Evolution of the genetic code. Emergence of DNA

This hypothesis can provide an opportunity to trace logically the process of the emergence of the DNA double helix. AT­enrichment in this hypothesis is main factor of evolution of DNA double helix from RNA double helix. The hypothesis that at the early stages there were just cytosine and guanine [1,2,3] makes the picture of the RNA world much poorer. It is difficult to form a great variety of tertiary structures based on nucleotides that tend to form almost exclusively Watson­Crick's pairs. This hypothesis can be said to restrict and, thus, to simplify the picture of the early RNA world. However, this hypothesis can provide an opportunity to trace logically the process of the emergence of the DNA double helix. The generally accepted picture of the RNA world suggests that DNA was invented to separate the function of information storage from the function of translation. The double copy is more stable towards interferences and mutations, but less accessible to enzymes. The emergence of DNA is often associated with the genome increase, suggesting that smaller organisms such as viruses can make do with RNA. This interpretation ignores one low­probability assumption – it is accepted de fide that a random replacement of ribose by deoxyribose resulted in the transformation of the previously single­strand RNA into the double helix. To make the copy more stable towards mutations! But this is pure teleology! Yet, nucleotides themselves already possess complementarity properties that allow them (in a miraculous way) to be incorporated into this double helix. If this did not occur at the early stages and complementarity was necessary for the nucleotide­to­nucleotide replication, how can one account for this amazing correspondence? The hypothesis suggesting that at early stages of the evolution of the genetic code there were just cytosine and guanine makes it possible to bring the emergence of DNA back to evolutionary philosophy. The complementary long chains based on guanine and cytosine only (CG­polyribonucleotides) are more apt to form the double helix than the complementary polyribonucleotides based on all four pairs. The reason is simple: guanine and cytosine form just Watson­Crick's pairs due to a significant energy benefit as compared to other variants. I can admit that different variants of oligomeric complementary RNAs based on cytosine and guanine can yield double helixes of different stabilities. It is important that for a random sequence of CG ribo­ oligomer the RNA double helix will be stable. The next …