|The roles of mRNA and tRNA in protein synthesis|
How this sequence of transcription and translation evolved was unclear. A possible explanation is the RNA world hypothesis. This states that prior to the formation of cellular life there existed life-like chemical systems in which energy and information was exchanged, but in the absence of membranes or proteins. Yet our understanding of how the RNA world transitioned into a more contemporary state of nucleic acids (DNA and RNA) in close association with amino acids, polypeptides and proteins has been woolly until recently. A team of researchers from the UNC School of Medicine has shown that this association likely existed early on in the process of forming life.
'Our work shows that the close linkage between the physical properties of amino acids, the genetic code, and protein folding was likely essential from the beginning, long before large, sophisticated molecules arrived on the scene,' said Charles Carter from UNC School of Medicine. 'This close interaction was likely the key factor in the evolution from building blocks to organisms.' By examining the physical properties of the 20 amino acids used by cells, such as their polarity, it was found that there was a relationship between those properties and the genetic code.
|Aminoacyl tRNA synthetases catalyse the |
formation of tRNAs bonded to amino acids
Aminoacyl tRNA synthetases catalyse the binding of amino acids to tRNA. The aminoacyl tRNA products then bind to mRNA and allowing the amino acids to bind together into a polypeptide, the precursor to a protein. 'Think of tRNA as an adapter,' said Carter. 'One end of the adapter carries a particular amino acid; the other end reads the genetic blueprint for that amino acid in messenger RNA. Each synthetase matches one of the twenty amino acids with its own adapter so that the genetic blueprint in messenger RNA faithfully makes the correct protein every time.'
The analysis showed that the two different ends of tRNA molecules contain independent codes or rules that specify which amino acid to select. The end of tRNA that carried the amino acid sorted amino acids according to size. The other end of the tRNA molecules selected amino acids according to polarity. 'Translating the genetic code is the nexus connecting pre-biotic chemistry to biology,' said Carter. We can now say that the genetic code and its translation to proteins evolved due to the simple physical properties of the amino acids involved. Ultimately this suggests that the RNA world is an incomplete hypothesis. Instead it should perhaps be renamed the Peptide RNA world.