Principles of Genetics and Breeding

1.2.3.9 Expression of genetic information

• The gene is expressed in the form of peptide or proteins composed of 20 different amino acids whose sequences are controlled by genes.
• If one base (A,T,G, or C) were to specify one amino acid, only 4 amino acids could be specified, and if two sequential bases could specify one amino acid, there would be only 4²=16 possible amino acids that could be specified. Hence at least 3 sequential base must specify each amino acid, and in this case, there are 64 different possible sequences composed of three sequential bases (4³).
• Actually, one amino acid has been shown to be specified by a sequence of three bases (triplet).
• 64 different triplets are enough to specify 20 different amino acids, so that one amino acid is represented by at least one or up to six triples. Leucine, for example, is specified by each of six triplets.
• This “genetic language” is common from microorganisms to mammals with only a few exceptions which makes it possible to express mammalian genes in E. coli.
• The sequences of amino acids include from the smallest peptide (two sequential amino acids) to large proteins (several thousand sequential amino acids). Further, consider that if 20 different amino acids are utilized, peptide composed of five sequential amino acids alone would include 20⁵ (=3,200,000) possible sequences. Thus, the variety of proteins is almost unlimited.
• The triplets mentioned above include not only amino acids specifying triplets but also start and stop signals.
• DNA contains a specific sequence to which the RNA polymerase binds (promoter), a sequence as a switch controlling protein synthesis (operator) and a sequence specifying the production of a substance that turns on or off the switch (repressor), etc. as well as structural genes specifying the sequences of amino acids.