Meiosis vs. Mitosis Chromosome count

[MedHopeful234]

I'm confused on the chromosome count on meiosis vs. mitosis

In mitosis: Prior to duplication we have 48 chromosomes----> DNA is duplicated in the S phase where each chromosome (single chromatid) becomes two sister chromatids (96 chromosomes)----> after mitosis (telophase) we have 48 chromosomes in two cells each with one chromatid

Meisosis: Prior to duplication we have 48 chromosomes ----> DNA duplicated in S phase where each chromosome (single chromatid) is now two sister chromatids (96) ----> meiosis I (telophase) is 48 chromosomes ----> meiosis II telophase is 23 chromosomes

My understanding of meiosis is wrong... apparently it's supposed to be 46 chromosomes prior to duplication---> 46 chromosomes AFTER duplication (WHY/HOW? 😵) ----> meiosis I leaves 46 (AGAIN HOW?) ----> meiosis II leaves 23 .....

This is driving me crazy... any thoughts would be amazing


Also, follow up on a Dat destroyer bio Q: If an organism has diploid number of 18, how many chromatids are visible at teh end of mitotic prophase...

The answer is 36. I got it correctly, but I want to make sure... this means that there is 36 CHROMOSOMES TOO correct? So 18 would be 2n and 36 would be 4n after DNA duplication.

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[LaughingGas]

I tried...

 

[MedHopeful234]

So wait... for Mitosis... do we have 4n after duplication or 2n like in meiosis (where # of chromatids double but not # of centromeres)....

It just seems like in Mitosis we actually duplicate the DNA but in meiosis we do not which doesn't make sense as to why there is an S phase

 

[customx]

LaughingGas - thank you so much. I've got my DAT in 12 days and this helps a lot.

 

[LaughingGas]

 

[MedHopeful234]

Hey Laughing Gas,

Thanks so much! you're awesome.

I just want to make sure though... In Interphase for mitosis, I specifically remember that 2n does becomes 4n... so 48 chromosomes become 96 before division... (this was actually a question in destroyer I believe)

What I don't understand is why this isn't the same for Meoisis.... Why does 2n stay 2n if the chromosomes are duplicated?

 

[LaughingGas]

Sorry, you are right. It becomes 4n or 2*2n as different textbooks apply. Sorry for the mistake.

 

[KGflyboy]

Replication does NOT increase the number of chromosomes. It increases the amount of DNA by making copies of the 46 chromosomes, but you still only have 46 chromosomes. The 4n refers to the number of copies DNA (both homologous and identical), therefore sister chromatids are counted when referencing the "n" number of a cell. However, sister chromatids produced by replication do NOT count as chromosomes. The more complete nomenclature would look like this, although it is often not written out the long way:

Pre-replication: 46/2n
Post replication: 46/4n

 

[MedHopeful234]

So before Meiosis AND Mitosis: 2n/46 chromosomes
After replication: 4n/46 chromosomes
Before Telophase (telophase I in meiosis) : 4n/92 chromosomes
After telophase (telophase I in meiosis) : each cell has 2n with 46 chromosomes again

 

[KGflyboy]

So before Meiosis AND Mitosis: 2n/46 chromosomes
After replication: 4n/46 chromosomes
Before Telophase (telophase I in meiosis) : 4n/92 chromosomes
After telophase (telophase I in meiosis) : each cell has 2n with 46 chromosomes again

Almost... the part about 92 chromosomes is off. There are never more than 46 in the stereotypical somatic cell. 92 copies of the DNA? Yes. 92 chromosomes? No.

At that point in the cycle (start of telophase I in meiosis), you have separated homologous chromosomes (still 46 in the cell even though each chromosome consists of 2 sister chromatids). 23 go into one cell, and 23 go into the other at the conclusion of meiosis I. At the conclusion of telophase I and cytokinesis, the cells each have 23 chromosomes (even though each chromosome consists of 2 sister chromatids).

Each daughter cell at this point would have: 23 chromosomes; 46 chromatids
Each daughter cell after meiosis II would have 23 chromosomes (same as above); 23 chromatids


I never realized how confusing this is, and it's mostly because people use terms in different ways. Some people use the 2n or n designation to refer to number of chromosomes. The professor referenced at the start of the thread confused you (and me at first) because he or she is using the n designation to refer to any copy of DNA, whether it is a chromosome or not (sister chromatid). I realize this can be a source of frustration because it could result in a missed test question due to the difference. I learned that the n number represented chromosomes (right or wrong is moot at this point). However, this professor teaches that the n number refers to total amount or quantity of DNA strands. Regardless of how n number is defined, the number of chromosomes is 46 in the typical somatic cell before AND after DNA replication because the new strands of DNA produced are referred to as chromatids, not chromosomes. So, as weird as it sounds, replication changes the amount of DNA but not the number of chromosomes. It essentially just makes each chromosome twice as big.

 

[KGflyboy]

I doubt those help enough to be worth the time it took to create, but oh well. I wanted to point out the number of chromosomes along the way as well as the "n" number. The n number is were I (and your previous teachers) differ from your professor. 2n/4n stuff.

 

[MedHopeful234]

Thanks so much KGflyboy! You're awesome 🙂

We are saying the same thing I realized. I think the distinction I realized was that I was focusing on chromosome count BEFORE Telophase occurs, as in during anaphase when the chromosomes are separated but NOT yet in separate cells... hence it would be 2x the chromosome count and DNA count... but after telophase it would be just 1x the chromosome count and DNA count

 

[KGflyboy]

You are welcome.

However, even at the particular phase you refer to, you still would not have twice the number of chromosomes you started with. You still have 46 (23 from mama and 23 from daddy), which is decreased to 23 by the formation of the new nuclei in Telephase I. You have 2x the DNA, but the same number of chromosomes as a normal cell.

The 92 number that popped up earlier in the thread and referred to chromosomes was not quite accurate, but it would have been if it used the word "chromatid."

To further complicate things, don't forget that when learning the cell cycle we speak of diploid cells because they are the norm, but we actually have many cells in our body that do in fact have more (some heart cells for example), but a typical body cell never increases beyond 46 at any point in the cycle despite replication, anaphase, telephase, etc.

 

[MedHopeful234]

Hey,
So I read about this a bit more and I think my confusion was on the fact that in meiosis you have homologous chromosomes that separate in Meiosis I (after tetrad formation and crossing over) and then the chromatids separate in Meiosis II whereas in mitosis you have the chromatids from the homologous chromosomes separating individually.

You are correct in that there is not 96 chromosomes in anaphase I of meiosis, but I've seen multiple sources (and a test question from bootcamp that specifically states the chromosomes (NOT chromatids) to be 96 (4n) during anaphase of MITOSIS).... This is because the nuclear membranes have not yet formed and the cells have not gone through cytokinesis to fully separate. The reason this is not the case in meiosis I is that in meiosis I you separate the actual homologous chromosomes and not the chromatids, so the chromosome number doesn't "double". However in meiosis II you do separate the chromatids and thus observe the same "doubling" effect as you do in mitosis.

Overall here is what I figured out: (assume 2n is chromosome number and not DNA)
Example: 2n= 46

Mitosis:
Before Duplication: 2n (single chromatid as a single chromosome)
After duplication: 2n (duplicate chromatids as a single chromosome)
Interphase = 2n
Prophase = 2n
Metaphase = 2n
Anaphase = 4n
Telophase = 4n
Cytokenisis = 2n
After cytokinesis there are two daughter cells, each of which has 2n chromosomes

Meiosis:
Before Duplication: 2n (single chromatid as single chromosome)
After Duplication: 2n (duplicate chromatid as a single chromosome)
Interphase I= 2n
Metaphase I= 2n Anaphase = 2n
Telophase I= 2n
Cytokenisis I = n
After cytokinesis 1 there are two daughter cells, each of which has n chromosomes with 2 chromatids each.

Metaphase 2 = n
Anaphase 2 = 2n
Telophase 2 = 2n
Cytokenisis 2 = n
After cytokinesis 2 there are four daughter cells, each of which has n chromosomes with only 1 chromatid each.

You are welcome.

However, even at the particular phase you refer to, you still would not have twice the number of chromosomes you started with. You still have 46 (23 from mama and 23 from daddy), which is decreased to 23 by the formation of the new nuclei in Telephase I. You have 2x the DNA, but the same number of chromosomes as a normal cell.

The 92 number that popped up earlier in the thread and referred to chromosomes was not quite accurate, but it would have been if it used the word "chromatid."

To further complicate things, don't forget that when learning the cell cycle we speak of diploid cells because they are the norm, but we actually have many cells in our body that do in fact have more (some heart cells for example), but a typical body cell never increases beyond 46 at any point in the cycle despite replication, anaphase, telephase, etc.

 

[KGflyboy]

That all looks really accurate and makes sense. I never really consider the chromatids in anaphase or mitosis to be chromosomes, even after they are pulled apart, but I can see why some would. Conventional texts consider them to no longer be chromatids and be known as full fledged chromosomes after the completion of the whole cycle when the cytoplasm has split to form 2 cells (thus bypassing a stage of 92 chromosomes within a single cell, but I suppose it's just semantics at that point. My guess about the reason many will not consider them chromosomes yet is because they are identical copies and not genetically distinct from the "other half." Once the cytoplasm separates, there is no other half, thus they represent a distinct chromosome in the daughter cell as opposed to when they lacked a distinct genetic identity in the anaphase cell.

I can see and agree with your logic behind the conclusion that the cell has 92 within that time though. Interesting.

 

[BeyonceFan1226]

Hey everyone! I am having trouble understanding the graphics above and I was wondering if anyone can confirm if the information below is correct:

Meosis I: separation of homologous chromosomes
Interphase: Start with 23 chromosomes/46 chromatids and this duplicates to 46 chromosomes/92 chromatids
Telophase: You end with 23 chromosomes/46 chromatids

Meosis II: separation of sister chromatids
No Interphase in Meosis II
- Start with 23 chromosomes/46chromatids
-End with 23 chromosomes/23 chromatids

*Nondisjunction can occur in Anaphase I or Anaphase II?
*DownSyndrome is due specifically to nondisjunction in Anaphase II?

 

[MoeC]

Hey everyone! I am having trouble understanding the graphics above and I was wondering if anyone can confirm if the information below is correct:

Meosis I: separation of homologous chromosomes
Interphase: Start with 23 chromosomes/46 chromatids and this duplicates to 46 chromosomes/92 chromatids
Telophase: You end with 23 chromosomes/46 chromatids

Meosis II: separation of sister chromatids
No Interphase in Meosis II
- Start with 23 chromosomes/46chromatids
-End with 23 chromosomes/23 chromatids

*Nondisjunction can occur in Anaphase I or Anaphase II?
*DownSyndrome is due specifically to nondisjunction in Anaphase II?

http://www.auburn.edu/academic/classes/zy/vert_embryo/html/PloidyandMeiosis.html

Hope this helps

 

[FeralisExtremum]

Hey everyone! I am having trouble understanding the graphics above and I was wondering if anyone can confirm if the information below is correct:

Meosis I: separation of homologous chromosomes
Interphase: Start with 23 chromosomes/46 chromatids and this duplicates to 46 chromosomes/92 chromatids
Telophase: You end with 23 chromosomes/46 chromatids

Interphase comes before actual meiosis, so I'd probably list that first for clarity. By the end of interphase you've duplicated your DNA, so starting in meiosis I you have 46 chromosomes with 92 chromatids. By the end of meiosis I, you have two cells: each has 23 chromosomes with 46 chromatids. Overall, what you have looks correct.

Meosis II: separation of sister chromatids
No Interphase in Meosis II
- Start with 23 chromosomes/46chromatids
-End with 23 chromosomes/23 chromatids

I'm not sure if no interphase in meiosis II is correct - but there is no S phase, which means we don't duplicate the DNA again. The rest of what you have here looks correct. Starting where left off in meiosis I, we have 2 cells, each has 23 chromosomes w/ 46 chromatids. By the end of meiosis II we have 4 cells: each has 23 chromosomes with 23 chromatids.

*Nondisjunction can occur in Anaphase I or Anaphase II?
*DownSyndrome is due specifically to nondisjunction in Anaphase II?

Yes, nondisjunction can occur during anaphase I or II. Down Syndrome can occur due to nondisjunction in Anaphase I or II - either way could potentially give you a gamete with an extra chromosome 21.