Those who are not familiar with biology, genetics are interested in how the cells of the body "understand" that some should become hair, others bones, others - brains, etc.? Organs are formed sequentially, some continue to form throughout life, somehow the command "start formation" and "complete formation" must be given. And if these teams are not formed from a single center, chaos will arise.
Where is this center then?
This question is not childish at all. This is actually not one, but several questions, and they touch upon all the most important problems, the solution of which is being dealt with by a large, very complex and rapidly developing science - developmental biology. It is simply impossible to answer these questions well and in detail in a few words. The answers to them are contained in large and thick books and thousands of scientific articles. Much in this science is still unclear, and new discoveries are made almost every day.
But some general principles can be tried to explain.
Let's start with the "single center", without which "chaos" will arise. Surprisingly, this is not the case. Many dividing cells can behave quite intelligently and form complex structures, even if they do not have a single control center. Such processes are called "self-organization". Unfortunately, the human mind is so structured that it is terribly difficult for him to understand such processes. When we come across examples of self-organization, it always seems to us some kind of inexplicable miracle. For example, how do beautiful ice patterns on glass or snowflakes form from randomly moving water vapor molecules? Where is the "snowflake program" or its "blueprint" stored? There is no drawing anywhere, but the program exists, these are the physical properties of the water molecule, on which the formation of ice crystals depends.
But back to the clump of cells - the tiny embryo that formed from the egg as a result of the first few divisions. Every cell in the embryo has the same genome (set of genes). The genome determines all the properties of a cell, this is its "program of behavior". The program for all cells of the embryo is the same. However, cells soon begin to behave in different ways: some turn into skin cells, others into intestinal cells, and so on. This is due to the fact that cells exchange information - they send each other chemical signals and change their behavior depending on what signals they received from their neighbors. Signals can also be physical: cells can "feel" their neighbors where they are pulling or pushing. In addition, some signals come from the outside world. For example,embryonic cells in plants sense gravity and take it into account when deciding how to behave. For example, those cells that have neighbors only from the top begin to turn into a root, and those with neighbors only from the bottom - into a stem. Finally, the ovum may have a simple "marking" from the very beginning: one of its poles may differ from the other in the concentration of some substances.
The behavior program for all cells is initially the same, but it can be quite complex and consist of several separate sets of rules. Which of the sets of rules a given cell will execute depends on the signals received by the cell. Each separate "rule" looks something like this: "if such and such conditions are met, do such and such an action." The main actions that cells do is turn certain genes on or off. Turning on or off the gene changes the properties of the cell, and it begins to behave differently, to react differently to signals.
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How is it that cells that have the same program of behavior and are seemingly in the same conditions still behave differently? The fact is that the cells of the embryo are actually in different conditions - it just happens by itself in the process of cell division. Someone turned out to be inside, someone outside, someone below, someone on top, in someone the concentration of substance A is high (because this cell was formed from that part of the egg cell where there was a lot of this substance), and in whom -that substance A is small.
Cells may also have a "division counter" that tells them how many times the egg has already divided. This counter is also chemical: initially there were certain substances in the egg, the supply of which is not replenished during the development of the embryo, and by how many of these substances remained in the cell, one can understand how many divisions have passed since the beginning of development.
The cell behavior program may contain, for example, the following commands:
“If you are outside, and if the concentration of substance A in you is such and such (is within such and such limits), and if the concentration of substance B around you is zero, and if 10 divisions have passed since the beginning of development, then start to excrete substance B."
What will the execution of such a command lead to? It will lead to the fact that at a certain moment (after ten divisions) a single cell appears on the surface of the embryo that secretes substance B. It will be located at a strictly defined distance from one of the poles of the embryo, because in our example, substance A served for the initial oocyte marking. Therefore, by the concentration of substance A, the cell can determine at what distance it is from the poles of the embryo. Why is there only one such cell that secretes substance B? But because there was an instruction: "If the concentration of substance B around you is zero." As soon as the first cell in which the stated conditions are fulfilled begins to release substance B, the concentration of this substance will cease to be zero, and therefore other cells will not start to release it.
And what happens if we remove the instruction “If the concentration of substance B around you is zero” from the program? Then substance B will begin to be secreted not by a single cell, but by a whole strip of cells encircling the embryo at a certain distance from the poles. The width of the belt and its position (closer or further from the pole where the concentration of A is maximum) will depend on what concentrations of substance A are indicated in the instruction "If the concentration of substance A in you is such and such."
Now our embryo is marked out much more complicated and interesting than before. He has a "front part" in which there is a lot of A, and the concentration of B increases from front to back; it has a central belt, where the concentration of B is maximum; and it has a back, where there is little A and where the concentration of B decreases from front to back. Our embryo has subdivided into sharply delimited parts, in which the cells are in different conditions and therefore will carry out different subroutines of their original general program.
We have subdivided the embryo into anterior, middle and posterior sections. They can become, for example, the head, torso and tail. But I would also like to understand where his back will be, and where his stomach is. How to do it? It's very simple, we have already gone through this. An instruction is needed that leads to the appearance of only one cell or a small group of cells secreting some substance (for example, B) on any "side" of the embryo, somewhere in the middle between the head and tail. And let this substance B start the program for the growth of a beautiful green dorsal ridge where there is a lot of it, and the program for the formation of a soft pink tummy where it is scarce.
When the embryo is already so well and in detail "marked", each group of cells can easily determine where it is and activate the subroutine prepared for this case (a set of rules of behavior).
During the development of the embryo, it is true that here and there special "control centers" appear - groups of cells that release one or another substance, which serves as a signal for other cells and affects their behavior. But at the same time, all cells still behave in strict accordance with the original genetic program, which is the same for all. Control centers arise by themselves, through self-organization, no one intentionally inserts them there. And no "unified centralized leadership", let alone meaningful, reasonable, is required for this.
In the development of real animals, everything is more complicated than in our imaginary example, but, oddly enough, not by much. For example, in most animals, about a dozen signaling substances are used for the "longitudinal marking" of the embryo (in our example, we managed to do two - A and B). A special group of genes, the so-called Hawks genes, are responsible for the production of these substances. And to separate the embryo into tissues (nervous, muscle, epithelial, etc.), another three dozen other signaling substances are used - they are called microRNAs. But these are only the most important regulators of development, and there are still many auxiliary ones, and scientists have not yet figured out all of their properties and functions.
The signaling substances that govern the behavior of the cells of the embryo are very powerful. For example, if you cut off the tail of a tadpole and drop one of these substances onto the wound, then instead of a new tail, the tadpole will grow a bunch of small legs. Such cruel experiments were carried out at the beginning of the 20th century. Then geneticists got down to business, who learned to change the work of genes in individual parts of the embryo. Including those genes that produce substances - development regulators. One of the most interesting discoveries of geneticists is that the genes that control development are very similar in all animals. They can even be transplanted from one animal to another and they will work. For example, if you take a mouse gene that turns on the mouse eye subroutine and make it work in a fly's leg bud,then an eye begins to form on the fly's leg. True, not a mouse's eye, but a fly's.
So, we realized that there is no "blueprint" of an adult organism in the genome, but only a program for the behavior of an individual cell. The adult organism "self-organizes" simply due to the fact that each cell strictly follows the same program of behavior. Mathematicians say that it would be much more difficult to code a blueprint of an adult animal in the genome than such a program. This program, oddly enough, itself is much simpler than the resulting organism. And also, if our development proceeded not through self-organization on the basis of a program, but according to a blueprint, it would be much more difficult for us to evolve.
A hundred years ago, when scientists still did not know the laws of embryo development, much in evolution seemed incomprehensible to them. For example, some scientists wondered how, in the process of evolution, all four legs could lengthen at the same time - after all, for this, they reasoned, it was necessary that mutations simultaneously change the length of all four legs at once! Indeed, if a drawing of an adult organism were recorded in the genome, then it would be necessary to make four corrections to this drawing in order to increase the length of four legs. Now we know that development proceeds according to a program in which it is enough to make only one change for the length of all four limbs to change, and change in the same way.
Alexander Markov