A nerve cell and a skin cell in your body came from the same first cell by mitosis, so they carry the same DNA. Yet they look and work nothing alike. The difference is which genes each cell uses, which builds on translation.
Key idea
Why it works
The lac operon
In the 1960s François Jacob and Jacques Monod worked out how the bacterium E. coli controls its genes for digesting lactose, a milk sugar. The genes sit together under one switch, called an operon.
Worked example: Predicting the switch
- Normally the repressor blocks transcription when lactose is absent.
- This repressor cannot sit on the DNA, so nothing blocks RNA polymerase.
Answer: On, all the time, even without lactose.
Worked example: Another mutant
- Lactose normally pulls the repressor off the DNA.
- This repressor ignores lactose and stays on.
Answer: The genes stay off, even with lactose.
Eukaryotes use many such DNA-binding proteins, called transcription factors, often in combinations. They also regulate at other steps: how DNA is packed, how RNA is processed, and how long a protein lasts. (The lac genes are also turned up most strongly when glucose, the bacterium's first choice, is scarce.)
Common mistake