Relationship Between the Phase Diagram of Life and Protein Distribution

A significant bias in the nucleotide composition of the first and second letters of the codon is a major characteristic of living organisms. There is a deep relationship between the bias in nucleotide composition and the constant distribution of proteins described in Chaps. 9 and 10 . Fluctuations in the DNA sequence due to random mutations determine the distribution of proteins, and the process is as follows. When the nucleotide composition deviates significantly from a completely random composition, a unique mode of fluctuation occurs in the DNA sequence. For example, a bias in the nucleotide composition of the second letter of a codon will result in a large hydrophobic and amphiphilic fluctuation in the amino acid sequence due to the bias in the physical properties of the amino acids in the codon table. When the mode of this fluctuation reaches equilibrium, the proportion of membrane proteins is constant (approximately 1/4). The first letter of the codon, on the other hand, relates to the structural freedom of the amino acid segment, which generates the molecular recognition unit of the protein. In other words, if nucleotide composition is maintained within the phase diagram of life, membrane proteins and molecular recognition units will occur in approximately constant proportions. At the end of this chapter, we also discuss the intracellular factors that maintain a biased nucleotide composition.