A new study by researchers from the Institute for Research in Biomedicine (IRB) Barcelona reveals the evolution of human life by shedding light on the origins and expansion of our genetic code. The team discovered that this genetic code - the universal set of rules that all of the Earth's organisms use to translate genetic sequences of DNA and RNA into amino acids - has a fundamental limit to its evolution that it hit more than 3,000 million years ago.
The team discovered that the genetic code evolved to include a maximum of 20 amino acids and that it is unable to grow any larger than this due to a limit in its transfer RNAs, which are the molecules that act to interpret the language of genes and that of the proteins that the code serves as a blueprint to create.
This fundamental limit led to an abrupt halt in the complexity of life more than 3,000 million years ago, prior to the evolution of bacteria, eukaryotes and archaebacteria, which all use the same genetic code to craft proteins using genetic information.
The authors discovered that with more than 20 amino acids, the mechanisms that translate genetics into protein structures would fall into disarray and cause constant mutations and the mistranslation of genetic information.
"Protein synthesis based on the genetic code is the decisive feature of biological systems and it is crucial to ensure faithful translation of information," said Lluís Ribas, a researcher from IRB Barcelona and lead researcher of the study.
Upon examination of the genetic code, the team realized that transfer RNA (tRNA) is the main reason that the genetic code became saturated. tRNAs are the molecules that carry amino acids to ribosomes to be made into proteins based on what's encoded in the genetics. However, the L-shape cavity on the ribosome means that there is very little possibility for variation between the tRNA molecules.
"It would have been to the system's benefit to have made new amino acids because, in fact, we use more than the 20 amino acids we have, but the additional ones are incorporated through very complicated pathways that are not connected to the genetic code," Ribas said. "And there came a point when nature was unable to create new tRNAs that differed sufficiently from those already available without causing a problem with the identification of the correct amino acid. And this happened when 20 amino acids were reached."
The findings were published in the April 29 issue of Science Advances.
