Israeli researchers have identified a molecular mechanism that helps explain one of the great paradoxes of evolution: how it is possible for man to perform such complex functions with a relatively small number of genes
Merit Sloin
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The DNA is constantly changing, and the changes that apply to it are responsible for the great diversity in nature, the creation of new species and the survival of existing species. The main mechanism in the cell that contributes to variation in nature is the creation of new genes. As far as we know today, this is done by doubling existing genes. When a gene is duplicated, two copies are made of it: the original copy maintains the original function, while the new copy may undergo changes that give it new functions. The new gene can be present in DNA for an extended period of time without causing any effect. If at any stage new environmental conditions are created in which it has an advantage, it becomes necessary for the survival of the species.
According to this model, the more genes there are, the greater the number of functions; And as we go up the evolutionary ladder, the number of genes in the genome will increase. But the decoding of the human genome showed that this is not the case. Before the decoding of the genome, it was estimated that a person has about 100,000 genes, but now it is known that the number of human genes is lower than 30,000. For comparison, parasitic intestinal worms, which do not have a head and do not perform complex functions, have 18,000 genes.
The discovery that humans have a relatively small number of genes raised the question of how human complexity arose. How did the functions of thinking, communication, memory, learning and more develop in a system where there are so few genes. What is the unique mechanism developed in primates that made this possible, and how less than 30,000 genes supervise the operation of the approximately 90,000 proteins that build and maintain the human body.
A research group from the School of Medicine at Tel Aviv University, led by Dr. Gil Ast, discovered a new mechanism by which new proteins are created in the body based on existing genes. The study, which was recently published in the journal "Science" provides answers to questions about human evolution, which had no answer until now.
Biologists struggle to explain the fact that only about one and a half percent of the human genome contains information for building proteins. In contrast, the majority of the genome consists of meaningless segments that repeat themselves hundreds and thousands of times and do not contain information to create proteins. Why does the body leave all this excess baggage and invest a lot of energy in its maintenance? For years, no answer was found to this question, and the excess genetic load was nicknamed "DNA garbage".
Among the repetitive segments in the DNA collection are short segments, which make up 11% of the total human genome. These segments are called Alu segments (1.4). A million copies of them circulate in the genome, all of them are almost identical, and they can only be found in humans and monkeys.
Finding the connection between these segments and the outbreak of diseases caused researchers to change their attitude towards these segments - they are no longer considered DNA garbage and researchers are beginning to recognize that they may affect the development of the organism. But until now no one knew how this happens.
The research team from Tel Aviv University was able to reveal for the first time the mechanism responsible for this and showed that these segments are actually created from nothing. They are responsible for creating new proteins by being "pushed" into the genes and creating new information without damaging the existing genetic information. Because these sequences are unique to primates, the new information likely contributes to some of our characteristics as humans.
These segments use a mechanism that exists in the cell, which differentiates between the main to handle the genetic material. In each cell, there are informative regions and meaningless regions on the surface of the DNA. The genes contain the operating instructions for building the proteins, and those who mediate between the two are RNA molecules. The DNA molecules create an exact copy of the genetic code stored in the DNA and transfer it to the ribosomes, the protein factories. Before moving to the ribosomes, the RNA is processed. In a special process known as splicing, meaningless areas are cut from it and removed, and the edges of the cut edges are joined together. Thus, only the meaningful information is expressed in the cell.
During the evolution of the human race, these fragments penetrated into the meaningless regions of the DNA and joined the DNA collection. Ast and his team found that, over time, the Alu segments began to accumulate mutations, and some of the mutations turned certain Alu segments into segments recognized by the splicing system as meaningful. Meaningful segments, as mentioned, are not eliminated by the splicing system, and the information they contain is translated into proteins.
However, Est and his team found that even when these segments are known to the splicing system, only in half of the cases does it treat them as meaningful segments, attaching them to existing genes and thus creating a new protein. In the second half of the cases, the splicing system cuts these segments and throws them away along with the other meaningless segments. In the first case, therefore, the combination of these segments with meaningful parts creates new information that leads to the creation of new proteins; In the second case, the information in the garden remains unchanged.
"We identified a mechanism in which these segments lead to the creation of new proteins while being careful not to damage the previous proteins," says Est. "If this mechanism did not exist and the old information was not preserved, the original proteins would have changed. Such a situation can cause the development of diseases." Indeed, the researchers found three diseases that were created in this way.
In the mechanism described by the researchers, there are therefore two products for the same gene. The original copy maintains the previous activity, while the new copy, to which this segment joined, can take on an activity that did not exist in the cell before. "According to our findings, some genes can give building instructions for two proteins, which means that a relatively limited number of genes can be used to create many proteins," says Est. "Since this unique system is only found in monkeys and humans, it explains how the human species is able to perform such complex functions with a small number of genes."
The processes uncovered by Est and his team also provide an explanation for the question of why the cell does not get rid of the enormous excess DNA in the genome. Within these surpluses are the reserves for the creation of the new information that drives the wheels of evolution.
In an accompanying article in the journal "Science," Vucic Maklowski of the Institute for Molecular Genetic Evolution at the University of Pennsylvania writes: "If we risk humanizing biological processes, we can say that evolution is too smart to waste valuable information. Therefore, the repeated DD-NA segments must not be called DD-NA Ashfat. Instead, they should be called the 'playground' of the genome, being a repository of ready-made segments that can be used for nature's evolution experiments."
