The lac repressor protein is bound to the operator region of the lac operon when there is no lactose in the medium. When introduced in lactose, it binds to an allosteric site on the repressor causing its to change conformation and release from the operator. This allows the polymerase that is bound to the promoter region to proceed transcription of the lac operon genes (LacZ, Lac A, and Lac Y). Translation of these genes forms enzymes that enable the organism to break down lactose for energy.
Tropical climate is a region of the earth surrounding the equator. When the earth is exposed to light from the sun it is right over the equator and up 23 degrees of latitude Tropic of Cancer in the northern hemisphere and Tropic of Capricorn in the southern hemisphere