mitosis/meiosis help

[user12]

I figure this might be a little better than searching the web since most of you guys are pretty smart and know your stuff.

So, supposing you have 46 chromosomes, like a human... when exactly does the diploid number change to haploid in meiosis... is it after anaphase I or after telophase I? and how many chromatids do you have in each cell after telophase I.... 46? so then at anaphase II you will have 46 chromosomes total? and with crossing over, the sister chromatids aren't identical, so you dont have 2 "sets" since set implies that the two sisters are identical. and then 23 randomly go to each daughter cell? and it is true that in mitosis, at anaphase you have 92 chromosomes?

this is all I want to know. I will appreciate any answers or insight you can give.

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[Czarcasm]

n

I figure this might be a little better than searching the web since most of you guys are pretty smart and know your stuff.

So, supposing you have 46 chromosomes, like a human... when exactly does the diploid number change to haploid in meiosis... is it after anaphase I or after telophase I? and how many chromatids do you have in each cell after telophase I.... 46? so then at anaphase II you will have 46 chromosomes total? and with crossing over, the sister chromatids aren't identical, so you dont have 2 "sets" since set implies that the two sisters are identical. and then 23 randomly go to each daughter cell? and it is true that in mitosis, at anaphase you have 92 chromosomes?

this is all I want to know. I will appreciate any answers or insight you can give.

Meiosis I is reductional division (splitting of homologous chromosome pairs), Meiosis II is equational division (splitting of recombinant chromatids). The reductional division occurs in Meiosis I, specifically Anaphase I. Remember "I Pee on MAT" (Interphase followed by Meosis: Prophase, Metaphase, Anaphase, Telophase). Metaphase is where alignment occurs (they line up in the middle), but anaphase is where the actual splitting occurs and at this point they're considered to be haploid. We're splitting the homologous chromosomes so we go from 2n to n, but remember each individual chromsome was in replicated form prior to meiosis (sister chromatids). As for your second question, "how many sister chromatids" after division, some would debate this answer. Personally, I would say zero because at metaphase, recombination occurs and so technically (although each chromosome is still in replicated form), you wouldn't refer to them as sister chromatids. Others though, would say 46 (I suppose you could just say chromatids, although I still have issues with that). And finally, by the end of meosis, you have 4 cells, each with an unreplicated amount of n number of chromosomes.

The max amount of chromsomes we'd have at any given point is 46 chromsomes (before metaphase of meiosis or mitosis). You would never consider replicated chromosomes (that is, sister chromatids) as twice as many chromosomes. So for instance, after interphase when each of the 46 chromsomes is replicated (having sister chromatids), it would be wrong to say we have 92 chromosomes! This is something most biology professors reiterate time and time again, so you have to be very careful. You don't count replicated chromosomes as additional chromosomes. It's the same amount, just in replicated form. Might seem confusing, but this is how it is and if anyone says otherwise, it's simply wrong.

 

[user12]

nMeiosis I is reductional division (splitting of homologous chromosome pairs), Meiosis II is equational division (splitting of recombinant chromatids). The reductional division occurs in Meiosis I, specifically Anaphase I. Remember "I Pee on MAT" (Interphase followed by Meosis: Prophase, Metaphase, Anaphase, Telophase). Metaphase is where alignment occurs (they line up in the middle), but anaphase is where the actual splitting occurs and at this point they're considered to be haploid. We're splitting the homologous chromosomes so we go from 2n to n, but remember each individual chromsome was in replicated form prior to meiosis (sister chromatids). As for your second question, "how many sister chromatids" after division, some would debate this answer. Personally, I would say zero because at metaphase, recombination occurs and so technically (although each chromosome is still in replicated form), you wouldn't refer to them as sister chromatids. Others though, would say 46 (I suppose you could just say chromatids, although I still have issues with that). And finally, by the end of meosis, you have 4 cells, each with an unreplicated amount of n number of chromosomes.

The max amount of chromsomes we'd have at any given point is 46 chromsomes (before metaphase of meiosis or mitosis). You would never consider replicated chromosomes (that is, sister chromatids) as twice as many chromosomes. So for instance, after interphase when each of the 46 chromsomes is replicated (having sister chromatids), it would be wrong to say we have 92 chromosomes! This is something most biology professors reiterate time and time again, so you have to be very careful. You don't count replicated chromosomes as additional chromosomes. It's the same amount, just in replicated form. Might seem confusing, but this is how it is and if anyone says otherwise, it's simply wrong.

I appreciate the help. but the thing with the 92 chromosomes.... if you consider mitosis. you start with 2 pairs of 23 chromosomes, you will then have 2 pairs of 23 sets of sister chromatids (92 chromatids total). when the chromatid is separated from its sister, it is then a chromosome, right? so after anaphase and before the cell divides, technically there would be 92 chromosomes in a single cell, for a very small amount of time. I don't understand why you cant count them as chromosomes. I agree that they aren't genetically different chromosomes, as in they aren't giving anything new that you didn't have before replications, but they are still chromosomes. but I doubt the mcat would ever go this far.

 

[Czarcasm]

I appreciate the help. but the thing with the 92 chromosomes.... if you consider mitosis. you start with 2 pairs of 23 chromosomes, you will then have 2 pairs of 23 sets of sister chromatids (92 chromatids total). when the chromatid is separated from its sister, it is then a chromosome, right? so after anaphase and before the cell divides, technically there would be 92 chromosomes in a single cell, for a very small amount of time. I don't understand why you cant count them as chromosomes. I agree that they aren't genetically different chromosomes, as in they aren't giving anything new that you didn't have before replications, but they are still chromosomes. but I doubt the mcat would ever go this far.

I also had the same concern as you when I learned this, but again, that's just something my professors have always reiterated time and time again. In mitosis, all replicated chromosomes are lined up in one big line at metaphase. At anaphase, they are pulled apart and so for that fleeting instant where they are separated, we wouldn't consider the cell having 96 chromosomes but rather, still 46 chromosomes (that's how it was taught to me; some review books say otherwise but I never had problems with my approach). Similarly, in meiosis (where homologous) chromosomes are lined up in pairs, and after anaphase, we'd have 23 chromosomes. The only instance where the number 92 might come up is if they ask how many chromatids are present at metaphase and they want to see if you understand what a chromatid is. This is probably the testing point they'd be more interested in - anything beyond that is probably way too debatable.

 

[Oh_Gee]

I also had the same concern as you when I learned this, but again, that's just something my professors have always reiterated time and time again. In mitosis, all replicated chromosomes are lined up in one big line at metaphase. At anaphase, they are pulled apart and so for that fleeting instant where they are separated, we wouldn't consider the cell having 96 chromosomes but rather, still 46 chromosomes (that's how it was taught to me; some review books say otherwise but I never had problems with my approach). Similarly, in meiosis (where homologous) chromosomes are lined up in pairs, and after anaphase, we'd have 23 chromosomes. The only instance where the number 92 might come up is if they ask how many chromatids are present at metaphase and they want to see if you understand what a chromatid is. This is probably the testing point they'd be more interested in - anything beyond that is probably way too debatable.

could you help with a good summary. i always get confused on this

mitosis: 2n-96 chromatids,46 chromosomes (interphase, which after replication) >>> 2n-46 chromatids, 46 chromosomes(after anaphase)
meiosis: 2n-96 chromatids,46 chromosomes(interphase)>>>>>> after anaphase (and then i get lost here)

 

[Czarcasm]

could you help with a good summary. i always get confused on this

mitosis: 2n-96 chromatids,46 chromosomes (interphase, which after replication) >>> 2n-46 chromatids, 46 chromosomes(after anaphase)
meiosis: 2n-96 chromatids,46 chromosomes(interphase)>>>>>> after anaphase (and then i get lost here)

A normal diploid cell in humans has 46 chromosomes (22 autosome pairs, 1 sex pair). One set (23 chromosomes) comes from your mother, the other set (of 23 chromosomes) from your father. The haploid number 'n' is the number of chromosomes in a single set. For us humans, n=23 and because we are diploid (that is, we have two sets), there are 46 chromosomes total (since 2n=46).

Consider what happens when a cell begins to divide (either through mitosis or meiosis). In both scenarios, we start of the same way: all the DNA within the cell must be replicated and this replication occurs during interphase. So on a microscopic level, prior to division we have 46 chromosomes (each a long piece of double stranded DNA). After this replication takes place, we now have a duplicate copies of this ds-DNA (that is 2 ds-DNA). We refer to this duplicate copy of ds-DNA for each chromosome as sister chromatids, and they are attached to each other by a protein called cohesin.

A point of clarification: Consider our chromosome #1. We receive one copy from our mom and one from our dad. These two copies can possess all sorts of different genetic differences for various genes (alleles) and so they are non-identical genetically. But because they both have the same genes (with slightly different alleles), we refer to them as homologous chromosomes (again, these aren't necessarily genetically identical). Sister chromatids however, are identical! For instance, after replication, we now how twice as much ds-DNA of chromosome #1 (dad), all with the same alleles, attached to one another. BUT, this does not mean we have x2 the chromosomes! There's still only 1 chromosome #1 (dad) and 1 chromosome #1 (mom); it would be wrong to say that after replication we have 46x2 chromosomes, so don't be confused.

This duplicate ds-DNA, along with the original ds-DNA that was present, begins to condense as we enter mitosis or meiosis. At this point, we still have the same number of chromosomes (46)! That has not changed. In other words, the identity of chromosome #1 (for example) is still the same. So then what changed? Well, after replication occurred, we now have two copies of each our of 46 chromosomes (sister chromatids); This means we have x2 the amount of DNA! And because we have 46 chromosomes (each now with a sister chromatid), there are 92 chromatids in total prior to the beginning of mitosis or meiosis.

So that's where the similarities end there. The key differences between mitosis and meiosis is first realizing how the homologous chromosomes arrange themselves prior to division. In mitosis, the homologous chromsomes line up in a single-file line. In meiosis however, the homologous chromosomes pair up (this allows for recombination to occur). Consider what happens when the cells divide. First focus in on mitosis. In mitosis, because homologous chromosomes are not paired together (as they are in meiosis), the first division results in separation of sister chromatids. We now have two cells (still with 46 chromosomes each) but now, with half as much DNA as we had originally. Each chromosome copy in the daughter cell is genetically identical to the original cell (ignoring any mutations or abnormalities). ... In meiosis, homologous chromosomes pair up; that is mom and dad chromosome #1 (each duplicated, with sister chromatids bound together), attach as a pair. So here, (after recombination occurs), the first division splits up these chromosome pairs and the result is half as many chromosomes. That is, we go from 2n to n (haploid cell). We call this equatorial division. But we're not done yet, since we still have excessive DNA that we need to get rid of. That's the purpose of the second division (reductional division). Like in mitosis, in meiosis II, all the chromosomes line up in a line so that when they divide, each duplicated piece of DNA is split apart. I avoid saying sister chromatids here because after recombination in meiosis I, the chromatids are no longer identical genetically. So the terminiology can get a little wishy washy.

Probably not the explanation you were looking for, but hope this helps.

 

[Oh_Gee]

A normal diploid cell in humans has 46 chromosomes (22 autosome pairs, 1 sex pair). One set (23 chromosomes) comes from your mother, the other set (of 23 chromosomes) from your father. The haploid number 'n' is the number of chromosomes in a single set. For us humans, n=23 and because we are diploid (that is, we have two sets), there are 46 chromosomes total (since 2n=46).

Consider what happens when a cell begins to divide (either through mitosis or meiosis). In both scenarios, we start of the same way: all the DNA within the cell must be replicated and this replication occurs during interphase. So on a microscopic level, prior to division we have 46 chromosomes (each a long piece of double stranded DNA). After this replication takes place, we now have a duplicate copies of this ds-DNA (that is 2 ds-DNA). We refer to this duplicate copy of ds-DNA for each chromosome as sister chromatids, and they are attached to each other by a protein called cohesin.

A point of clarification: Consider our chromosome #1. We receive one copy from our mom and one from our dad. These two copies can possess all sorts of different genetic differences for various genes (alleles) and so they are non-identical genetically. But because they both have the same genes (with slightly different alleles), we refer to them as homologous chromosomes (again, these aren't necessarily genetically identical). Sister chromatids however, are identical! For instance, after replication, we now how twice as much ds-DNA of chromosome #1 (dad), all with the same alleles, attached to one another. BUT, this does not mean we have x2 the chromosomes! There's still only 1 chromosome #1 (dad) and 1 chromosome #1 (mom); it would be wrong to say that after replication we have 46x2 chromosomes, so don't be confused.

This duplicate ds-DNA, along with the original ds-DNA that was present, begins to condense as we enter mitosis or meiosis. At this point, we still have the same number of chromosomes (46)! That has not changed. In other words, the identity of chromosome #1 (for example) is still the same. So then what changed? Well, after replication occurred, we now have two copies of each our of 46 chromosomes (sister chromatids); This means we have x2 the amount of DNA! And because we have 46 chromosomes (each now with a sister chromatid), there are 92 chromatids in total prior to the beginning of mitosis or meiosis.

So that's where the similarities end there. The key differences between mitosis and meiosis is first realizing how the homologous chromosomes arrange themselves prior to division. In mitosis, the homologous chromsomes line up in a single-file line. In meiosis however, the homologous chromosomes pair up (this allows for recombination to occur). Consider what happens when the cells divide. First focus in on mitosis. In mitosis, because homologous chromosomes are not paired together (as they are in meiosis), the first division results in separation of sister chromatids. We now have two cells (still with 46 chromosomes each) but now, with half as much DNA as we had originally. Each chromosome copy in the daughter cell is genetically identical to the original cell (ignoring any mutations or abnormalities). ... In meiosis, homologous chromosomes pair up; that is mom and dad chromosome #1 (each duplicated, with sister chromatids bound together), attach as a pair. So here, (after recombination occurs), the first division splits up these chromosome pairs and the result is half as many chromosomes. That is, we go from 2n to n (haploid cell). We call this equatorial division. But we're not done yet, since we still have excessive DNA that we need to get rid of. That's the purpose of the second division (reductional division). Like in mitosis, in meiosis II, all the chromosomes line up in a line so that when they divide, each duplicated piece of DNA is split apart. I avoid saying sister chromatids here because after recombination in meiosis I, the chromatids are no longer identical genetically. So the terminiology can get a little wishy washy.

Probably not the explanation you were looking for, but hope this helps.

after reading that,
mitosis: 2n-96 sister chromatids,46 chromosomes (interphase, which after replication) >>> 2n-46 chromatids, 46 chromosomes(after anaphase)
meiosis: 2n-96 sister chromatids,46 chromosomes(interphase)>>>>>> after anaphase I:1n-23 chromosomes,46 chromatids>>>>after anaphase II:1n-23 chromosomes, 23 chromatids

is that right? i changed sister chromatid to just chromatid after anaphase I

 

[Czarcasm]

after reading that,
mitosis: 2n-96 sister chromatids,46 chromosomes (interphase, which after replication) >>> 2n-46 chromatids, 46 chromosomes(after anaphase)
meiosis: 2n-96 sister chromatids,46 chromosomes(interphase)>>>>>> after anaphase I:1n-23 chromosomes,46 chromatids>>>>after anaphase II:1n-23 chromosomes, 23 chromatids

is that right? i changed sister chromatid to just chromatid after anaphase I

Looks right, but honestly, I rarely ever see people ask about the # of chromatids after crossing over and/or separation events simply because a chromatid implies one of two copies of ds-DNA. But in the unlikely scenario they were to ask you, I suppose that looks about right. Just be cautious though, because if they ask how many sister chromatids are present during meiosis II, the answer is zero. If they are a little bit vague and simply use the word chromatid, then I suppose you could say 46 - but I never encountered a situation asking that. It's really misleading.

 

[Oh_Gee]

Looks right, but honestly, I rarely ever see people ask about the # of chromatids after crossing over and/or separation events simply because a chromatid implies one of two copies of ds-DNA. But in the unlikely scenario they were to ask you, I suppose that looks about right. Just be cautious though, because if they ask how many sister chromatids are present during meiosis II, the answer is zero. If they are a little bit vague and simply use the word chromatid, then I suppose you could say 46 - but I never encountered a situation asking that. It's really misleading.

just trying to be prepared as possible