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Question about chromosome replication before mitosis

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

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Question about chromosome replication before mitosis

Postby tou » Sat Mar 07, 2009 5:00 pm

Hi

I am confused about the chromosome replication before mitosis occurs.

There are 46 chromosomes in every (ok, not all) cells of a human. After mitosis there are again 46 chromosomes in the parent and new cell. In all my book it says that the before the division each chromosome makes a copy of itself. Does that mean that before mitosis there are 92 chromosomes in the cell?? I understand the rest of mitosis, but not the beginning.

What happens exactly?

Thanks!!
tou
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Postby MrMistery » Sat Mar 07, 2009 5:40 pm

This is a very good question, tou. the answer lies in the much historical definition of a chromosome. In S phase, each chromosome makes a copy of itself, but the two copies (called sister chromatids) are glued together by proteins (I am guessing you are not into molecular biology so I'll spare you the details). Because they are glued together, they are not considered two chromosomes, primarily for the historic reason that when people first looked at chromosomes through brightfield microscopes they could only see one entity. Thus the replicated chromosome is nothing more than two chromosomes glued together, but that are still considered one chromosome from this historical reason, which really has no good explanation at the molecular level.
Now, in metaphase the chromosomes line up at the metaphase plate. At precisely the same time they split into the two daughter chromosomes (the sister chromatids separate by an awesome mechanism that I find extremely fascinating) and 46 daughter chromosomes go to one pole of the cell and the other 46 daughter chromosomes go to the other pole. DURING ANAPHASE YOU DO HAVE 92 CHROMOSOMES IN THE CELL. As the nuclear membranes form around the two daughter nucleus, each nucleus has 46 chromosomes within it. As cytokinesis pinches the cell in two, you are left with two cells, each with 46 chromosomes.
Thus, you do have 92 chromosomes/cell in anaphase and telophase, before cytokinesis is completed.

Hope that clears things up a bit
Andrei
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