meloj
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#1
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#1
So I'm revising for AQA AS biology, and here's my understanding so far:

1) There are 46 chromosomes in the original cell (23 homologous pairs)

2) The homologous pairs are split apart, with one chromosome from each moving to separate cells in the first split. This leaves 23 chromosomes in each new cell.

3) This is the part I'm confused with: In my textbook, it says these 23 chromosomes are each split into 46 chromatids, which split again. This leaves 4 new gamete cells with 23 chromatids each.

If the gamete cells are left with 23 chromatids, when 2 gametes fuse together, how does this restore the usual 46 chromosomes? As surely 46 chromatids would only create 23 chromosomes?

Maybe I'm getting confused, but I thought a chromatid was only half of the usual chromosome?

Thanks in advance. I've attached a picture from my textbook too, hope that helps.

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RMNDK
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#2
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#2
(Original post by meloj)
So I'm revising for AQA AS biology, and here's my understanding so far:

1) There are 46 chromosomes in the original cell (23 homologous pairs)

2) The homologous pairs are split apart, with one chromosome from each moving to separate cells in the first split. This leaves 23 chromosomes in each new cell.

3) This is the part I'm confused with: In my textbook, it says these 23 chromosomes are each split into 46 chromatids, which split again. This leaves 4 new gamete cells with 23 chromatids each.

If the gamete cells are left with 23 chromatids, when 2 gametes fuse together, how does this restore the usual 46 chromosomes? As surely 46 chromatids would only create 23 chromosomes?

Maybe I'm getting confused, but I thought a chromatid was only half of the usual chromosome?

Thanks in advance. I've attached a picture from my textbook too, hope that helps.

Name:  ImageUploadedByStudent Room1457553395.960053.jpg
Views: 111
Size:  158.7 KB


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Ah when the chromosome splits into the two separate chromatids, they are then called chromosomes in the daughter cells.

Think of it like this.
1. You have 46 chromosomes (singular form). They each have a pair. So they are arranged in 23 homologous pairs (mother and father)

2. The chromosomes then duplicate (DNA replication). Now at this point, each chromosome actually consists of two sister chromatids attached at a centromere. If you think about the singular form, there's 92 in total.

3. During meiosis I, the homologous pairs line up and then separate. This is just what you said, 23 chromosomes in each new cell. But remember that these chromosomes are not singular, they're two chromatids attached. So you've actually got 46 singular chromosomes.

4. During meiosis II, in each daughter cell, the chromosomes separate again. So as you said, each cell now has 23 chromatids which at this point are now called chromosomes.


So... when two gametes fuse together, you're going to get a total of 46 chromatids, or what we now call chromosomes. Chromosomes exist in the singular form, they only exist as that double form when DNA replicates.
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meloj
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#3
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(Original post by RMNDK)
Ah when the chromosome splits into the two separate chromatids, they are then called chromosomes in the daughter cells.

Think of it like this.
1. You have 46 chromosomes (singular form). They each have a pair. So they are arranged in 23 homologous pairs (mother and father)

2. The chromosomes then duplicate (DNA replication). Now at this point, each chromosome actually consists of two sister chromatids attached at a centromere. If you think about the singular form, there's 92 in total.

3. During meiosis I, the homologous pairs line up and then separate. This is just what you said, 23 chromosomes in each new cell. But remember that these chromosomes are not singular, they're two chromatids attached. So you've actually got 46 singular chromosomes.

4. During meiosis II, in each daughter cell, the chromosomes separate again. So as you said, each cell now has 23 chromatids which at this point are now called chromosomes.


So... when two gametes fuse together, you're going to get a total of 46 chromatids, or what we now call chromosomes. Chromosomes exist in the singular form, they only exist as that double form when DNA replicates.
So the chromosomes lined up at the beginning, before any splitting takes place, which are 2 chromatids attached, have actually been duplicated? So is a chromosome naturally the 2 chromatids attached or does that only happen when a cell is replicating?

Thank you so much for your explanation, sorry to be a pain! I find this stuff really confusing!


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RMNDK
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(Original post by meloj)
So the chromosomes lined up at the beginning, before any splitting takes place, which are 2 chromatids attached, have actually been duplicated? So is a chromosome naturally the 2 chromatids attached or does that only happen when a cell is replicating?

Thank you so much for your explanation, sorry to be a pain! I find this stuff really confusing!


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Yes you're right, the chromosomes that line up have actually been duplicated.

A chromosome is naturally just one big long strand of DNA (wrapped around histones) and coiled up into a single sausage structure.

The form you see where there are two sister chromatids attached at the centromere is only the result of DNA replication (the Synthesis Phase of the Cell Cycle).


I really don't blame you. I remember learning mitosis and meiosis for sooo long. I learnt more from websites and revision guides than AQA's own ****ing textbook.
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meloj
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#5
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(Original post by RMNDK)
Yes you're right, the chromosomes that line up have actually been duplicated.

A chromosome is naturally just one big long strand of DNA (wrapped around histones) and coiled up into a single sausage structure.

The form you see where there are two sister chromatids attached at the centromere is only the result of DNA replication (the Synthesis Phase of the Cell Cycle).


I really don't blame you. I remember learning mitosis and meiosis for sooo long. I learnt more from websites and revision guides than AQA's own ****ing textbook.
Thank you! That makes so much more sense now. I'm actually revising AS again for a resit. When I was learning it last year I just memorised it because I had to for the exam, so I'm glad I'm going back and actually understanding it. I would have gone on forever thinking a chromosome was naturally the two sister chromatids.

I understand they have to leave out some details at A-level but in my opinion they should really mention the chromosomes that line up have been duplicated, I had no idea. When you actually start to research some of the topics independently, it gets frustrating when it doesn't match up with what you have to say to gain the marks in the exam.

Anyway enough moaning from me. Thanks again!
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