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Do Chromosomes dissolve and disappear?

Genetics as it applies to evolution, molecular biology, and medical aspects.

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Do Chromosomes dissolve and disappear?

Postby guybrush » Mon Feb 22, 2010 10:51 am

Do chromosomes actually dissolve and disappear? I know the nuclear membrane disappears during mitosis/meiosis but i thought chromosomes paired up rather than dissolve?

With regards to Homologous Pairs. During meiosis they separate in Anaphase I, but in mitosis do they come together?
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Re: Do Chromosomes dissolve and disappear?

Postby mamoru » Mon Feb 22, 2010 11:42 am

guybrush wrote:Do chromosomes actually dissolve and disappear?

Short answer: No.

Long answer: Nooooooooooooooooooooooooooooooooooooo. ;)

Chromosomes are supercoiled DNA. When the cell is not dividing, they uncoil to an extent and are pretty much arranged as chromatin (which, mind you, is rather coiled as well, but more accessible than the supercoil bundles of chromosomes). So, they are technically always there, though not usually tightly packed into the chromosome shapes.

As for your other questions, the Wikipedia Article on Mitosis answers them all clearly. If you imagine the chromosomes in first step of the the simplified diagram at the top of the article to be a homologous pair, it should all become clear to you.

For a gorgeous (and accurate) rendering of how DNA is organized and a bit about how it replicates and separates (as well as is transcribed and translated), go here and download the first video, "Body Code". (or, I'm sure you can YouTube it, but most versions I've seen on YouTube are lower quality).

[edit]unless I misunderstood you, and by "dissolve" you meant stop being supercoil bundles but still exist as long molecules of DNA rather than "dissolve as in break down and dissociate". :?
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Postby guybrush » Mon Feb 22, 2010 1:04 pm

Thanks for the link.

The way it seems is that the pairs initially come together in both Mitosis and Meiosis, however, they then both separate to opposite ends of the cell by the spindle fibres.

The question is i have been asked is:
Homologous pairs of chromosomes come together in Mitosis/Meiosis: True or False






Thanks
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Re:

Postby mamoru » Tue Feb 23, 2010 12:33 am

guybrush wrote:Thanks for the link.

The way it seems is that the pairs initially come together in both Mitosis and Meiosis, however, they then both separate to opposite ends of the cell by the spindle fibres.

The question is i have been asked is:
Homologous pairs of chromosomes come together in Mitosis/Meiosis: True or False

I'm going to say "false", but I'm not entirely sure what you mean by "come together".

Go back to the Mitosis page I linked. In the first frame, the red chromosome and the white chromosome represent a homologous pair of uncopied DNA. Keep in mind, it wouldn't actually be like this in the cell, because the DNA would be arranged as chromatin rather than bundled into a chromosome at that point. Then, what you see is replication (which in real life would then be followed by the supercoiling into Chromosomes during Prophase), which gives the well-known X shape of the chromosome (it is actually two copies of the same chromosome attached together). So, in the second frame of that progression, you are still only seeing 1 homologous pair of chromosomes, but each "chromosome" is actually two copies of itself. During Anaphase, these copies are pulled to opposite sides of the cell, giving each daughter cell a homologous pair.

During Meiosis, there is something different. During Anaphase I, rather than having the replicated chromosomes separate into the daughter cells, you have the homologous pairs separating, so that the daughter cells then each have two copies (shown by the X shape) of ONE chromosome of a homologous pair. Then, in Anaphase II, the copies themselves are separated.

So, uhh... what exactly to you mean by come together? Do you mean how the chromosomes are arranged and then segregated during Meiosis?

The closest thing I can possibly think of for "coming together" would be during possible crossovers during Prophase I of meiosis (in which transpositions and translocations may occur between the halves of a homologous pair), or perhaps the arrangement for segregation during Metaphase I. Is this what you are referring to?
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Re: Do Chromosomes dissolve and disappear?

Postby guybrush » Fri Feb 26, 2010 5:24 pm

Hi,

I have been in biology today and spoke to lecturer about the homologous pairs.

You were correct that this is false in mitosis.

It is referring to the tetrads that form during interphase. So i was wrong by saying it was false in meiosis.

Thanks for your help.
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