Today, hardly anyone will be surprised by such concepts as heredity, genome, DNA, nucleotides. Everyone knows about the double helix of DNA and that it is she who is responsible for the formation of all the signs of an organism. But not everyone knows about the principles of its structure and subordination to the basic rules of Chargaff.
Offended Biologist
Not many discoveries are awarded the title of outstanding in the twentieth century. But the discoveries of Erwin Chargaff (1905-2002), a native of Bukovina (Chernivtsi, Ukraine), are undoubtedly one of them. Although he did not receive a Nobel Prize, he believed to the end of his days that James Watson and Francis Crick stole his idea of the double-stranded helical structure of DNA and his Nobel Prize.
Universities in Poland, Germany, USA and France are proud to have this outstanding biochemist teaching there. In addition to Chargaff's fundamental rules for DNA, he is known for another one - the golden rule. That's what biologists call it. And the golden rule of E. Chargaff sounds like this: “One of the most insidious and nefarious properties of scientific modelsis their tendency to take over, and sometimes supplant, reality . In simple terms, this means - do not tell nature what to do, and she will not tell you where you should go with all your claims. For many young scientists, this rule of Erwin Chargaff has become a kind of motto of scientific research.
Academic foundations
Recall the basic fundamental concepts necessary for understanding the following text.
Genome - the totality of all hereditary material of a given organism.
Monomers form polymers - structural units that combine to form high-molecular organic molecules.
Nucleotides - adenine, guanine, thymine and cytosine - monomers of the DNA molecule, organic molecules formed by phosphoric acid, a carbohydrate with 5 carbon atoms (deoxyribose or ribose) and purine (adenine and guanine) or pyrimidine (cytosine and thymine) grounds.
DNA - deoxyribonucleic acid, the basis of the heredity of organisms, is a double helix formed from nucleotides with a carbohydrate component - deoxyribose. RNA - ribonucleic acid, differs from DNA in the presence of ribose carbohydrate in the nucleotides and the replacement of thymine with uracil.
How it all started
A group of scientists at Columbia University in New York, headed by E. Chargaff in 1950-1952, was engaged in DNA chromatography. It was already known that it consists of four nucleotides, but no one has yet known about its helical structure.knew. Multiple studies have shown. That in a DNA molecule the number of purine bases is equal to the number of pyrimidine bases. More precisely, the amount of thymine is always equal to the amount of adenine, and the amount of guanine corresponds to the amount of cytosine. This equality of nitrogenous bases is Chargaff's rule for deoxyribonucleic and ribonucleic acids.
Meaning in biology
It was this rule that became the basis on which Watson and Crick were guided when deriving the structure of the DNA molecule. Their double-stranded helically twisted model of balls, wires, and figurines explained this equality. In other words, Chargaff's rules are that thymine combines with adenine and guanine combines with cytosine. It was this ratio of nucleotides that ideally fit into the spatial model of DNA proposed by Watson and Crick. The discovery of the structure of the deoxyribonucleic acid molecule prompted science to discover a wider level: the principles of variability and heredity, the biological synthesis of DNA, the explanation of evolution and its mechanisms at the molecular level.
Chargaff rules in their purest form
Modern science formulates these fundamental provisions with the following three postulates:
- The amount of adenine corresponds to the amount of thymine, and cytosine to guanine: A=T and G=C.
- The amount of purines is always equal to the number of pyrimidines: A + G=T + C.
- The number of nucleotides that contain pyrimidine in position 4 and 6purine bases, is equal to the number of nucleotides that contain oxo groups in the same positions: A + G \u003d C + T.
In the 1990s, with the discovery of sequencing technologies (determining the sequence of nucleotides in long sections), Chargaff's DNA rules were confirmed.
Children's headache
In high school and at universities, the study of molecular biology necessarily involves solving problems on the Chargaff rule. They only call these tasks the construction of a second DNA chain based on the principle of complementarity (spatial complementarity of purine and pyrimidine nucleotides). For example, the condition gives the sequence of nucleotides in one chain - AAGCTAT. The pupil or student is required to reconstruct the second strand based on the DNA matrix strand and the first Chargaff rule. The answer will be: GGATCGTS.
Another type of task suggests calculating the weight of a DNA molecule, knowing the sequence of nucleotides in one chain and the specific gravity of nucleotides. Chargaff's first rule of biology is considered fundamental to understanding the basics of molecular biochemistry and genetics.
For science, not everything is so simple
E. Chargaff continued to study the composition of DNA, and 16 years after the discovery of the first law, he divided the molecule into two separate strands and found that the number of bases is not exactly equal, but only approximately. This is Chargaff's second rule: in a separatestrands of deoxyribonucleic acid, the amount of adenine is approximately equal to the amount of thymine, and guanine - to cytosine.
Equality violations turned out to be directly proportional to the length of the analyzed section. Accuracy is maintained at a length of 70-100 thousand base pairs, but at lengths of hundreds of base pairs and less, it is no longer preserved. Why in some organisms the percentage of guanine-cytosine is higher than the percentage of adenine-thymine, or vice versa, science has not yet explained. Indeed, in ordinary genomes of organisms, an equal distribution of nucleotides is rather an exception than a rule.
DNA does not reveal its secrets
With the development of genome sequencing techniques, it was found that a single strand of DNA contains approximately the same number of complementary single nucleotides, base pairs (dinucleotides), trinucleotides, and so on - up to oligonucleotides (sections of 10-20 nucleotides). The genomes of all known living organisms obey this rule, with very few exceptions.
Thus, two Brazilian scientists - biologist Michael Yamagishi and mathematician Roberto Herai - used set theory to analyze the nucleotide sequences needed for a sequence to lead to the Chargaff rule. They derived four set equations and tested 32 genomes of known species. And it turned out that fractal-like patterns are true for most species, including E. coli, plants and humans. But the human immunodeficiency virus and a parasitic bacterium that causes rapid wiltingolive trees, do not obey the laws of Chargaff's rule at all. Why? No answer yet.
Biochemists, evolutionary biologists, cytologists and geneticists are still struggling with the mysteries of DNA and the mechanisms of inheritance. Despite the achievements of modern science, humanity is far from unraveling the universe. We overcame gravity, mastered outer space, learned how to change genomes and determine the pathology of the fetus in the early stages of embryo development. But we are still far from understanding all the mechanisms of nature that it has been creating for billions of years on planet Earth.
