The origin of the genetic code

Abstract

Standardni genetički kod (SGC) je šifra prevođenja sekvence nukleotida u sekvencu aminokiselina na način da je svaki od mogućih tripleta (nizova od 3) nukleotida – kodon pridružen nekoj od 20 bioloških aminokiselina. SGC je gotovo univerzalan u živom svijetu, zbog čega znamo da je kao takav postojao i u posljednjem univerzalnom zajedničkom pretku (LUCA-i). Unatoč tome što je SGC najkonzerviranija informacija u biologiji, pitanje njegovoga porijekla do danas je nerazriješeno. U ovome radu izložene su vodeće teorije o evoluciji SGC: stereokemijska teorija koja postulira da je stereokemijski afinitet između kodona ili antikodona i pripadajuće aminokiseline uzrok njihovome pridruživanju; koevolucijska teorija koja kaže da je kod prvotno raspolagao s malim brojem aminokiselina prisutnih u okolišu, nakon čega su ostale kasnije dobivene metaboličkim putevima iz potonjih i zatim uključene u kod pri čemu su im pridruženi kodoni srodni onima njihovih prekursora; teorija minimizacije greške koja tvrdi da je otpornost stanice na mutacije i pogreške u translaciji bila predvodeći selekcijski pritisak koji je gurao evoluciju SGC. Svaki od spomenutih pristupa ima u obliku u kojemu su najčešće izloženi mane koje se čine nepremostivima te se budućnost istraživanja porijekla SGC vjerojatno nalazi u integriranom eksperimentalnom pristupu koji u obzir uzima sve od spomenutih faktora, a i evoluciju aaRS i ostatka translacijske mašinerije, kao relevantne u ranoj evoluciji i porijeklu SGC.The standard genetic code (SGC) is a code by which sequences of nucleotides are translated into sequences of amino acids such that each nucleotide triplet (sequence of three) called a codon is assigned to any one of 20 biological amino acids. The SGC is nearly universal in all extant life forms, which is why we can safely presume it had been established as such at least as far back as the last universal common ancestor (LUCA). Although the SGC is perhaps the greatest information constant in all of biology, the question of its origin remains unanswered. In this paper, main theories of its evolution and origin are demonstrated: the stereochemical theory, which hypothesizes that a stereochemical affinity was the driving force of adjoining codons to their cognate amino acids; the coevolution theory which posits that the code originally had fewer amino acids at its disposal, and the rest of the 20 were derived from this original set via assorted metabolic pathways, after which codons that previously had belonged to their precursors were assigned to them; the error minimization theory which says that the pressure to resist the deleterious effects of mutations and translation errors was the main selective pressure to drive SGC evolution. Each of the described theories or the approaches by which they are tested have their flaws, so the future of research on the origin and evolution of the SGC appears to be in an integrated approach which takes into account all of the above, as well as the evolution of the translation apparatus, as relevant in the early evolution of the SGC