One of the main scientific problems that scientists around the world are working on is the origin of life on Earth. Over the past decades, many successes have been achieved in this area, for example, the concept of the RNA world has been developed. However, it is still unknown how exactly the molecules that served as the first "building blocks" of life arose. The journal Science published an article that answers perhaps the most important question: where did the nucleotides that make up RNA come from. "Lenta.ru" reveals the details of the study and talks about its meaning.
According to modern scientific concepts, life originated from organic compounds that reacted with each other to create key molecules - nucleosides. The nucleoside is known to be formed by the sugar ribose or deoxyribose and one of five nitrogenous bases: adenine, guanine, thymine, cytosine, or uracil. Nucleosides are precursors of nucleotides, of which, in turn, DNA and RNA are composed. For a nucleoside to turn into a nucleotide, an additional component is required - phosphoric acid residues.
Why do nucleosides come to the fore? This question is answered by a scientific concept known as the hypothesis of the RNA world, which believes that it was RNA that stood at the origins of life. The molecules of ribonucleic acids were the first to carry out the catalysis of chemical reactions in the primary soup, learned to copy themselves and each other and, most importantly, carry hereditary information. These RNAs are called ribozymes. If any RNA molecule had the ability to synthesize its own copies, then this property was passed down from generation to generation. Sometimes copying was accompanied by errors, as a result of which new RNAs acquired mutations.
Mutations could seriously harm the catalytic properties of molecules, but they could also alter RNA, giving it new abilities. For example, scientists have found that some mutations speed up the self-copying process, and the altered ribozymes after a while begin to dominate over the "normal" ones. Molecular biologists led by Brian Pegel of the Scripps Research Institute in California have observed how the enzymatic activity of ribozymes increased 90-fold during a three-day evolution in a laboratory. Therefore, even if ribozymes were initially not very active, molecular evolution could turn them into ideal catalytic machines.
Nevertheless, the hypothesis of the RNA world runs into a number of difficulties. For example, it is not known how the abiogenic, that is, without the participation of living organisms, synthesis of the first ribozymes could occur. While many arguments have been found in favor of the RNA world, the key question - how it came about - remains a stumbling block.
Some scientists suggest that the chemical compounds from which nucleosides were formed could not arise in terrestrial conditions, but were brought to the planet from space. It is worth noting, however, that the problem is associated with purine nucleosides - adenosine and guanosine, containing adenine and guanine, respectively. For pyrimidine molecules containing cytosine, thymine or uracil, synthesis pathways are known that could well exist at the origin of life. Domino-like chemical reactions lead to the formation of large quantities of the required pyrimidines.
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Scientists have proposed a possible pathway for the formation of purine nucleosides, but it can lead to the appearance of many other compounds, among which the required nucleosides would be only a small fraction. Just brushing off purines will not work, since they are not only integral components of RNA and DNA, but also form adenosine triphosphate (ATP), which is involved in the metabolism of energy and substances in the body, and guanosine triphosphate, which serves as an energy source for protein synthesis.
A simple way to form a nucleoside like adenosine is to combine adenine with ribose in the presence of NH4OH. Ribose attaches to one of the adenine nitrogen atoms, only it has several of them, and only nitrogen in the ninth position should participate in the synthesis of adenosine. In addition, it turns out that this nitrogen atom is not very reactive. This means that if the hypothesis of the RNA world is correct (which is more than likely), there must be some other way of synthesizing adenosine and guanosine in the primary broth.
In a new study, scientists have proposed a different pathway for the synthesis of purine nucleosides that solves the problem and strengthens the position of the concept of the RNA world. It all starts with aminopyrimidine molecules, which are easily formed from a compound as simple as NH4CN. This happens through the formation of guanidine, it then reacts with aminomalonitrile, resulting in the formation of a tetraaminopyrimidine molecule. It easily oxidizes in an oxygen-containing environment, but remains stable in the oxygen-free atmosphere that was characteristic of the Earth before the birth of life. In addition to tetraaminopyrimidine, other similar molecules can be formed: triaminopyrimidinone and triaminopyrimidine. All these compounds are readily soluble in water.
Most importantly, for all three aminopyrimidines, only a certain nitrogen atom is reactive, and this solves the problem of participation in the reaction of other atoms, which is characteristic of adenine. The acidified environment leads to the fact that nitrogen atoms in the ring attach protons and block all external amino groups, except for one located in the fifth position. When a mixture of aminopyrimidines and formic acid is heated, only one possible compound is formed - formamidopyrimidine. The reaction yield is 70 to 90 percent.
Formamidopyrimidine, despite its similarity to purines, is devoid of their disadvantages. The nitrogen atom in the ninth position, as it turned out, is the most reactive, and the reaction with ribose in an alkaline medium always leads to the same result: the synthesis of carbon skeletons for purine nucleosides. Interestingly, formamidopyrimidine is actively involved in the formation of ribose from glycolaldehyde and glyceraldehyde, facilitating the synthesis of nucleosides in an ammonia environment. In general, scientists have succeeded in discovering a pathway for the formation of nucleotide precursors from the simplest ammonia derivatives. Such derivatives were recently found on the Churyumov-Gerasimenko comet, which confirms the point of view about the active participation of comets in supplying the Earth with everything necessary for the emergence of life.
However, chemical evolution raises many more questions, and to answer them will require the efforts of researchers around the world. The complete picture of abiogenesis should describe not only the emergence of nucleotides and other organic molecules without the participation of living organisms, but also their interaction in the conditions of the early Earth, the interaction that led to the formation of the first cells.
Alexander Enikeev
