Figure 1. Suppression of chain-terminating mutations by secondary mutations in a transfer ribonucleic acid (tRNA) anticodon. (a) A tRNA molecule brings a lysine residue to the ribosome to add to the growing polypeptide chain. The tRNA molecules interpret the genetic code by specifically matching mRNA codons to corresponding amino acids. (b) A mutation that changes an amino acid-encoding codon to a stop codon results in a truncated protein. This occurs because no tRNA in the cell can base pair with the mutant codon. Assuming this protein is essential, such a mutation would likely be lethal to the organism. (c) A matching mutation in a tRNA gene which mutates the anticodon to recognize the stop codon results in suppression of chain termination. If the lysine residue is not essential to the protein's function, the organism would be able to survive.

Figure 2. Isopropyl--d-thiogalactoside (IPTG), a genetic ‘switch’. (a) When IPTG is not present in the growth medium of the cell culture, the lac repressor protein is bound to the IPTG-driven promoter. The repressor protein stops the RNA polymerase from binding and transcribing the transgene. (b) When IPTG is present in the growth medium, it binds the repressor protein, causing a conformational change. This change makes it impossible for the repressor to bind DNA. With the repressor released from the promoter, RNA polymerase can bind and transcription can occur.