Proteins

Genes and proteins are products of evolution. Over the course of evolution, the nucleotide sequences of genes undergo numerous changes. First, duplications (or deletions) may lead to creation of additional copies (or removal) of genes or gene fragments. Second, local mutations: substitutions, insertions and deletions within genes may result in changes to the amino acid sequence of proteins they encode. Thus, the initially identical copies of duplicated genes over time accumulate divergent mutations that make their sequences progressively dissimilar. Not all positions of protein-encoding genes are equally susceptible to mutation, as some amino acid residues may be very important for protein function, stability, or folding and may thus be more constrained in the residue types allowed. Therefore, although mutations are random, in nature we observe only such protein variants, in which sequence changes have been ‘accepted’ by the evolutionary pressure. Proteins with mutations that cause detrimental changes in structure and/or function are usually eliminated. If the protein is important to the integrity of the organism, the organism that bears the mutant gene dies, and the structurally/functionally compromised variant ceases to exist; or if it is not important, then the inactivated gene may be eventually ‘purged’ from the genome by random deletions. On the other hand, if the mutant variant brings an additional and beneficial new function to the organism, it is likely to be retained and further ‘optimized’ towards the activity favored by the selective pressure.