The Emergence of Microtubules
A flashback to everything that has been explained here seems to give us the impression that the Cell-to-be resembles a hard solid object, particularly at the time when it first emerged in the Carbon body, which is known to be hard. However, this is definitely untrue for the compounds formed in the body of a Carbon granule: they are midway between solid and liquid.
Can the Microtubule be characterized as such: neither purely solid nor purely liquid? But what is a Microtubule, anyway?
Microtubules are one of the components of the cytoskeleton. Microtubules are hollow cylinders about 23 to 25 nm in diameter (lumen = approximately 15nm in diameter), most commonly comprising 13 protofilaments which, in turn, are polymers of alpha and beta tubulin. They have a very dynamic behavior, binding GTP for polymerization. They are commonly organized by the Centrosome. (Source: Internet).
The discussions that follow will, however, be confined to those Microtubules between two Centrosomes (the polar Microtubule), and those between the Centrosome and the Kinetochore at the Centromere of the Chromosome (the Kinetochore Microtubule).
At the time the Cell-to-be first begins to divide itself, its inner part has already been in a condition midway between solid and liquid. Such is its elasticity then that in its division it looks as if it were a lump of dough being torn apart by a baker. Its central part gets elongated and narrowed (illustration 1). It is possible that around the centre or the core there are compounds resembling slime such that at the time the division occurs the Cell-to-be looks like what one can see in illustration 1.
It is this slime that will later transform itself into Microtubules, very much like the ones we have today. Depending on its condition, there is also a possibility that during the division all that becomes of the slime is but only a strand of Microtubule fiber or perhaps four strands, thus resembling something more like what is shown in illustration 1d in which only the core is depicted.
In the illustrations below greater emphasis is placed on the Microtubule’s-to-be evolutionary journey in its attempt to develop into its present form.
In Illustration 2, the thread-like slime is seen to be making every attempt to connect both Centrosomes, the way the Polar Microtubule does. Apart from that, the thread-like slime is also seen to connect the Centrosome with the Chromosome, thus turning itself into a Kinetochore Microtubule. This is particularly true during the Metaphase and the Anaphase.
Illustration 3 is a representation of the actual Microtubule in its simple form. Illustrations 4a and 4b show division that occurs at the time when the Cell-To-Be first emerged.
In Illustration 5, a Microtubule originating from a Centrosome is seen to move towards the Centromere, and then get caught at the Kinetochore located at the Centromere. (Review text under the heading The DNA-to-be inside the Chromosome-to-be detaches itself from the Core). As it turns out, all microtubules, like DNAs, undergo the process of twisting too.
In the Microtubule, however, the twists are seen to be extremely tight and close—thus proving the truth of the assumption that the time when the Cell-to-be divides is the time when an abundance of the inner parts of the Cell get twisted. In other words, there is no denial then that the vertical axis did form an angle with the two Cells-to-be during the division that took place when it first emerged.
Why then is a Microtubule hollow? The answer is simply that such a hollow is a mere result of its twists being flat. For a Microtubule-to-be to come out flat, it must be either one that originates from a flat Centrosome-to-be, or one that comes out from the front part of the Centrosome-to-be. Being flat it twists, in a way as is illustrated above, and in the course of its evolution a hollow is formed in the middle of its twists. Not only this; later, with every one of the rolls touching each other, they eventually get attached to each other the way they do now.
However, to attribute the hollow to the Centrosome’s-to-be being flat apparently sounds less convincing. Another possibility could be that in the course of its evolution the Centrosome-to-be moves in spiral curves such that a hollow is formed inside it. Certainly the spirals formed in its movement is extremely small, e. g. approx.23 nanometer in diameter.(Illustration 7)
Let’s now go into more details about the other possibility as to why Microtubules are hollow.
In Illustration 8 the body, then still blanketed by Carbon, divides itself in such a way that the division is directed towards the reader. As s result, one of the split off part is at the back, while the other is in front. In the meantime, this divided body spins sideways. There are, however, two circumstances that can make it impossible for the body to absolutely achieve accurate measurement
First: It is just impossible for the body to split itself up with the dividing line absolutely parallel to the vertical line without forming an angle. This has been discussed in the text under the heading “The twisting of the DNA.”
Second: It is just impossible for the body to split itself up with the two central points of the Centrosome-to-be being in one straight horizontal line. A1 and A2 must certainly shift, even if the shift is only to a very minute measure. Viewed from the front, while the process of division is going on, A1 and A2 do not seem to overlap each other; rather, when projected as in illustration 9b, one seems to be “keeping a distance” from the other. This “distance” was a calculated attempt that had begun from the time it (the Centrosome-to-be) first separated until the time the Microtubule broke up during the Cytokinesis phase. Thus, apart from being led to spin, A1 and A2 maintain a distance from each other as shown in illustration 9b. (Relate all this with the text under the heading “Impossibilities and Possibilities” in the pages that follow)
Unlike illustration 8, which shows the whole Carbon body, Illustrations 9a and 9b focus only on the Centrosome-to-be. The fact that a hollow is formed is an implication that both Centrosomes-to-be must certainly be keeping a distance from each other when projected as in illustration 9b, where both cores are seen to be encircling each other (relate this with illustration 10b).
No hollow would certainly be formed if A1 and A2 (the parts resulting from the self-division of A in the course of its evolution) were on the same axis; however, given the fact that the two split up parts continue to maintain a distance between them, it is naturally possible for such a hollow to be formed in the Microtubule-to-be. This, however, does not necessarily mean that with the divided parts keeping a distance from each other the Microtubule must have a hollow. Yet, one thing is certain: with the presence of such hollow, the Centrosomes-to-be must certainly be keeping a distance from each other when projected.
It would just be impossible for A1 and A2 to be absolutely on a single axis (illustration 10a). The dramatic division of the body causes both of them to shift and secure their positions as shown in illustration 9b.
What is it that makes the centre of the Centrosome-to-be so important? It is simply because the central point serves as the standard of measurement when referring to the number of Microtubules-to-be being produced. Look at illustration 10a, where the Centrosome-to-be is seen to produce a Microtubule-to-be (further clarified with the broken lines). It can also be seen in the illustration that A1 and A2, are overlapping if viewed from the side (see the eye in the illustration).
It is worth noting down here that the twists of the Microtubule, viewed from a certain standpoint, is not at all the same as those of the DNA-to-be. In the case of the Microtubule-to-be, the twists shrink again soon after the completion of the division of the Centrosome-to-be. It is only at the beginning of the division following their shrinkage that almost nothing could be seen of the twists—perhaps they then twist by only 0.01 degree. But the direction that the twists will take has already taken form. This direction of the twists is one obtained at the time when the Cell-to-be undergoes natural division for the first time. Thus it is this division and the twists, with their directional lead, that have made the present day twists to be as they are now. Illustrations 3, 6, 9, and 10 show the emergence of the Microtubule.
The influence of the division of the cell-to-be when it divided for the first time and during which it began to develop twists, similar to those of the DNA, on the Microtubule. (Refer to the Additional Illustration below entitled “The Twisting of the DNA”).
The development of the twists meant here is by no means one that occurred during metaphase; rather, it was one that occurred evolutionarily.
It is necessary to emphasize here that twists on today’s microtubule is a result of not only the characteristics of the molecules themselves when they form a chain but also the influence of the position of the cell division when they divided for the first time.
Similarly is the case with the molecules of the DNA which formed twists in its chain; here too the twists can be seen occurring from the position of the combined molecules that had served to arrange the Microtubules. But the form of the twists was still highly influenced by the position it first took when the division occurred for the first time.
Nevertheless, whatever the possibility could be, there is yet no room to doubt that the Microtubule today is as such because during the first natural division, both split-up parts of the Cell-to-be form an angle with each other, thereby enabling them to perform a variety of spins. Apart from this, the shifting of both the split-up Centrosomes-to-be sideways has led to the Microtubule’s-to-be having a hollow in its middle. (Relate all these with the text under the heading “Impossibilities and Possibilities” in the pages that follow).
Certainly the shape of the Microtubule as it really is today is not as neat and simple as has been illustrated and described here. Rather it is more like what is shown in illustration 11.
One should, therefore, not be surprised should one come to know that all the inner parts of the present day Cell are equally not as neat and simple as illustrated and described here. In addition to that, with its present condition full of the dynamic movements of Microtubules, it is also commonly referred to as the motor of proteins. Such is its condition, we therefore find it difficult to perceive the simplicity of the Microtubule of the past. So long is the route that the Cell-to-be has to take that in the course of its journey so many new compounds are formed and so many changes, in either their combinations or their characteristics, continue to take place.