* Genes and Proteins: Genes contain the instructions for building proteins. Proteins are the workhorses of the cell, carrying out a wide variety of functions.
* Enzymes: Many proteins are enzymes, which catalyze (speed up) specific chemical reactions. Substance X is likely produced by an enzyme.
* Mutations: Mutations are changes in the DNA sequence. If a mutation occurs within the gene that codes for an enzyme, it can alter the protein's structure and function. This could prevent the enzyme from being produced correctly or from working properly, thus preventing the production of substance X.
In summary: A change in the gene responsible for producing the enzyme that creates substance X is the most likely cause of the bacterial cell's inability to produce it.
