The Genetic Code

The genetic information required for the construction of proteins follows from the type and arrangement of amino acids in the protein. The encoding of this information in DNA, the genetic code, is determined by the arrangement of the four bases (contituting the four different nucleotides)within the DNA and is the same in all living things. The variable sequence of the various bases determines the specific informational content of the genes that forms the blueprint for millions of different protein molecules, just as the placing of the letters of the alphabet in an intelligent sequence determines the informational content of a book.

The double helix consists of the bases adenine (A), cytosine (C), guanine (G), and thymine (T), and the sugar deoxyribose (Z), and phosphate bridges formed by acid phosphate radicals. Each base combines with a sugar and a phosphate radical to form a nucleotide. Using the analogy of a rope ladder, the sugar and phosphate units form the sides (ropes), and the bases the rungs of the ladder. By chemical affinity adenine always forms a base pair with thymine, and guanine with cytosine, connected to each other by bridges of hydrogen bonds (H). The distances between rungs and the radius of the double helix are given in nanometers (1 nm = 10−9m = one billionth of a meter). (After Beske)

Three bases at a time in varying combinations define one informational unit, a “word”—also called a triplet or a codon—that must be translated into one of the 20 amino acids present in proteins. For instance, the combination of the bases guanine (G), adenine (A), and thymine (T)—GAT in abbreviated form—contains the information for the amino acid asparagine; and the triplet AAG is the code for lysine. The amino acids present in the cytoplasm are combined according to the sequence of the base triplets, to form the corresponding protein molecules (see below). Consequently, the four building blocks provide a total of 43 (4 × 4 × 4 = 64) possible combinations (informational units = “words”). Of these, 61 are used in the instructions to build proteins. The remaining triplets indicate the beginning and end of a protein molecule or a gene. The program for the construction of a protein consisting of, say, 340 amino acids therefore includes 340 such base triplets (or codons). The complete set of these triplets is called a gene (factor).
Thus, a gene determines how many amino acids constitute a protein, and in what sequence these must be arranged. One gene contains on average 300−3000 base triplets. Itmay take several genes to determine a single characteristic.