DNA consists of a code language with just four letters, known as bases, making up a variety of words, known as codons, that are three letters in length. The four bases are strung together linearly along chromosomes, and in humans make up the 3-billion base pairs worth of sequences in the human genome. How did scientists first learn to interpret the genetic code? Some of the first clues appeared serendipitously as researchers studied aspects of gene expression. Later, researchers probed in earnest, setting up specific experiments to decipher the code used in RNA molecules. Marshall Nirenberg and his colleagues gathered the bulk of the data.
In the 1960s, Marshall Nirenberg and his colleagues determined the language of the genetic code. Their meticulous work paved the way, decades later, for interpreting the sequences of the entire human genome and the genomes of many other organisms. The scientists designed specific RNA sequences to test the possible code words, or codons, in the genetic code. Through these types of experiments, we now know that of the 64 possible codons, 61 of them correspond to specific amino acids. Three codons code for no amino acids, are known as stop codons, and are found at the end of a coding sequence in a messenger RNA molecule.
Textbook Reference: Concept 10.3 The Genetic Code in RNA Is Translated into the Amino Acid Sequences of Proteins