Some confusion with mitosis Watch

JordanL_
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I know the 4 stages and what happens during them, I know why mitosis happens etc, I'm just confused about something.

In the interphase, the DNA replicates. If for example there are two chromosomes - one pair - X and Y, that will become X X Y Y: you now have two identical pairs.

When you get to the anaphase, the spindle fibres have attached to each chromatid: each strand of each double helix, so each half of each chromosome. It then pulls the chromosomes apart so you end up with 4 halfs of X (2 pairs of identical chromatids) and 4 halfs of Y (same again).

What happens now? Because I originally thought that half an X and half a Y would be pulled to each side. They'd then create the other chromatid using the complimentary base pairs and the nuclear envelope would form.

But if that's the case, then you're left with two halves of an X and two halves of a Y floating around in the original cell.

I think I've confused myself necessarily and I've probably just made a minor mistake somewhere, but could anyone explain where I'm going wrong?
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Chief Wiggum
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I think you would just have 2 X chromatids, and 2 Y chromatids after replication. Then when they separate, you have one X and one Y chromosome in each "new cell" (each of these chromosomes having previously been a chromatid).

That's completely from memory though, so I could be wrong...
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Ashnard
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What Chief said.

Also, note that the DNA helix does not separate during mitosis. The spindle attaches to the centromere of each chromatid so that sister chromatids are pulled apart from each other during anaphase (i.e. pair of sister chromatids -> 2 separate chromatids). The base pairing within each chromatid is maintained throughout mitosis.
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ash92:)
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(Original post by JordanL_)
I know the 4 stages and what happens during them, I know why mitosis happens etc, I'm just confused about something.

In the interphase, the DNA replicates. If for example there are two chromosomes - one pair - X and Y, that will become X X Y Y: you now have two identical pairs.

When you get to the anaphase, the spindle fibres have attached to each chromatid: each strand of each double helix, so each half of each chromosome. It then pulls the chromosomes apart so you end up with 4 halfs of X (2 pairs of identical chromatids) and 4 halfs of Y (same again).

What happens now? Because I originally thought that half an X and half a Y would be pulled to each side. They'd then create the other chromatid using the complimentary base pairs and the nuclear envelope would form.

But if that's the case, then you're left with two halves of an X and two halves of a Y floating around in the original cell.

I think I've confused myself necessarily and I've probably just made a minor mistake somewhere, but could anyone explain where I'm going wrong?
You're on the right lines, just maybe a bit confused at the end. I'll try to explain it:

Let's say you have a chromosome pair called a, and a chromosome pair called b. Now with the sex chromosomes, we can't call them 'both x' or 'both y' because the pair isn't identical - we have xy forming a pair instead of xx or yy.

After interphase, we have a, b, x and y as follows (where a "-" between 2 letters denotes a chromosome pair):
----------------------------
| aa-aa xx-yy bb-bb |
----------------------------
chromosomes in one cell
.
There are 2 copies of each chromosome that was present before replication. So the pair of a chromosomes has doubled from 2 to 4, as have the b chromosomes.
There was only one x and one y (as they both together form one pair), so no there are 2 Xs and 2 Ys.


Let us pretend that the diagram above is one nucleus. The 2 duplicates (ie the sister chromatids) are joined by the centromeres. Then we have anaphase, after which the sister chromatids are pulled apart to opposite ends of the cell. At each end, around each gathering of chromosomes, a nucleus forms. After the nucleus has formed, the whole cell splits into 2. So the chromosomes would be divide like this:

-------------- --------------
| a-a x-y b-b | | a-a x-y b-b |
-------------- --------------
Nucleus 1. Nucleus 2.

(Again, a "-" between 2 letters denotes a chromosome pair, not sister chromatids).

I hope this wasn't too confusing. Sorry for the rubbish diagrams, but I was using my phone.

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JordanL_
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Thankyou! I get it now. I was getting confused because I'd read that the chromatid was one 'strand', and I thought that meant one strand of a double-helix. So I got confused thinking that each chromosome was actually pulled apart. So just to check, a chromatid is just one DNA molecule - one double-helix?
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ash92:)
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(Original post by JordanL_)
Thankyou! I get it now. I was getting confused because I'd read that the chromatid was one 'strand', and I thought that meant one strand of a double-helix. So I got confused thinking that each chromosome was actually pulled apart. So just to check, a chromatid is just one DNA molecule - one double-helix?
Yup a chromatid is one double helix (not to forget the compacting process to squash it all together, making the chromosomes sausagey shape.

"Chromatid" is used when DNA replication occurs and the 2 double helices are joined. Each of the joined copies is called a chromatid. Together, the 2 are called "sister chromatids".

Hope this helps.


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JordanL_
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Yeah it helps, I see where I was going wrong. Thanks a lot!
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