Zakia Bayguzhina
DNA Reveals Its Secret
Erwin Chargaff (1905–2002) – it is with his name, more precisely with his research, that the secret of DNA, which we have to uncover, is connected.
Between 1949 and 1951, biochemist Erwin Chargaff and his group made a major discovery. They determined the quantitative ratio of purines and pyrimidines in DNA. In the future, the spatial structure of the molecule will be built, where the principle of complementarity of nitrogenous bases will be observed, and the founder of this is Erwin Chargaff. DNA contains the code of heredity, and in order to protect it, it was necessary for just such a structure of purines and pyrimidines.
So, all the components of DNA were found, the main provisions were formulated, it remained only to collect all the accumulated material to create the spatial structure of DNA. However, one important component was still missing… It was a DNA X-ray. X-ray diffraction studies are related to the names of Rosalind Franklin (1920–1958) and Maurice H. F. Wilkins (1916–2004).
On April 25, 1953, the journal, Nature, published an article by Francis Crick (1916–2004) and James Dewey Watson (born 1928) on the spatial structure of DNA. There was only one page of text, but it was with this publication that the era of Molecular biology began.
All ingenious things are simple, we see this repeated over and over again. Nature does not like complicated paths – and in this molecule, which has existed for billions of years, this principle is justified more than ever.
All living organisms, as well as some viruses, have DNA. It is known that it is the carrier of hereditary information. But is this its only function? Maybe there are others that we don't know about yet? Why, for example, is the DNA polymer of one human cell, if untangled, so long – about two meters? Or another question – why are genes such a small part of this molecule? Why, in this case, do we need the rest of the DNA, which is often called "junk"? But this cannot be! If nature has created such a long creation-connection, then why did it need it?
The hereditary information stored in the DNA polymer is well protected, the nitrogenous bases are hidden inside the molecule. The chains, consisting of phosphoric acid and deoxyribose residues, form a strong external structural backbone of the molecule.
Various types of DNA are currently known. But we are interested in the double-stranded B-form.
The structure of DNA is as follows: two antiparallel polynucleotide chains are twisted about the axis. On the periphery of the molecule there are carbohydrate-phosphate chains, inside – nitrogen-containing heterocycles. DNA contains four nitrogenous bases: purine – adenine and guanine and pyrimidine – thymine and cytosine. But there are exceptions, for example, some viruses have another pyrimidine derivative – uracil. The nitrogenous bases of one antiparallel chain are connected to a specific base of the other chain, observing Chargaff's rule: adenine combines with thymine, guanine with cytosine. This arrangement is called complementary. Pairing of nitrogenous bases is carried out using hydrogen bonds.
Scheme of arrangement of complementary nitrogenous bases in DNA.
To understand the principle of complementarity, let's look at the formulas for nitrogenous bases.
From our school chemistry course, we know that the ordinal number of a chemical element according to D. I. Mendeleev corresponds to the magnitude of the nuclear charge, as well as the number of electrons. Let's count the number of electrons in each nitrogenous base in DNA.