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Mitosis and Meiosis

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Mitosis and Meiosis

Postby Elvira » Mon Mar 10, 2008 10:43 am

Does anyone know why root tips are used to show mitosis? I can't seem to find this info anywhere.
(i'm new to this, so if i've done anything wrong here, please tell me)
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Re: Mitosis and Meiosis

Postby blcr11 » Mon Mar 10, 2008 1:17 pm

Root tips, I would guess, grow rapidly. Anywhere you have rapid growth, you can expect to see lots of cells undergoing mitosis. You won't see any meisosis in root tips. You'll need to look at cells from the gonads to see meiosis.
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Postby MrMistery » Mon Mar 10, 2008 6:50 pm

actually, plants can grow(and quite fast) without mitosis, when cells accumulate water. Root tips are studied for mitosis because in a root mitosis occurs only in a specialized region - the tip.
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Postby Cat » Tue Mar 11, 2008 2:45 am

MrMystery, please, explain what you mean by "plants can grow(and quite fast) without mitosis, when cells accumulate water", I don't agree (cell wall will not stretch!)

In regard to root tips:
major collections of constantly dividing cells are stem cells (called meristematic cells in plants) and root tips contain root apical meristem. Root tips are preferable for studying mitosis because they are easy to excise/stain and cells are generally larger (easier to see) compared to shoot apical meristem for example; some species of plants, such as Narcissus, are preferred to others because of the variable chromosome morphology.
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Postby MrMistery » Tue Mar 11, 2008 10:20 pm

oh yes, the cell wall will definitely stretch! That is how a plant cell grows in size: it accumulates water in the central vacuole, leading to an increase in volume. The cell wall will stretch, but of course it is a directed stretch, dictated by the number of bonds between cellulose microfibrils and the orientation of the microfibrils. Unfortunately it is too much too explain here. Look up molecular mechanism of auxin and giberelins, expansins and the like. Also note that in the long term synthesis of new cell wall material is required.
Also see this:
http://www.ncbi.nlm.nih.gov/books/bv.fc ... iggrp.3614
http://www.ncbi.nlm.nih.gov/books/bv.fc ... iggrp.3613
And poke around, the mechanisms are really interesting...
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Re: Mitosis and Meiosis

Postby Cat » Wed Mar 12, 2008 1:14 pm

I still do not agree with your statement - "plants can grow (and quite fast) without mitosis, when cells accumulate water". While your comment about cell wall capable of stretching is correct in some cases, it is not a general rule. Cell wall function is to protect the plant cell from lysis in hypotonic environment. In most cases where cell wall does stretch, it precedes either mitosis or cell differentiation. More accurate comment on my part should have been ‘cell wall will not stretch indefinitely’.
While theoretically it could be possible for plants to survive without mitosis, it is definitely impossible for them to grow. Easy experiment to that effect: cuting off shout apical meristem will result in no more growth of that shoot, no matter how much you water. It will also result in increased appearance of lateral shouts (which involve mitosis) due to the loss of apical dominance.
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Postby MrMistery » Wed Mar 12, 2008 1:57 pm

I said plant CAN grow without mitosis, not plants DO grow without mitosis. Mitosis is necessary of course. What I meant by "a plant can grow without mitosis" was that increase in size without resorting to mitosis, by taking up water in cells. See cabbage bolting under giberelin influence as an example.
The cell wall stretching is an absolutely general rule. Mitosis occurs only in meristems, where the resulting cells are very small. Few cells retain that small size, they need to grow. And when a plant cell grows, the cell wall needs to stretch. It is a natural stage in the differentiation of a plant cell for it's wall to stretch. In fact that is how the shape of a cell is determined: the cell wall in the young cell has microfibrils oriented in a particular fashion so that when the cell accumulates water the cell wall will stretch in a particular way giving the cell its shape.
Of course the cell wall will not stretch indefinitely, but think of it this way: if the cell wall would be unable to stretch, the consequences for the plant would be dramatic.
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Postby Cat » Wed Mar 12, 2008 2:26 pm

Cabbage bolting is flowering stalk production and, therefore, need mitosis.
If you were talking about ‘swelling’ as growing, then I would understand what you are talking about. As it is, however, I do not. Another interesting point you made – “Mitosis occurs only in meristems, where the resulting cells are very small. Few cells retain that small size, they need to grow.” Do you equate cell growth with overall growth of the plant? (If you do, it might explain my confusion!) The number of cells that ‘grow’ is limited to the number of cells produced by mitosis. Mature differentiated cells rarely (if ever) ‘grow’ much.
Also, the amount of meristematic tissue is quite large and widely distributed throughout plant. Most plant cells can dedifferentiate to become meristem cells as well when needed (under the right conditions, of course).
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Postby MrMistery » Wed Mar 12, 2008 4:19 pm

Yeah, but most plant cells 'grow'(elongate) when differentiating.
I equate growth of the plant to mitosis and elongation of the cells. Here is an example of what happens when cell elongation occurs deficiently. Cell elongation is very important.
Image
A very relevant example is the root: mitosis is restricted to the apical meristem in the root tip, but most 'growth' results from these newly formed cells take up water and elongate, which happens mostly in the elongation zone.
Image
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Postby Cat » Wed Mar 12, 2008 4:49 pm

Arhhh… At last, we are on the same page! We were talking about different things and I agree with what you said in that sense. You see, here I was trying to imagine a whole plant growing without mitosis taking place… That is impossible, of course.
There is one little misconception that you have though, you said that “mitosis is restricted to the apical meristem in the root tip” and that is not true. For the root or stem to grow in width, they need mitosis in the other meristems to occur. You might want to look up ground meristem, procambium, protoderm, and types of lateral meristems.
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Postby MrMistery » Wed Mar 12, 2008 6:57 pm

I was referring to a primary root. As for the ground meristem, procambium and protoderm i don't think mitosis occurs there. From what I have always understood, mitosis occurs in the initial cells of the RAM, with each division of the initials being asymmetric. Those cells(which my old botany teacher calls segment cells - I doubt the term is still used) which make up the primary meristems are simply cells starting to differentiate, but which still have a common structure that is distinct from that of the differentiated organ.

Whatever the case, I admit that i was not thinking thoroughly when i wrote that sentence, I was speaking in principle.

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Andrew
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Postby Cat » Thu Mar 13, 2008 1:57 am

"As for the ground meristem, procambium and protoderm i don't think mitosis occurs there" - yes it does! Those tissues are primary meristematic tissues, you can think of them as adult stem cells.
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