Studies have shown that more soy protein might be needed to maintain nitrogen balance when compared to egg-white protein, and that the difference may be eliminated by the addition of methionine to the soy diet. This indicates that sulfur amino acids can be limiting in soy. Similarly, the limiting amino acid in wheat protein is lysine. The Recommended Daily Allowance (RDA) for protein is estimated for people consuming complete proteins that prevent negative nitrogen balance due to the inadequate intake of limiting indispensable amino acids. The RDA recommendations also assume that the diet has enough nonprotein energy, such as carbohydrates and fats, to prevent the carbon skeletons of amino acids from being used to meet energy needs.
That RNA can act as a template and also now exhibits catalytic activity fuelled hypotheses for the evolution of an “RNA world”. 34 In this scenario RNA is the primary polymer of life that replicates itself. DNA and proteins were later refinements. So the first genes were short strands of RNA that reproduced themselves, perhaps on clay surfaces. This conjecture is strengthened by the fact that in cells today there are segments of some eukaryotic pre-rRNAs which can cleave themselves off and join the two cut ends together to reform the mature rRNA. In 1982 Thomas Cech and his colleagues at the University of Colorado discovered this can take place in the absence of protein in the ciliated protozoan Tetrahymena thermophila . 35 Just as remarkable are the small nuclear RNAs (snRNAs), which complex with protein to form small nuclear ribonucleoproteins (snRNPs; pronounced “snurps”). Particles called spliceosomes convert pre-mRNA to mRNA. 36 Other ribozymes include the hammerhead variety and RNAse P, which generates the 5 ' ends of tRNAs. The former are found in certain plant viruses. Origin-of-life theories see prebiotic significance in these ‘vestigial’ post-translational mechanisms.