ABSTRACT
The sequences of amino acids in naturally occurring proteins have been selected by evolution for their favorable thermodynamic, kinetic, and functional properties. Variations in protein sequences are continuously generated via several molecular mechanisms. When variations that do not impair essential functions occur in germinal or replicating cells, they are transmitted to the progeny and generate diversity in the population, whereas variations that do impair essential functions disappear. Variations can be caused by substitution of one DNA base with another, replication of whole regions of a genome, and insertion and deletion of bases. In diploid organisms, which have two chromosome sets and, therefore, two copies of the genetic material, a mechanism known as crossing-over (i.e., the exchange of regions between two homologous chromosomes) can also introduce diversity.
