Today it is unlikely that anyone will be surprised by such concepts as heredity, genome, DNA, nucleotides. Everyone knows about the double helix of DNA and what exactly it is responsible for the formation of all signs of the body. But not everyone knows about the principles of its structure and subordination to the fundamental 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), undoubtedly, relate precisely to such. Although he did not receive the Nobel Prize, until the end of his days he believed that James Watson and Francis Crick had stolen his idea of a double-stranded helical structure of DNA and his Nobel.
Universities in Poland, Germany, the United States and France are proud that this outstanding biochemist taught at them. In addition to the fundamental rules of Chargaff for DNA, he is known for another - the golden rule. That's what biologists call him. And the golden rule of E. Chargaff sounds like this: “One of the most insidious and nefarious properties of scientific models is their tendency to take over, and sometimes supplant, reality.” In simple terms, this means - do not tell nature what it needs to do, and it will not tell you where you should go with all its claims. For many young scientists, this Erwin Chargaff rule has become a kind of motto for scientific research.
Academic background
Recall the basic fundamental concepts necessary for understanding the subsequent text.
Genome - the totality of all the hereditary material of a given organism.
Monomers form polymers - structural units that combine into high molecular weight organic molecules.
Nucleotides - adenine, guanine, thymine and cytosine - are monomers of a 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) bases.
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 by 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, led by E. Charguff, was involved in DNA chromatography in 1950-1952. It was already known that it contains four nucleotides, but no one knew about its spiral structure. Multiple studies have shown. That in the DNA molecule the amount 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 the Charguff rule for deoxyribonucleic and ribonucleic acids.
Value in biology
It was this rule that became the pillar that Watson and Crick were guided in deriving the structure of the DNA molecule. Their double-stranded, spiral-wound model of balls, wire and figures explained this equality. In other words, Chargaff’s rules are that thymine binds to adenine, and guanine binds to cytosine. It was this ratio of nucleotides that fit perfectly into the spatial model of DNA proposed by Watson and Crick. The discovery of the structure of the molecule of deoxyribonucleic acid pushed science to discoveries of a wider level: the principles of variability and heredity, biological DNA synthesis, explanation of evolution and its mechanisms at the molecular level.
Pure Chargaff rules
Modern science formulates these fundamental principles with the following three postulates:
- The amount of adenine corresponds to the amount of thymine, and cytosine corresponds to guanine: A = T and G = C.
- The number of purines is always equal to the number of pyrimidines: A + G = T + Ts.
- The number of nucleotides that contain in position 4 of the pyrimidine and 6 purine bases is equal to the number of nucleotides that contain in the same positions of the oxo group: A + G = C + T.
In the 1990s, with the discovery of sequencing technologies (determining the sequence of nucleotides in long sections) of DNA, the rules of Chargaff were confirmed.
Students 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 a sequence of nucleotides in one chain - AAGCTAT. The student or student is required to restore the second chain based on the DNA template chain and the first Chargaff rule. The answer will be: GGATTSGTS.
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 in biology is considered fundamental to understanding the basics of molecular biochemistry and genetics.
For science, not everything is so simple
E. Charguff 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 accurate, but only approximately. This is the second rule of Chargaff: in a single strand of deoxyribonucleic acid, the amount of adenine is approximately equal to the amount of thymine, and guanine to cytosine.
Violations of equality were directly proportional to the length of the analyzed area. Accuracy is maintained at a length of 70-100 thousand nucleotide pairs, but at lengths of hundreds of pairs and less than bases, it is no longer maintained. 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 the ordinary genomes of organisms, an equal distribution of nucleotides is more likely an exception than a rule.
DNA does not reveal its secrets
With the development of genome sequencing techniques, it was found that a single DNA strand contains approximately the same number of complementary single nucleotides, pairs of nucleotides (dinucleotides), trinucleotides, and so on - up to oligonucleotides (plots of 10-20 nucleotides). The genomes of all known living organisms obey this rule, with very few exceptions.
So, two Brazilian scientists - the biologist Michael Yamagishi and the mathematician Roberto Herai - used set theory to analyze the nucleotides needed for the sequence so that they led to the implementation of the Chargaff rule. They derived four equations of sets 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 the parasitic bacterium that causes the fast withering of olive trees do not at all obey the laws of the Chargaff rule. Why? No answer found yet.
Biochemists, evolutionary biologists, cytologists and genetics today are struggling with the mysteries of DNA and the mechanisms of inheritance. Despite the achievements of modern science, mankind is far from the solution to the universe. We overcame gravity, mastered outer space, learned 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 created billions of years on planet Earth.