All basic information about an organism is recorded in its DNA. DNA molecules are long, thin threads that encode instructions for the construction and operation of body structures. In humans, their total length is about three meters; in each cell. In an elegant natural solution, the instruction threads are divided into separate information blocks – chromosomes; conditionally – volumes of instructions. Blocks are different in size (length), and they contain different information. There are 23 such blocks in human cells. More precisely, 23 pairs – a total of 46. The information is duplicated (albeit with some nuances). The set of chromosomes in most cells is diploid, denoted by: 2n.
- This is what a human karyotype looks like – a complete collection of volumes of instructions. Each volume is in two variations – pairs of chromosomes. In one of the pairs, the variation is very significant, but only in one sex.
In chromosomes, DNA strands are extremely tightly packed. Packed in such a way that all almost three meters of polymer are successfully placed in the core of a small cell. Packaged with the help of special protein molecules, on which DNA is strung, and with which they are plastered. Packed in an orderly manner, so that if necessary, the appropriate fragment can be quickly unpacked and “read”. “Read” means that according to the available instructions, the required number of protein molecules will be built to perform the specified functions in the body.
In addition to issuing direct instructions, DNA strands in a cell can copy themselves (they have their own instructions encoded for this). Copying is necessary so that when the cell divides, each daughter has a complete set of all instructions. When the DNA is copied, each chromosome of the 23 pairs contains two identical strands. 46 chromosomes, each with two strands of DNA. In the process of cell division, which is called “mitosis”, the strands of chromosomes must be distributed absolutely equally. This is an extremely complex, risky and beautiful procedure.
For an even distribution between the daughter cells, the chromosomes must first be further compacted. To compact so much that they become visible under a microscope. It is in such a condensed “parade” form, from two joined longitudinal halves – sister chromatids, in which form they are primarily known, and not in their working form “ball of threads”.
- The DNA strand is compacted 10,000 times, forming a chromosome. The sealing is very orderly. Histones are molecular coils on which the DNA thread is wound in order to be ordered and compact.
The dance, or parade of chromosomes, needs space, so many internal structures of the cell are temporarily disassembled, freeing up space. The nuclear envelope, which previously separated the working, tangled chromosomes from the rest of the cell, dissolves to then “magically” reassemble itself around the chromosomes in the daughter cells. The structures of the endoplasmic reticulum are factories where protein molecules are produced according to the instructions received from DNA, break up into small droplets and are pushed to the periphery. The Golgi complex, a structure of extensive tanks with synthesis products, also partially disintegrates. During mitosis, everything else in the cell is pushed to the background.
Chromosomes are aligned in the central plane of the cell. The so-called “metaphase plate” is formed. Of course, they are not built by themselves. They are key in this process, but their dance is more like a puppet dance. Large and complex molecular mechanisms – kinetochores – are attached to the central parts of compacted chromosomes, called centromeres. Each separate daughter chromosome has its own kinetochore attached. On one side it holds the chromatid, on the other side it is attached to the thread-rails. Threads-rails extend through the cell, perpendicular to the plane in which the chromosomes are lined up. Threads-rails are attached to the poles in the organizational centers – centromeres.
Threads-rails are attached to the poles in the organizational centers – centromeres. A division spindle is formed from these threads. The forces arising on these threads move the chromosomes and create the amazing choreography of mitosis.
- The most vividly secreted dance of chromosomes is revealed by the methods of fluorescence microscopy.
When the spindle is just forming, threads from the organizational centers at the poles grow randomly, facing each other. Having encountered the kinetochore of chromosomes, they grab it and push it until the kinetochore of the sister chromosome encounters the thread from the opposite side. Then the pulling and pushing begins. The threads of the spindle sometimes increase, then shorten from different sides, until the oppositely directed forces are balanced. It is then that the chromosomes are lined up in one plane perpendicular to the spindle – the metaphase plate.
Lined up at the metaphase plate, they freeze for a brief moment, after which the kinetochore machines on each sister half of the chromosome turn on. Kinetochores, like locomotives, pull sister chromatids along rails in different directions, to opposite poles of the dividing cell. Thus, each of the daughter cells receives its own set of chromosomes after the doubling of the mother cell. This happens in every cycle of cell division.
Many complex molecular machines are involved in this important process: kinetochore motors, spindle threads, etc. And, unfortunately, in various elements of each of these systems, failures and troubles can occur. For instance, one of the kinetochores in an oppositely directed pair may fail to turn on, or lose contact with its spindle filament “rail,” or release its chromatid. Then its counterpart will pull the undivided sister chromatids to its pole. It is clear that in this case, one of the child cells will not receive one of the volumes of instructions, and the other will receive two volumes. For both of them, this will become a significant problem both for functioning and for survival in general (an extra volume of instructions is not just a load, but an active participant in the processes that will be significantly disrupted in such an imbalance).
When such disruptions occur not just during the division of cells in the body, but during the formation of germ cells, the problem becomes even more dramatic, since the zygote – the cell from which a totally new organism must be formed – will receive a chromosomal imbalance.
The spontaneous risk of such violations always exists and it is not easy to predict. The fine structures of the fission spindle can be destroyed both by a random high-energy particle that rushed from the universe, and by a toxic molecule that can enter the body. And given the complexity of the process, it seems rather surprising that everything usually works smoothly and reliably.
With older reproductive age in the female body (especially over 35 years), the amount of energy in the mitochondria decreases, and because of this, the probability of incorrect separation of chromosomes in eggs increases, the probability of errors in the transfer of chromosomal material to the future embryo increases. That is why it is so important for us to apply in time for the possibility of the most effective conception of chromosomally healthy embryos. For this reason, we recommend saving your oocytes or embryos obtained at a younger age (up to 35 years) to be sure of the possibility of having healthy children at an older age.