 92 chromatids. Meiosis I (Anaphase I, in particular) is the only stage that reduces the number of chromosomes in the cell(s).

When using the "N" notation, a lot of sources (incl. FA, apparently) aren't always precise about what they are talking. Usually it's just a shorthand for the [amount of] DNA content of the cell.

I also agree that this degree of precision isn't likely to be very important for Step 1 🙂

 

[kunfu]

Ok, so i know this isn't that important to know for step but this is driving me crazy. First aid shows that spermato and oogenesis go through a 4N phase. This is completely wrong is it not? Meiosis doesn't go through a tetraploid stage does it? And on top of that when the cell is in meiosis 1 the chromatid number is 92 right, not 46? 46*2=92 Am i the only one that thinks first aid completely screwed these up or am i ******ed? It should be: meiosis I= 46 chroms at 2N with 92 chromatids....meiosis II: 23 chroms at N with 46 chromatids...right?

Your forgetting a key point, BEFORE going into meiosis 1 the cell passes through S phase which doubles its chromosome content hence the 4N, once it divides its back to 2N.

 

[kunfu]

Also notice that is says SISTER chromatids. This essentially means 92 chromatids like you were saying.

One other point between Meiosis 1 and Meiosis II DNA does not duplicate. It only duplicates before Meiosis 1.

 

[xx216xx]

i think first aid and i are just having a communication problem even though we're talking about the same thing

I see now that it says 46 sister chromatids so that would be 92 chromatids, but I still disagree in labeling a diploid cell as 4N. The cell is never 4N. That is calling it tetraploid, which never happens, you never have 4 sets of 23 chromosomes. Before S phase you have 2 sets of 23 chromosomes and after S phase you still have 2 sets of 23 chromosomes, the only thing that changes is that you doubled the amount of DNA. This is all really irrelevant for step most likely but it bugs the cell bio major in me.