General question on Meiosis II and Diploid Number

[zut212]

Q1: How is it that after Meosis I, the daughter cells are now haploid? The way that I look at it is this: During Meosis I, the chromosones have already twice the amount of DNA, which are connected by centromeres. This is like just before prophase in mitosis. The chromosone looks like the letter "X", and there is a centrosome at the junction. This "X" is one chromosone with twice the amount of DNA. Anyways, during Meosis I, two "X"s get really close together. They exchange genetic material with one another during cross over at the chiasmata.

During separation, the "X" shaped double-DNA chromosone goes off to form two different daughter cells (albeit with very diverse genetics now). The daughter cells are now diploid still, I think. Please clarify.

Q2: Do all eukaryotes, which are healthy, have an even number of chromosones? If not, how do odd-numbered eukaryotes undergo crossover?

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[Silverfalcon]

Q1: How is it that after Meosis I, the daughter cells are now haploid? The way that I look at it is this: During Meosis I, the chromosones have already twice the amount of DNA, which are connected by centromeres. This is like just before prophase in mitosis. The chromosone looks like the letter "X", and there is a centrosome at the junction. This "X" is one chromosone with twice the amount of DNA. Anyways, during Meosis I, two "X"s get really close together. They exchange genetic material with one another during cross over at the chiasmata.

During separation, the "X" shaped double-DNA chromosone goes off to form two different daughter cells (albeit with very diverse genetics now). The daughter cells are now diploid still, I think. Please clarify.

I had similar question today, but I get it now.

Basically, think this way. Viewing as "X"s and stuffs is likely to make you confused so just realize that after S phase, there is a diploid cell. When it undergoes Meiosis I, it will produce two daughter cells of haploid because Meiosis I is a reductional division, meaning that ploidy becomes reduced. Meiosis II is like mitosis in the sense that ploidy does not change. Hence, Meiosis II is called equatorial division.

Not sure if it answered all of your questions for this one, but that's the simple logic behind Meiosis.

 

[docelh]

Q1. They have to be homologs to be considered diploids.

Q2. I have no idea what you're talking about.

 

[zut212]

I had similar question today, but I get it now.

Basically, think this way. Viewing as "X"s and stuffs is likely to make you confused so just realize that after S phase, there is a diploid cell. When it undergoes Meiosis I, it will produce two daughter cells of haploid because Meiosis I is a reductional division, meaning that ploidy becomes reduced. Meiosis II is like mitosis in the sense that ploidy does not change. Hence, Meiosis II is called equatorial division.

Not sure if it answered all of your questions for this one, but that's the simple logic behind Meiosis.

There is something that I sort of disagree with you on. Let's use somatic cells and MITOSIS as an example (not MEIOSIS). At the S stage of interphase, the number of chromosones stays the same, but the AMOUNT OF DNA HAS DOUBLED. The linear DNAs are attached at their "waist line". They resemble the letter "H" or "X". One "H" or "X" is two identical strands of DNA. The Chromosone is still diploid.

After G2, and during mitosis, the number of daughter cells has doubled, and the "X" or "H" shaped chromosones have become split at the centromeres. They are no longer "X" or "H" shaped anymore. The chromosones are now "I" shaped. The cells are still diploid!

Using this logic in Meiosis I, the "X" shaped diploid chromosones simply exchanged material, and the number of cells doubles. This is analgous to MITOSIS: We started with twice the amount of chromosones (all shaped like "X" or "H"). This was diploid. When the daughter cells formed, it was still diploid, but the chromosones no longer resemble "X" or "H", but instead, an "I" (but a very mixed up chromosone due to crossing over).

 

[Phantastic]

There is something that I sort of disagree with you on. Let's use somatic cells and MITOSIS as an example (not MEIOSIS). At the S stage of interphase, the number of chromosones stays the same, but the AMOUNT OF DNA HAS DOUBLED. The linear DNAs are attached at their "waist line". They resemble the letter "H" or "X". One "H" or "X" is two identical strands of DNA. The Chromosone is still diploid.

After G2, and during mitosis, the number of daughter cells has doubled, and the "X" or "H" shaped chromosones have become split at the centromeres. They are no longer "X" or "H" shaped anymore. The chromosones are now "I" shaped. The cells are still diploid!

Using this logic in Meiosis I, the "X" shaped diploid chromosones simply exchanged material, and the number of cells doubles. This is analgous to MITOSIS: We started with twice the amount of chromosones (all shaped like "X" or "H"). This was diploid. When the daughter cells formed, it was still diploid, but the chromosones no longer resemble "X" or "H", but instead, an "I" (but a very mixed up chromosone due to crossing over).

I had this problem forever. But you really have to listen here:

Tetrads (X shaped structure) are duplicated chromosomes (so they are sister chromatids). In Mitosis, every chromosome (so that means all 46) line up and each tetrad gets broken in two, pulling each sister to each new cell. We've maintained our ploidy number here, because each daughter has two of each chromosome.

In Meiosis I, homologous chromsomes consisting of sister tetrads line up on the metaphase plate. They look like X - X (as apposed to the X only of mitosis or meiosis II). These tetrads are kept together, but separated from each other in anaphase I. When these homologous tetrads get broken up into each daughter cell, necessarily each daughter cell will be haploid. This is because the tetrad that each contains merely contains two chromatids of the same original chromosome (haploid), even though there do exist 2 chromatids of the information. (Note, the reason that the two daughters that form from Meiosis II are genetically different even though they are from the same sister chromatid is due to crossing over (between X - X) of homologous chromosomes in Prophase I)


Give it some serious thought.

edit: How about an example. If you were born with trisomy 21 (Down's syndrome) usually caused by nondisjunction of chromsome 21 resulting in a gamete with 3 copies of chromosome 21, would you be triploid (3 chromosomes) or would you still be diploid?

 

[MCAT guy]

Give it some serious thought.

edit: How about an example. If you were born with trisomy 21 (Down's syndrome) usually caused by nondisjunction of chromsome 21 resulting in a gamete with 3 copies of chromosome 21, would you be triploid (3 chromosomes) or would you still be diploid?

your ploidy with respect to only chromosome 21 would be triploid, but your real ploidy would be diploid as your other 21 chromosomes have a diploid copy and the two sex chromosomes.

Chromosome 21: 3n
Chromoses 1-20,22: 2n
Sex: 2n

Correct?

 

[fizzgig]

Q1: How is it that after Meosis I, the daughter cells are now haploid? The way that I look at it is this: During Meosis I, the chromosones have already twice the amount of DNA, which are connected by centromeres. This is like just before prophase in mitosis. The chromosone looks like the letter "X", and there is a centrosome at the junction. This "X" is one chromosone with twice the amount of DNA. Anyways, during Meosis I, two "X"s get really close together. They exchange genetic material with one another during cross over at the chiasmata.

During separation, the "X" shaped double-DNA chromosone goes off to form two different daughter cells (albeit with very diverse genetics now). The daughter cells are now diploid still, I think. Please clarify.

Q2: Do all eukaryotes, which are healthy, have an even number of chromosones? If not, how do odd-numbered eukaryotes undergo crossover?

already covered but multiple explanations might help. i fought with this a bit too.

usually you have 23pairs, 46 chromosomes like this |
upon duplication, you get 23pairs, 46 chromosomes like this X
mitosis, they line up and get pulled to opposite ends, separating the X's at the centromeres like this <-| |->

-meiosis, you duplicate and start again with 23pairs, 46 chromosomes like X
-then, each PAIR gets together to do crossing over. now you have 23 rows and 2 columns of chromosomes (each row looks like X X, and is still 2 chromosomes, so you're still diploid). in mitosis before the split you'd have 1 column and 46 rows because homologues wouldn't be paired up side by side.

-now when the first split occurs, each X goes to a side like this <-X X->
remember the X X pair sitting there was NOT 'one chromosome' but two still. so when you pull them apart, you end up haploid after meoisis I. each daughter has 23 chromosomes, one newly mixed up homologue each, and that chromosome still looks like this X (still replicated, 2 chromatids).
-if this is confusing remember that each X is a chromosome and each pair of X's is getting split up, one to each daughter, thus haploid daughters.

-now meoisis 2 happens a lot like mitosis except with half the chromosomes. each X lines up in a vertical column and is physically split at the centromere, leaving you STILL with 23 chromosomes in the end, but now in the usual unduplicated form of |

 

[Silverfalcon]

There is something that I sort of disagree with you on. Let's use somatic cells and MITOSIS as an example (not MEIOSIS). At the S stage of interphase, the number of chromosones stays the same, but the AMOUNT OF DNA HAS DOUBLED. The linear DNAs are attached at their "waist line". They resemble the letter "H" or "X". One "H" or "X" is two identical strands of DNA. The Chromosone is still diploid.

Don't take this personally, but I disagree with you strongly on this. First of all, until prophase of mitosis, you don't even see the shape of DNA because they have yet to be condensed. That's why I dislike the term "X" or "H," as you say. Furthermore, I know that you mentioned somatic cells and hence, diploid as the ploidy, but there is really no need to go onto ploidy for mitosis. It's an equatorial process - ploidy does not change.

After G2, and during mitosis, the number of daughter cells has doubled, and the "X" or "H" shaped chromosones have become split at the centromeres. They are no longer "X" or "H" shaped anymore. The chromosones are now "I" shaped. The cells are still diploid!

The number of daughter cells does not double. What happens is that "the chromosomes duplicated during S phase." This is the correct phrase that I just got off from textbook. And again, with the whole deal about "shape," you don't even see any shape until the beginning of prophase when chromatin fibers become condensed into discrete chromosomes.

Using this logic in Meiosis I, the "X" shaped diploid chromosones simply exchanged material, and the number of cells doubles. This is analgous to MITOSIS: We started with twice the amount of chromosones (all shaped like "X" or "H"). This was diploid. When the daughter cells formed, it was still diploid, but the chromosones no longer resemble "X" or "H", but instead, an "I" (but a very mixed up chromosone due to crossing over).

You can't compare Mitosis and Meiosis I. This is absurd, actually. No textbook - or prep book (but I'll emphasize textbook as they are based on researches) - that I have read compares Mitosis to Meiosis I. They are very different, and the only logical comparison is between Mitosis to Meiosis II.