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Mitosis

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Mitosis

Postby OoBiologyoO » Fri Nov 10, 2006 8:53 pm

Hi!

Apparently, before counting the # of chromosomes in onion root cells, we can make predictions about the number we are going to find. How is this possible? I am not asking for an answer necessarly, but some pointers would be great!

THANK YOU!

S.
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Postby druid » Fri Nov 10, 2006 10:06 pm

Possibly you can predict the number of chromozomes you'll see by searching the web for "Allium cepa chromosome number".
It's almost surely you'll find in 2 seconds that 2*n = 16

:)
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Postby OoBiologyoO » Fri Nov 10, 2006 10:10 pm

Well, the problem is that we are supposed to predict the number of chromosomes by ourselves, meaning without using existing data...
Apparently, something about onion root cells should lead us to a number close to 18...
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Postby druid » Fri Nov 10, 2006 10:16 pm

OoBiologyoO wrote:Apparently, something about onion root cells should lead us to a number close to 18...


No.
Apparently due to actively dividing cells at root tips, you'll catch cells at mitosis with 4*n number of chromosomes ( at metaphase for example ).
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Postby OoBiologyoO » Fri Nov 10, 2006 10:25 pm

Ok, so this would be one way of predicting. We can look at cells in mitosis, and count the chromosomes, and then divide...
Would there be any other way?
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Postby druid » Fri Nov 10, 2006 10:40 pm

OoBiologyoO wrote:Ok, so this would be one way of predicting. We can look at cells in mitosis, and count the chromosomes, and then divide...
Would there be any other way?


It seems i don't really understand your question.
Do you mean the following task: "Given name of a species, predict number of chromosomes it has." ?
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Postby OoBiologyoO » Fri Nov 10, 2006 10:47 pm

No, basically in class we looked at cells in mitosis from an onion root.
We found that, on average, there were 18 chromosomes per cell in anaphase.
But they ask us, if we didn't have a microscope, how we could predict the number of chromosomes we were going to find...
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Postby druid » Fri Nov 10, 2006 11:19 pm

Well, that you've got 18 on avarage is due to blurred vision ;).
Number of chromosome in species is constant ( Usually incorrect number of chromosomes leads to death of the cell ).

Onion's haploid set is 8 chromosomes, diploid set is 8*2=16 chromosomes.
At anaphase you cannot distinguish between sister chromatids thus the apparent number of chromosomes is not changed, that is it remains 16.

Without microscope you can predict the number of chromosomes at any stage of cell cycle by the following rule ( for diploid organism ):

[interphase]--> 2N --[synthesis]--> 4N ---[mitosis]--> 2N ---> [interphase]

where N is haploid number of chromosomes.
Anyway you have to know N from observation or database.
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Postby OoBiologyoO » Sat Nov 11, 2006 5:27 pm

Actually, I may have mixed up 16 and 18, so yes you might be right there!

So, there is no way of knowing how many chromosomes an onion cell has, unless we look with a microscope or search for existing data?
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Postby druid » Sat Nov 11, 2006 6:13 pm

OoBiologyoO wrote:So, there is no way of knowing how many chromosomes an onion cell has, unless we look with a microscope or search for existing data?


If you discover something else, please post it here.
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Postby MrMistery » Sat Nov 11, 2006 7:17 pm

there is a way of aproximating the degree of poliploidy in higher plants, maybe your teacher expects of answer of this type. though i have never heard of poliploid onion..
"As a biologist, I firmly believe that when you're dead, you're dead. Except for what you live behind in history. That's the only afterlife" - J. Craig Venter
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Postby druid » Sun Nov 12, 2006 12:15 am

I would answer so:

1. If the cell observed is in
phase=[interphase..gap1] it has 2*8=16 chromosomes.

2. If the cell observed is in
phase=(synthesis..metaphase) it has 2*8*2=32 chromosomes which appear to be 16 chromosomes.

3. If the cell observed is in
phase=[anaphase..cytokinesis] it has 16+16=32 chromosomes which will be observable because they have been separated by the spindle and moving to opposite directions.

Reminder:
[, ] including ends
(, ) excluding ends

If something's gone wrong with cell devision another numbers can appear, so the above statements are only hypoteses.
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