The X-shaped Chromosome
Nothing could possibly occur without a cause. The same thing holds for the Chromosome of today which is shaped like an X during metaphase. Illustration 1 shows the process by which those compounds exposed to external influences are pulled apart or, to put it another way, the process by which the compounds of the Chromosome-to-be containing the DNA-to-be inside it are dismantled. Certainly the X shape of the Chromosome of today must have originated from the most initial condition in which the cell finds itself during its self-division, i.e. the condition of the cell-to-be when it divides itself for the first time. Here, however, a stronger emphasis will be laid upon the issue of how such a shape could have been formed than on the initial movement of the Nucleotide and the chemical changes occurring in it.
Naturally the X shape that Chromosome of today have mush have been one that they got from the earliest condition of the divided Cell, i.e. the condition of the Cell-to-be when it first divided itself. In addition to this, a further analysis will be made here as to how the Chromosome got its X shape from the initial movement of the Nucleotides.
In the illustration the compounds are drawn to resemble circular gray stains (A, B, and C), whereas the core of the ACP is colored black. As already described earlier, in the division of a ACP crowded with a variety of compounds, it is the core that divides first and foremost. Only after the core divides will all other compounds follow suit later. Illustration 1 shows the position of the three Chromosomes-to-be with a core that has earlier been split up. The broken black line depicts the condition when there occurs mutual attraction between the two divided cores and the three Chromosomes-to-be, which gradually ends up in the dismantling of the compounds. It seems as if A, B, and C are divided into two.
Here, if a line is drawn to pass through A, B, C and the two separated cores, the end result is always a trapezium, no matter where the Chromosomes-to-be are. But, the question remains: How close is the relationship between the X shape of the Chromosome today and the diagonals of the trapezium? It needs to be emphasized here that as far as the types of compounds formed in the ACP are concerned, solids and liquids vary greatly.
A look back at conditions when the centre started splitting itself up tells us that such separation means that the activated ACP had then been destructed and that what was left was only the various compounds in the body of the Cell-to-be. (see illustrations 4 and 5 in Chapter 2: The Compression/Condensation and Development of the Body).
In Illustration 2, expansion at the core of X, which is about to divide itself, attracts everything that was originally connected with the core, thus causing X1 and X2, now divided, to seemingly appear as if they were fighting against each other for control over everything around them during their division. Further to this, A (the area prone to external influence) also divides itself, thus we have A1 and A2. In Illustration 2, it can be seen that X1 and X2 are trying to take possession of A1 and A2. As it is the core that develops first and foremost, whereas the body only follows suit later, the upper side of the trapezium therefore becomes shorter than its lower side. This further causes the upper part of the diagonals to be shorter than its lower part—the former apparently representing the short arm, the latter the long arm, while the points of intersections of the diagonals may well be said to resemble the centromere of the Chromosome. Could it be that such similarity is just a coincidence or is there anything else that has caused the centromere to look like the diagonals of the trapezium?
Illustrations 3 and 4 are meant to provide further explanations in order to convince the reader that the X shape of the Chromosome has something to do with the level of tension and the rareness of a material.
Let’s take a look at Illustration 3. In Illustration 3a, the rectangle is assumed to be the DNA-to-be inside the Chromosome-to-be. Now let’s replace the rectangle with a rectangular rubber sheet, the four corners of which are so stretched that a trapezium is formed—one similar to that formed during the dismantling of compounds (Illustration 3b). As soon as the four corners are stretched and a trapezium is formed, the inner part rarefies that it almost resembles the diagonals of the trapezium illustrated. The DNA-to-be is so positioned to resemble the left and right sides of the trapezium (colored blue). If such a thing happens to the rubber sheet, then those parts will become thinner and tense. Thus, there is very little difference between the rubber sheet and the compounds in the Cell-to-be. At the time the compounds are dismantled, the particles rarefy and tension occurs inside the compounds. With their particles compressed, the four rectangles change their shapes into diagonals with the particles rarefied at area X.
Illustration 4 shows the presence of the DNA-to-be (represented by the blue broken lines) in it. The outer part is surrounded by an area, which though tense, is yet sparse with particles.
Later when it is about time for the Chromosome-to-be to split itself up, it is the inner part where the DNA-to-be is located that divides first and foremost
What is interesting about Illustration 5 is the fact that the trapezium has to break itself up right in the middle of the intersections, between the two diagonals, prior to the division of the Cell-to-be, when centers Xi and X2 are trying to take possession of their respective shares. Thus a Chromosome-to-be splits itself up into two chromatids-to-be, such that each gets half of the short arm, the centromere, as well as the long arm at each chromatid. This is understandable, because both the twin X1 and X2 are equally capable of taking possession of A, which has by now become A1 and A2.
As a matter of fact it is not quite appropriate to say that the point referred to as the intersection of the diagonals (A), assumedly the centromere-to-be, lies exactly in the middle, because here it is positioned somewhat further down.
What is involved here is the density and the tension of the material. This is so because with the lower corners stretching farther than the upper corner, the thin area is made to shift lower down (see point B, which represents the centromere-to-be in Illustration 6). Not only this, with the lack of homogeneity of the stretched area, determination of the position of the centromere-to-be is made impossible. If half of the upper part is more elastic than the lower part, for example, the centromere-to-be part will then stretch further down prior to the division, though later, after the division and after it shrinks back, it will occupy a position further up.
Thus it is clear that the intersection of the diagonals of the trapezium is not exactly the point where the centromere-to-be will later be formed.
Another possible explanation about the formation of those “defects” that could possibly turn into a centromere (illustration 6b) in the Chromosome, is that there has been some stagnation in the process of the dismantling of the compounds of the Chromosome because some other process of dismantling is simultaneously taking place nearby. (Relate this with illustration 8).
In illustrations 2, 3, 4, 5, and 6a, the upper part of the trapezium is intentionally made to appear wider in order to make it easier for the viewer to see its shape. In reality A1 and A2 are attached to each other; nonetheless, the trapezium they form is still obvious. The upper part of the trapezium is in measurements of Angstrom, whereas the lower part is in measurements of Nano or Micron.
What then are we to say of those Chromosomes of which the centromere is situated in the middle? What if the centromere is located at the very far end of the upper part? What about those Chromosomes that have either plenty of centromere or none at all?
It is from the moment the core begins to divide until the time the compounds of the Chromosome-to-be separate that tension and rareness of the material inside it prevails, thus resulting in the two divided parts being totally split apart as is shown in illustration 5. It seems that the position of the centromere largely depends not only on the elasticity of the DNA-to-be but also on the elasticity of its “wrap”. (See illustration 8).
The varied positions of the centromere are thus highly determined by the degree of ease with which the compounds dismantle. The implication here, therefore, is that the degree of elasticity of the DNA-to-be and the degree of homogeneity of the wrap represent essential factors in the determination of the position of a centromere. If, as is shown in illustration 2, the short arm is—as its name verily implies—shorter than the long arm, which serves as the submetacentric—as it does in human beings—the dismantling of the DNA-to-be will then become rather difficult.
A centromere that is situated almost at the centre (metacentric) signifies that the core of the Cell-to-be divides with ease and that the division happens at almost the same time as the division of the DNA-to-be inside the Chromosome-to-be. A centromere that is metacentric is also an indication that the compounds are more easily dismantled. In an acrocentric centromere—one that is located at the end—such dismantling is more difficult, though not as difficult as in a telocentric centromere—one that is located at the end at such extremity that its presence is invisible. In such a case as this, it is quite possible that the compounds are very elastic and that ends A1 and A2 do not divide with ease. (Illustration 7c and 7d)
Illustration 8 shows how the protein-coated DNA-to-be (colored blue) first starts to divide and then replicates, whereby a pair of Chromosomes-to-be, very much like the ones we have today, is formed.
Following its division, the tension in the DNA-to-be gradually reduces, consequently causing the DNA-to-be to contract. Since in its initial development things had worked sideways, the contraction therefore works sideways too.
The divided Chromosomes-to-be then develop new shapes (illustration 8d). But why is it then that the protein coat re-compresses such that it eventually encloses the divided DNA-to-be? This is understandable as (illustration 8a) the protein coat has since its initial development established so close a relationship with the DNA-to-be that it seems to be so attached to the DNA-to-be. With the division of the DNA-to-be, the skin expands, such that after the DNA-to-be is totally divided, the skin will stick back to the DNA-to-be, though what is left now is only half of it. (Illustration b, c, d). This is a Chromosome-to-be which contains only half of the DNA-to-be.
This re-containment of the DNA-to-be may not be quite perfect, as it is not that easy for the traces left by the division to disappear. Traces such as these represent a kind of a “defect” in the body of the Chromosome-to-be, which is why their presence in an area can cause the area to be no longer perfect.
]In illustration 8d, the arrow points to the said defect.
Illustration 8e shows the condition of the DNA-to-be and its counterpart in the process of replication, eventually turning intact though their defects still stay with them.
Illustrations 8a through to 8e represents a map of one generation with illustration 8e being mapped at the time when the DNA-to-be is in its metaphase and replication has been accomplished.
Illustration 8f depicts the DNA-to-be in its metaphase following a long period of evolution.
It is this very defect that will later cause the microtubule that originates from the centrosome to be hooked and that will later become the site where the centromere makes its appearance.
Illustration 8f is an illustration of what followed after 8e had undergone a period of evolution so long that a Chromosome—very much like the one we have today—was formed.
The process of the dismantling of the compounds is the most dramatic event in a Cell-to-be, as it is a process which invariably leads to the occurrence of numerous changes in the body of the Cell-to-be.
It is perhaps necessary to clarify here that as far as the initial emergence of the Cell-to-be is concerned, the “upper” part refers to the short arm that is connected with the surface of the Cell-to-be body, whereas the “lower” arm refers to the long arm connected with the centre of the Cell-to-be.
A look into the various possibilities concerning the formation of the Centromere-to-be in the Chromosome-to-be may well tell us that it is the “tension” and “rareness” between the core and the upper end of the Chromosome-to-be that determines where the intersections are to be located so as to closely resemble the diagonals of a trapezium, thus forming the Centromere-to-be. In the sentences here much mention is made of the word “Chromosome-to-be”, though the Chromosome-to-be meant here was originally oftentimes referred to only as DNA-to-be. This is so, because by now the DNA-to-be is being blanketed such that it gradually begins to appear as a Chromosome-to-be.
Now, what if in the process of the dismantling of all the compounds there occurs some disturbances in the formation of the Centromere-to-be? To put it another way, what could possibly happen if at the time when only partial dismantling had been accomplished there suddenly occurred some disturbances to the tension such that the whole process came to a halt and became temporarily stagnated because there were some other process of dismantling going on at the same time nearby. Should this happen, we could perhaps expect that sooner or later there would emerge a Chromosome with a number of Centromeres.
A pair of Chromosomes takes the shape of an X, and as thus it closely resembles the diagonals of a trapezium. The Chromosome has had its X shape since it was first formed, though its very formation itself had been preceded by the formation of the DNA. During the re-contraction following the dismantling of the compounds, there emerged a Chromosome-to-be taking the shape of a semi-X. It was only after the replication of the DNA-to-be that the pair of semi-Xs was able to incite the X-shaped Chromosome-to-be to make its appearance.
Here our discussions will be confined to only those that are submetacentric, i.e. the ones similar to the Chromosomes of a human being as is shown in 7b.