Houses usually have a light source in every room, but it would be a waste of energy to leave every light on all the time, so there are switches to turn off the lights in rooms that are not in use. sometimes one switch controls several lights in the same room. likewise, prokaryotic cells can turn genes on and off based on environmental factors. sometimes related genes are grouped together with one switch. this group of genes, along with the sections of dna that regulate them, is called a