these complex and varied chemicals have many different roles. a molecule of DNA can hold the coded plans for thousands of different proteins and the length of DNA that codes for each of them is known as a gene. proteins are assembled by linking together chemical units called amino acid in an exact order. there are 20 different amino acids but DNA has to specify them using just four different chemical letters or bases (p.34). so how does it do this? the answer was suggested not by a biologist but by the astronomer George Gamow in 1954. he thought DNA might use words made up pf three letters and as events proved his guess was right