the lac operon in E. coli contains three structural genes: z, y, and a . The z gene specifically codes for the enzyme beta-galactosidase (\beta -gal), which is responsible for the hydrolysis of lactose into its monomeric units, galactose and glucose . The bacteria utilize these monomers as a source of energy . If the z gene is mutated, the cell cannot synthesize functional beta-galactosidase, rendering it unable to break down lactose . Consequently, in a medium where lactose is the only available energy source, the bacteria will fail to grow . Mastering these operon mechanics is a high-priority strategy, as NCERT textbooks provide the foundation for approximately 95% of NEET Biology questions .