The Synthesis of Proteins upon Ribosomes.

The information conveyed by the sequence of the four main nucleotides in genes (the genetic code) is used to order the sequence of the 20 different amino acids in proteins. In this process the four letter nucleotide code is translated into the 20 letter amino acid alphabet. Until several years ago, there existed much confusion about the molecular basis of the reading of the genetic code. Now, however, there instead exists the prevalent belief to which I also subscribe that the main biochemical devices underlying the transfer of genetic information to polypeptide chains are known. This happy state of affairs is in large part due to the following experimental and conceptual insights : 1) The clean demonstration that an RNA intermediate is used to convey the genetic information of DNA to the sites of protein synthesis. Even though this is an old idea, it became a hard fact when the in vitro enzymatic synthesis of RNA molecules on DNA templates was demonstrated (WEISS and NAKAMOTO, 1961 ; HURWITZ et al., 1960 ; STEVENS, 1960 ; CHAMBERLIN and BERG, 1962). These experiments showed that the enzyme RNA polymerase forms complementary copies of single DNA strands. Most importantly, in vitro only one of the two complementary DNA strands is copied (MARMUR et al., 1963). Thus the information needed to order amino acid sequences is carried by single stranded RNA molecules. 2) The insight in 1956 by Francis CRICK (1958) that most unmodified amino acids would not be attracted by RNA molecules, This led him to predict that the amino acids would first be attached to specific adaptor molecules, which in turn would bind to the RNA template. This prediction was quickly verified when HOAGLAND and ZAMECNIK (1958) found that the amino acids, prior to incorporation into protein, are attached to small RNA molecules (commonly known as soluble RNA (sRNA)) (Figure 1). Subsequent experiments showed that sRNA molecules are specific for a given amino acid and that it is the sRNA component of the AA ­ sRNA complexes which