Figure 1. tRNA determines which amino acid goes into protein. Depicted are two ribosomes at Cys codons (UGU, UGC) on an mRNA; Cys-tRNA^Cys normally decodes these codons, inserting cysteine in protein, but after treatment with Raney nickel, Cys-tRNA^Cys is converted to Ala-tRNA^Cys, which now inserts alanine in protein at positions normally occupied by cysteine.

Figure 2. (a) Cloverleaf diagram is the two-dimensional folding pattern of the transfer RNA (tRNA) molecule. Nucleotide bases found almost universally in the same position in all tRNA sequences are indicated. The ladder-like stems are made up of complementary bases in different parts of the polynucleotide chain that pair up and form hydrogen bonds, causing the chain to fold back on itself. The number of nucleotides in the various stems and loops is generally constant except for two parts of the D loop designated and (which consist of 1–3 three nucleotides in different tRNAs) and the variable loop (which usually has four or five nucleotides but may have as many as 21). Abbreviations: A, adenosine; G, guanosine; C, cytidine; U, uridine; R, adenosine or guanosine; Y, cytidine or uridine; T, ribothymidine; and y, pseudouridine. (b) Folding pattern of the polynucleotide chain in yeast phenylalanine tRNA is diagrammed. The sugar-phosphate backbone of the molecule is represented as a coiled tube, with the cross rungs standing for the nucleotide base pairs in the stem regions. The short rungs indicate bases that are not involved in base–base hydrogen bonding. The colours refer to the cloverleaf diagram in part (a). Reproduced from Rich A and Kim SH (1978) The three-dimensional structure of transfer RNA. Scientific American 238: 52–62.