Biology- Chromosome and chromatid confusion

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Zahid~
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I've been taught that a replicated chromosome is made from two chromatids attached to a centromere.
After anaphase, spindle fibres pull the replicated chromosomes by there chromatids and the centromeres split and thus we are left with two chromatids on either side of the cell which later undergoes telaphase and cytokenises.
So then when and how does that single chromatid become a normal (un-replicated) chromosome!?!
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Tillybop
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(Original post by Zahid~)
I've been taught that a replicated chromosome is made from two chromatids attached to a centromere.
After anaphase, spindle fibres pull the replicated chromosomes by there chromatids and the centromeres split and thus we are left with two chromatids on either side of the cell which later undergoes telaphase and cytokenises.
So then when and how does that single chromatid become a normal (un-replicated) chromosome!?!
Remember that before the cell divides it replicates it's DNA, so it has two sets of DNA. These two sets will form the chromosome with two chromatids, so that one goes to each new cell.

When the cell is just normal it only contains chromatin, which is the unwound version of a chromosome. The DNA will then replicate during the middle part of interphase, and after this the chromosomes will form. There are now two sets of DNA present, so when the cell divides, one set will go to one cell, and one set will go to the other cell.

After cell division the chromosomes disappear, and chromatin is present once more. Therefore the chromosome spends little time as just a single chromatid.

Hope that makes sense
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Retrodiction
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(Original post by Zahid~)
I've been taught that a replicated chromosome is made from two chromatids attached to a centromere.
After anaphase, spindle fibres pull the replicated chromosomes by there chromatids and the centromeres split and thus we are left with two chromatids on either side of the cell which later undergoes telaphase and cytokenises.
So then when and how does that single chromatid become a normal (un-replicated) chromosome!?!
A chromosome is both a chromosome and a chromatid. We have 23 pairs of chromosomes, so 46 overall. A chromosome is only referred to as a chromatid when we're discussing it in the context of the process of separation by the spindle.

I might not be expressing this particularly clearly, but basically only once the chromosomes are lined up at the equator of the cell are they referred to as sister chromatids, as they have been condensed into discrete objects paired up with another version of the same chromosome.
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Hype en Ecosse
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(Original post by Tilly-Elizabeth)
Remember that before the cell divides it replicates it's DNA, so it has two sets of DNA. These two sets will form the chromosome with two chromatids, so that one goes to each new cell.

When the cell is just normal it only contains chromatin, which is the unwound version of a chromosome. The DNA will then replicate during the middle part of interphase, and after this the chromosomes will form. There are now two sets of DNA present, so when the cell divides, one set will go to one cell, and one set will go to the other cell.

After cell division the chromosomes disappear, and chromatin is present once more. Therefore the chromosome spends little time as just a single chromatid.

Hope that makes sense
Chromatin is a mix of DNA and proteins in any structure - not just when it's messy and unreplicated. Both free, unreplicated DNA during interphase and the replicated chromosomes of metaphase are both made of chromatin. The large DNA structures, even when unreplicated and unwound in a messy chaotic fashion, are still called chromosomes. They're just not the familiar metaphase chromosomes that you'd see on a karyogram. You can think of chromosomes being made of chromatin, if you like.

OP, the difference between a chromatid and chromosome is really just weird terminology. You start off with an unduplicated chromosome. Once this chromosome replicates, you end up with a duplicated chromosome made up of two sister chromatids. When you pull the chromatids apart, you've got two unduplicated chromosomes again. These unduplicated chromosomes are identical to the two sister chromatids - they're just no longer joined together at a centromere so we don't call them chromatids any more!

The word chromosome refers to a single DNA/protein structure. In replication, the DNA is copied, but they're still just one big structure (due to the connection at the centromere). Hence why call it one chromosome (with two chromatids) rather than 2 joined chromosomes.
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Tillybop
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(Original post by Hype en Ecosse)
Chromatin is a mix of DNA and proteins in any structure - not just when it's messy and unreplicated. Both free, unreplicated DNA during interphase and the replicated chromosomes of metaphase are both made of chromatin. The large DNA structures, even when unreplicated and unwound in a messy chaotic fashion, are still called chromosomes. They're just not the familiar metaphase chromosomes that you'd see on a karyogram. You can think of chromosomes being made of chromatin, if you like.

OP, the difference between a chromatid and chromosome is really just weird terminology. You start off with an unduplicated chromosome. Once this chromosome replicates, you end up with a duplicated chromosome made up of two sister chromatids. When you pull the chromatids apart, you've got two unduplicated chromosomes again. These unduplicated chromosomes are identical to the two sister chromatids - they're just no longer joined together at a centromere so we don't call them chromatids any more!

The word chromosome refers to a single DNA/protein structure. In replication, the DNA is copied, but they're still just one big structure (due to the connection at the centromere). Hence why call it one chromosome (with two chromatids) rather than 2 joined chromosomes.
Ah right thanks
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