Why 20 amino acids instead of 64?
The reason why there are only 20 amino acids commonly found in living organisms instead of the full 64 possible combinations of nucleotide triplets (codons) in DNA and RNA is due to several factors involving the evolutionary history of life on Earth.
1. Evolutionary selection: Through natural selection, only those amino acids that provide a clear functional advantage to organisms are commonly utilized. Over time, certain amino acids have shown to be more structurally and functionally beneficial for life processes. It is believed that these amino acids were selected early in evolutionary history, and as life evolved, the genetic code became more fixed with these specific amino acids
2. Conserved genetic code: The genetic code, which determines the relationship between codons and amino acids, is highly conserved across all known life forms. This similarity suggests that the genetic code was established early in evolutionary history and has remained relatively unchanged. This suggests that there is a selective advantage to the specific combinations of codons and amino acids found in the genetic code
3. Structural and functional constraints: The biochemical properties of amino acids play a crucial role in protein folding, stability, and functionality. The 20 amino acids commonly found in proteins provide a diverse range of properties such as hydrophobicity, hydrophilicity, charge, and size. These properties enable the formation of complex protein structures and functions. The incorporation of additional amino acids would introduce potential structural and functional disruptions to protein stability and activity
4. Toxicity and efficiency: The utilization of a large number of amino acids could be potentially toxic to an organism. Additionally, the synthesis and incorporation of a greater variety of amino acids would require a more complex and energy-intensive metabolic system. The efficiency of protein synthesis and regulation would also be compromised by increasing the number of amino acids
Therefore, the presence of 20 amino acids in living organisms is a result of a combination of evolutionary selection, genetic stability, functional constraints, and practical considerations related to toxicity and metabolic efficiency
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