A Taste Tester
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This worker, the Lac repressor, is one of the
body's taste testers. The Lac repressor tells the body's cells when there's lactose (milk
sugar) around, so they can get ready to eat it. One of the Lac
repressor's domains can grab on to a
molecule of lactose. Another of the Lac repressor's domains can grab
on to DNA, specifically
the promoter for the Instruction that makes
lactase, the worker that breaks down lactose for digestion.
Usually, there's no lactose around, so the Lac repressor clings tightly to the lactase promoter. This prevents the cell from making lactase, as the cell's dispatchers can't see the promoter that tells them where the Instruction starts. This is the situation shown in the picture: the Lac repressor is shown in shades of blue (one shade for each domain) and the DNA that makes up the lactase promoter shown in red and orange. (In the body, this bit of DNA is part of a much longer strand containing not only the lactase Instruction, but thousands of other Instructions. The rest of the DNA is hidden for clarity.) When the Lac repressor grabs on to a molecule of lactose, it lets go of the lactase promoter. This allows the cell to make lactase, which breaks the lactose down. The Lac repressor's grip on lactose isn't perfect. When the lactose slips away, the Lac repressor grabs back onto the lactase promoter. As the amount of lactose in the cell falls, less and less of it bumps into the Lac repressor, so the Lac repressor spends more time hanging on to the lactase promoter. This reduces the amount of lactase being made. When there's finally no lactose in the cell, the Lac repressor goes back to hanging on the lactase promoter all the time, stopping the production of lactase.
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Part Three: Improving the Book
Appendix E: A Gallery of Workers