pelaitsc45
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Hi, I'm going over meiosis and it shows that cells divide 4 times in meiosis. What I don't understand is why cells divide 2 times instead of just once. If the 23 pairs split to form 23 in each gamete won't it achieve the same result, just less gametes? Why does it need to split twice? They both produce half the normal amount.
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alkyone
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It's just the way it works. It's not as simple as 23 chromosomes go on one side and 23 to another. There is is lot of organisation involved, and you must make sure you have one of each copy of the 23 chromosomes in each gamete. In fact the first division is not like a mitotic division, it is the sister chromatids that go together in each cell. So you copy your chromosomes, divide, and the two paternal chromosomes go in one cell and the two maternal to the other cell. Then those divide to give you the final haploid cells.
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pelaitsc45
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(Original post by alkyone)
It's just the way it works. It's not as simple as 23 chromosomes go on one side and 23 to another. There is is lot of organisation involved, and you must make sure you have one of each copy of the 23 chromosomes in each gamete. In fact the first division is not like a mitotic division, it is the sister chromatids that go together in each cell. So you copy your chromosomes, divide, and the two paternal chromosomes go in one cell and the two maternal to the other cell. Then those divide to give you the final haploid cells.
I'm doing GCSE so those words don't mean much to me lol but I guess I'll just have to accept that it's just the way it works. As always...
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Nayzar
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In very simple terms:

Mitosis:
DNA Replicates then cell divides
Each daughter cell has normal number of chromosomes

Meiosis:
DNA replicates then cell divides
The 2 new cells divide again without replication
Each of the 4 daugher cells has half the normal number of chromosomes (compared to a body cell). These are called gametes (or sex cells)

A daugher cell just means the cell created by replication. ie. one cell splits into two daughter cells

In meiosis division happens twice so that each cell can have half the number of chromosomes. This is because when two gametes join together in fertilisation, they create a cell with a full set of chromosomes, which eventually divides into an embryo
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tereziscool
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Haha, I'm just studying this right now. What a coincidence.
There are TWO DIVISIONS in meiosis NOT FOUR.
There are FOUR CELLS PRODUCED AS A RESULT OF MEIOSIS.

This is because meiosis produces gametes (sex cells) in the ovaries and testes which have half the usual number of 46 chromosomes, so that's 23 chromosomes. So, when the sperm fertilises the egg cell (note that sperm and egg cells are gametes) the zygote which is the name for a fertilised egg cell, will now combine both 23 chromosomes from the sperm and 23 from the ovaries to produce 46 chromosomes. Which ALL BODY CELLS CONTAIN.

It may be a bit confusing at first, but, just go over it and feel free to ask any more questions! ^-^
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longshot100
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(Original post by pelaitsc45)
Hi, I'm going over meiosis and it shows that cells divide 4 times in meiosis. What I don't understand is why cells divide 2 times instead of just once. If the 23 pairs split to form 23 in each gamete won't it achieve the same result, just less gametes? Why does it need to split twice? They both produce half the normal amount.
They need to multiply so that they can swap sections of DNA with other chromosomes to give variation in the half set of DNA in gametes, otherwise if the same two parents had a child, the two children would both be genetically identical.


I think this should help answer your question, because if they only divided once to form two gametes, there'd be no way to swap the dna sections to give variation in the DNA included in each gamete

Ask me specific questions about what I said if it doesn't make sense to you btw
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pelaitsc45
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(Original post by Nayzar)
In very simple terms:

Mitosis:
DNA Replicates then cell divides
Each daughter cell has normal number of chromosomes

Meiosis:
DNA replicates then cell divides
The 2 new cells divide again without replication
Each of the 4 daugher cells has half the normal number of chromosomes (compared to a body cell). These are called gametes (or sex cells)

A daugher cell just means the cell created by replication. ie. one cell splits into two daughter cells

In meiosis division happens twice so that each cell can have half the number of chromosomes. This is because when two gametes join together in fertilisation, they create a cell with a full set of chromosomes, which eventually divides into an embryo
(Original post by tereziscool)
Haha, I'm just studying this right now. What a coincidence.
There are TWO DIVISIONS in meiosis NOT FOUR.
There are FOUR CELLS PRODUCED AS A RESULT OF MEIOSIS.

This is because meiosis produces gametes (sex cells) in the ovaries and testes which have half the usual number of 46 chromosomes, so that's 23 chromosomes. So, when the sperm fertilises the egg cell (note that sperm and egg cells are gametes) the zygote which is the name for a fertilised egg cell, will now combine both 23 chromosomes from the sperm and 23 from the ovaries to produce 46 chromosomes. Which ALL BODY CELLS CONTAIN.

It may be a bit confusing at first, but, just go over it and feel free to ask any more questions! ^-^
Yeah, I understand what happens in meiosis and the end result, I just don't understand why it needs to divide twice. See, I'm using the CGP guide and at the end of the meiosis process, 4 gametes are produced, however, two are the exact same, so it seems pointless that it divides twice because all it seems to be doing is just producing more gametes, not more different ones. I know I'm wrong, I just don't understand why.
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pelaitsc45
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(Original post by longshot100)
They need to multiply so that they can swap sections of DNA with other chromosomes to give variation in the half set of DNA in gametes, otherwise if the same two parents had a child, the two children would both be genetically identical.


I think this should help answer your question, because if they only divided once to form two gametes, there'd be no way to swap the dna sections to give variation in the DNA included in each gamete

Ask me specific questions about what I said if it doesn't make sense to you btw
Okay, I get what you're saying, but this is just in one person, so the gamete will still combine with another person's gamete producing someone with a combination of different chromosomes, therefore there is still variation?
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Nayzar
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(Original post by pelaitsc45)
Yeah, I understand what happens in meiosis and the end result, I just don't understand why it needs to divide twice. See, I'm using the CGP guide and at the end of the meiosis process, 4 gametes are produced, however, two are the exact same, so it seems pointless that it divides twice because all it seems to be doing is just producing more gametes, not more different ones. I know I'm wrong, I just don't understand why.
Ah thats a good question. Basically in one part of meiosis, some of the genes actually get swapped around. So you actually end up with 4 different gametes. But thats A level so I wouldnt bother going further than that for now
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alkyone
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(Original post by pelaitsc45)
I'm doing GCSE so those words don't mean much to me lol but I guess I'll just have to accept that it's just the way it works. As always...
Sorry, let me have another go. Now the truth about meiosis is that it is awfully complicated and we don't know everything about it yet. Each person has 2 sets of 23 chromosomes in their DNA, one from their mother (maternal) and one from their father (paternal). First thing that happens in meiosis is make copies of these, so you end up with two paternal and two maternal. At this stage you can get the maternal chromosome and the paternal chromosome exchange pieces of DNA, so that you get greater variation in the cells produced, and so that for each chromosome the baby in the end gets DNA from both grandparents on this side of the family. During the first division you get the two paternal chromosomes stuck on each other and the two maternal chromosomes stuck on each other. So in the resulting cells you have two sets of maternal chromosomes and two sets of paternal chromosomes. Then you divide again to form 4 cells with 1 set of 23 chromosomes.
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pelaitsc45
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(Original post by Nayzar)
Ah thats a good question. Basically in one part of meiosis, some of the genes actually get swapped around. So you actually end up with 4 different gametes. But thats A level so I wouldnt bother going further than that for now
Ah okay, I get that now, thanks. So each gamete is genetically different to the parent?
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pelaitsc45
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(Original post by alkyone)
Sorry, let me have another go. Now the truth about meiosis is that it is awfully complicated and we don't know everything about it yet. Each person has 2 sets of 23 chromosomes in their DNA, one from their mother (maternal) and one from their father (paternal). First thing that happens in meiosis is make copies of these, so you end up with two paternal and two maternal. At this stage you can get the maternal chromosome and the paternal chromosome exchange pieces of DNA, so that you get greater variation in the cells produced, and so that for each chromosome the baby in the end gets DNA from both grandparents on this side of the family. During the first division you get the two paternal chromosomes stuck on each other and the two maternal chromosomes stuck on each other. So in the resulting cells you have two sets of maternal chromosomes and two sets of paternal chromosomes. Then you divide again to form 4 cells with 1 set of 23 chromosomes.
And each gamete in the set is different to the other, right?
Because that's what I was getting confused over since the revision guide shows both gametes in the set as the same.
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Nayzar
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(Original post by pelaitsc45)
Ah okay, I get that now, thanks. So each gamete is genetically different to the parent?
Yes. Pretty much every single gamete ends up with a different genetic structure
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alkyone
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(Original post by pelaitsc45)
And each gamete in the set is different to the other, right?
Because that's what I was getting confused over since the revision guide shows both gametes in the set as the same.
They are different because the exchange of genetic material is random. This by the way happens in one single parent, so when we talk about maternal and paternal chromosomes we mean the chromosomes coming from the resulting baby's grandparents if that makes sense.

Also apart from this variation being produced this two divisions thing might be necessary to line up the chromosomes correctly so that you actually get all 23 chromosomes in each cell. But as I said there is a lot we don't know about how meiosis works.
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Nayzar
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(Original post by pelaitsc45)
And each gamete in the set is different to the other, right?
Because that's what I was getting confused over since the revision guide shows both gametes in the set as the same.
The ones that are "the same" in the book may be relatively similar, but different, because of this DNA crossing over that happens
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pelaitsc45
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(Original post by Nayzar)
The ones that are "the same" in the book are similar but different, because of this DNA crossing over that happens
Right, thanks a ton.
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longshot100
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(Original post by pelaitsc45)
Okay, I get what you're saying, but this is just in one person, so the gamete will still combine with another person's gamete producing someone with a combination of different chromosomes, therefore there is still variation?
The swapping of sections means that there is more variation in gametes. The same happens in the other parent.

Therefore if the same two humans reproduce again, the gametes that fuse will not be the same as the ones that fused to make the first child, you see?

So imagine that you had one kid with your partner, and your gametes didn't switch DNA sections during meiosis. All of your gametes would have a more limited combination of DNA sections if they didn't swap sections.

So let's say you decide to have another kid with the same partner. Without proper meiosis, the kid would have a good chance of being identical (or very similar) to the first, whereas if you had proper meiosis, it would be very hard to be extremely genetically similar.

Because of the difference, in the harsh world out there, if the first kid wasn't fit enough, at least the second one would survive. If both were the same, neither would survive.

TL;DR- the way meiosis occurs means there is more variation in gametes, which means more variation in offspring.
More variation in offspring= better chance of some of your offspring surviving = better chance of the race surviving due to variation.

Sorry if I ended up repeating or paraphrasing some of the things I said before.
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pelaitsc45
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(Original post by longshot100)
The swapping of sections means that there is more variation in gametes. The same happens in the other parent.

Therefore if the same two humans reproduce again, the gametes that fuse will not be the same as the ones that fused to make the first child, you see?

So imagine that you had one kid with your partner, and your gametes didn't switch DNA sections during meiosis. All of your gametes would have the same combination of DNA, or at most, two different combinations.
So let's say you decide to have another kid with the same partner. Without proper meiosis, the kid would be identical to the first, whereas if you had proper meiosis, it would be different.

Because of the difference, in the harsh world out there, if the first kid wasn't fit enough, at least the second one would survive. If both were the same, neither would survive.

TL;DR- the way meiosis occurs means there is more variation in gametes, which means more variation in offspring.
More variation in offspring= better chance of some of your offspring surviving = better chance of the race surviving due to variation.

Sorry if I ended up repeating or paraphrasing some of the things I said before.
Oh yeah, I never thought about if that. Thanks. I'm pretty sure I get it now
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alkyone
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(Original post by longshot100)
The swapping of sections means that there is more variation in gametes. The same happens in the other parent.

Therefore if the same two humans reproduce again, the gametes that fuse will not be the same as the ones that fused to make the first child, you see?

So imagine that you had one kid with your partner, and your gametes didn't switch DNA sections during meiosis. All of your gametes would have the same combination of DNA, or at most, two different combinations.
So let's say you decide to have another kid with the same partner. Without proper meiosis, the kid would be identical to the first, whereas if you had proper meiosis, it would be different.

Because of the difference, in the harsh world out there, if the first kid wasn't fit enough, at least the second one would survive. If both were the same, neither would survive.

TL;DR- the way meiosis occurs means there is more variation in gametes, which means more variation in offspring.
More variation in offspring= better chance of some of your offspring surviving = better chance of the race surviving due to variation.

Sorry if I ended up repeating or paraphrasing some of the things I said before.
It is true that crossing over generates more diversity, but it is not true that otherwise you would only have two possibilities though. You have 23 chromosomes and no way of telling them how to line up, so during meiosis the paternal and maternal chromosomes are assorted independently. That is to mean that just because in a cell you have chromosome 4 as a paternal chromosome it doesn't mean that they all need to Be from your father, and can have different combinations of paternal and maternal chromosomes. This means that without crossing over you have 2^23 possible combinations.
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longshot100
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(Original post by alkyone)
It is true that crossing over generates more diversity, but it is not true that otherwise you would only have two possibilities though. You have 23 chromosomes and no way of telling them how to line up, so during meiosis the paternal and maternal chromosomes are assorted independently. That is to mean that just because in a cell you have chromosome 4 as a paternal chromosome it doesn't mean that they all need to Be from your father, and can have different combinations of paternal and maternal chromosomes. This means that without crossing over you have 2^23 possible combinations.
Just checked my own post, and yeah I did say "at most two", didn't I? I was just trying to make a point and I was focusing more on trying to make my overall explanation legible.
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