Mitosis and Meiosis
March 10, 2008 at 10:43 am #9260ElviraParticipant
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)
March 10, 2008 at 1:17 pm #82694blcr11Participant
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.
March 10, 2008 at 6:50 pm #82704
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.
March 11, 2008 at 2:45 am #82724
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.
March 11, 2008 at 10:20 pm #82740
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…
March 12, 2008 at 1:14 pm #82756
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.
March 12, 2008 at 1:57 pm #82758
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.
March 12, 2008 at 2:26 pm #82761
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).
March 12, 2008 at 4:19 pm #82767
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.
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.
March 12, 2008 at 4:49 pm #82769
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.
March 12, 2008 at 6:57 pm #82777
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.
March 13, 2008 at 1:57 am #82786
"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.
March 13, 2008 at 10:18 am #82794
I know what primary meristems are, but as initials are said to be restricted to the RAM, I always assumed that mitosis only takes place there. However, I must admit that nobody has ever explained these things to me in detail, everything I know is from what I read in one book and in free articles around the internet.
March 13, 2008 at 10:50 am #82796
Ok, I just realized that by some strange coincidence I have a botany ebook on my sister’s laptop. Here is a picture that would suggest that what I said earlier is accurate: the region of cell division is in the apical meristem, with the primary meristems being the place where the cells start to differentiate and which eventually give rise to the respective. Also, logically thinking, there would be no reason for the cells of the primary meristems to keep dividing as the cells are somewhat differentiated – it would be much easier to restrict division to the RAM.
If I somehow got things mixed up, I welcome corrections.
March 13, 2008 at 7:44 pm #82809
“Also, logically thinking, there would be no reason for the cells of the primary meristems to keep dividing as the cells are somewhat differentiated – it would be much easier to restrict division to the RAM.” – No, it would not. If only apical meristem cells could divide, none of the plants would be able to grow in width – that is what primary and secondary meristems are for.
The region of cell division is identified as the major collection/concentration of dividing cells in one place. It does not preclude cell division from occurring everywhere else.
You are mostly correct though: apical meristems give rise to primary meristems, and primary to secondary… Each one is somewhat differentiated and set to produce only certain types of cells, just like adult stem cells in humans.
The main difference they have from human adult stem cells is that primary meristems can fairly easily dedifferentiate into apical meristem cells on some occasions. You can observe this when you try rooting a cutting. In absence of root, some cells from the stem primary meristem tissues would dedifferentiate into root apical meristem cells that would produce new roots.
March 13, 2008 at 9:32 pm #82813
Let’s leave out secondary meristems – i think we can agree mitosis takes place there, but I was referring to primary structure here.
Primary meristems are for growing in width? I honestly don’t think that’s the case. By looking at the structure, I can see cell lines(as in lines made out of cells on top of each other, not stabilized cell lines) radiating from the RAM. Which suggests to me that the cell divisions in the RAM take place in a particular way, with longitudinal divisions taking place much more frequently than horizontal ones. In fact, I even remember reading that somewhere(don’t remember where though).
Sure, mitosis will occur in other places. It takes place in the perycicle giving rise to lateral roots and in the epidermis giving rise to trichomes etc. It may even take place once in a while in primary mersitems. But my hypothesis is that it takes place so rarely compared to cell divisions in the RAM that it is negligible, and that the natural course is the restriction of initials to the RAM. Sure, it’s hard to draw a clear line between the RAM and primary meristems, but in general at least.
PS: I am enjoying the discussion 😀
March 13, 2008 at 10:57 pm #82814
I enjoy this discussion too. 😀 I hope Elvira does not mind…
O.K. Here is a better picture of the meristem locations in the root:
Also, look at this website below and let me know what you think.
http://www.botany.uwc.ac.za/SCI_ED/grad … rimary.htm
I understand there is some disagreement as to which meristematic tissues – primary or secondary – contribute more to annual plant growth in width. In the woody plants it is unanimously secondary (or lateral) meristems. In either case all meristems divide and more rapidly than those of the promeristem – heart of apical meristem.
March 14, 2008 at 10:28 am #82824ElviraParticipant
Thanx a lot, you’ve been loads of help! 🙂
March 15, 2008 at 4:10 pm #82845
so the protoderm, ground meristem and procambium are part of the apical meristem? That does explain why I understood it the wrong way: cause everywhere it said that the apical meristem is the region where mitosis occurs – and then it said what tissue each primary meristem forms, never specifying the RAM is just one big mersitem complex… Maybe you should write a clear botany book that teacherless students like myself could learn from 😆
March 17, 2008 at 3:25 pm #82881
I’ll think about it…
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