Genetic Code

The genetic code tells a cell how to interpret the chemical information stored inside deoxyribonucleic acid (DNA). This information is in the form of a sequence of chemicals that tell a cell which proteins to make. Without the genetic code, the cell would be unable to interpret the DNA sequence, and therefore could not make the proteins that build cells and make them work.

By the early 1950s, scientists knew that genes were made of DNA, and that specific proteins were made by specific genes. DNA is found in the chromosomes in the nucleus of cells, and it controls the characteristics of living things by means of a chemical code of instructions. The structure of the DNA molecule was found to resemble a twisted ladder called a double helix. The rungs on this ladder are called "bases" and are the coded instructions. These instructions are written with only four chemicals—adenine (A), thymine (T), guanine (G), and cytosine (C)—that make up what might be considered a four-letter alphabet. These bases must pair up a certain way (A only with T, and G only with C). Each pair of bases is called a nucleotide. It is the order of these nucleotides along the DNA that spells out the instructions for making proteins, which control the characteristics of organisms.

Proteins are chains made of twenty different units called amino acids, and it is the order of the amino acids that determines what type of protein will be produced. After the 1950s discovery of the molecular structure of DNA, the question that drove geneticists during the 1960s was: "How is a gene, whose information is contained in the sequence of only a four-letter alphabet (A, T, G, C) able to code enough messages for twenty different amino acids?" If a single base coded for one amino acid, only four amino acids could be made. If two bases coded for one amino acid, then a maximum of sixteen arrangements was possible. However, if the four bases somehow combined in groups of three to form one amino acid, sixty-four combinations were possible. After a great deal of difficult research, this triplet code called a "codon," proved to be the answer. The explanation somewhat resembles that of the Morse code, which is able to code all twenty-six letters of the alphabet using only two symbols—a dot and a dash. It does this by using different combinations of dots or dashes to code for each letter of the alphabet. With DNA, the answer lies in the codons or triplet code. Each codon is three bases long and has an exact meaning. In other words, a group of three bases in a certain order forms the codon for a specific amino acid. Therefore, the sequence GAG would specify the amino acid glutamic acid.

Once the idea of a triplet code was discovered, years of work resulted in what might be called a working dictionary of codes. It was found that of the sixty-four possible combinations, sixty-one of the codons are actually used to form the twenty amino acids. This means that some amino acids can be specified by more than one type of codon. It also means that the remaining three codons do not code for any amino acid but instead act as punctuation in a long message. Thus, these three codons can signal the end of a genetic "sentence" and therefore the completion of a code. It makes sense that, just as a paragraph of words has punctuation guiding the reader, a continuous sequence of hundreds of thousands of bases needs punctuation to make it a meaningful set of precise instructions. These three codons therefore not only end a code, but are thought to also signal something like, "I am not a gene," or "I am not a gene but one is coming soon." Interestingly, however, no commas or internal punctuation are found within the code. Punctuation is limited to stop or start signals at the beginning or end of a continuous run of triplets. The code was also found to be linear, meaning that just like a sentence, its sequence of bases are supposed to be read from a fixed starting point (the beginning). Once this genetic code was broken, it was found that the code is universal. That is, the very same three-letter codons specify the exact same proteins for all living things—from humans to bacteria. All life is therefore guided or directed by a common language that is the genetic code written in all DNA.

[See alsoDNA; Gene; Gene Theory; Genetics ]