In the Beginning was Information

6.2 The Genetic CodeWe now discuss the question of devising a suitable coding system.For instance, how many different letters are required and how longshould the words be for optimal performance? If a certain codingsystem has been adopted, it should be strictly adhered to (Theorem8, par 4.2), since it must be in tune with extremely complextranslation and implementation processes. The table in Figure 19comprises only the most interesting 25 fields, but it can be extendedindefinitely downwards and to the right. Each field represents aspecific method of encoding, for example, if n = 3 and L = 4,we have a ternary code with 3 different letters. In that case aword for identifying an amino acid, would have a length of L = 4,meaning that quartets of 4 letters represent one word. If we nowwant to select the best code, the following requirements shouldbe met:– The storage space in a cell must be a minimum, so that the codeshould economise on the required material. The more lettersrequired for each amino acid, the more material is required, aswell as more storage space.– The copying mechanism described above, requires n to be aneven number. The replication of each of the two strands of DNAinto complementary strands thus needs an alphabet having aneven number of letters. For the purpose of limiting copyingerrors during the very many replication events, some redundancemust be provided for (see Appendix A 1.4).– The longer the employed alphabet, the more complex the implementingmechanisms have to be. It would also require morematerial for storage, and the incidence of copying errors wouldincrease.In each field of Figure 19 the number of possible combinations forthe different words appears in the top left corner. The 20 aminoacids require at least 20 different possibilities, and according toShannon’s theory the required information content of each aminoacid could be calculated as follows: For 20 amino acids the averageinformation content would be i A ≡ i W ≡ ld 20 = log 20/log 2 =4.32 bits per amino acid (ld is the logarithm with base 2).94