Question 54 - Biological and Biochemical Foundations of Living Systems Practice Test for the MCAT

The DNA of prokaryotes is organized into a circular chromosome supercoiled in the nucleoid region of the cell cytoplasm. Proteins that are needed for a specific function, or that are involved in the same biochemical pathway, are encoded together in blocks called operons. For example, all of the genes needed to use lactose as an energy source are coded next to each other in the lactose (or lac) operon.

In prokaryotic cells, there are three types of regulatory molecules that can affect the expression of operons: repressors, activators, and inducers. Repressors are proteins that suppress transcription of a gene in response to an external stimulus, whereas activators are proteins that increase the transcription of a gene in response to an external stimulus. Finally, inducers are small molecules that either activate or repress transcription depending on the needs of the cell and the availability of substrate.

Lactose digestion in E. coli begins with its hydrolysis by the enzyme ββ-galactosidase. The gene encoding ββ-galactosidase, lacZ, is part of a coordinately regulated operon containing other genes required for lactose utilization.