Answer
Genes don’t simply produce their products at a steady, incessant
pace, like a 24-hour nonstop manufacturing plant. They are turned
on and off from day to day, even hour to hour, as their products are
needed or not, and many genes are permanently turned off in any
given cell. The genes for hemoglobin and digestive enzymes, for
example, are present but inactive in liver cells.
There are several ways to turn genes on or off. We cannot
consider all of them here, but an example can convey the general
principle. Consider a woman who has just given birth to her first
baby. In the ensuing days, the hormone prolactin stimulates cells
of her mammary glands to synthesize the various components of
breast milk, including the protein casein—something her body has
never synthesized before. How is the gene for casein turned on at
this point in her life? Figure 4.12 shows the steps leading from
prolactin stimulation to casein secretion.
Work Step by Step
Gene Regulation Pg 124
