SME
Grade 9 Science · Problem type
Genes switched on and off
Use this for “Why does a bacterium make the enzymes for lactose only when lactose is there?”
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With no lactose, the repressor sits on the operator and blocks RNA polymerase.
The gene is off: no mRNA, so no lactose enzymes are wasted.
Assumptions
•
Every cell has the same genes, but it reads only some of them: those genes are expressed.
•
RNA polymerase binds the promoter in front of a gene and copies the gene into mRNA.
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Proteins on the DNA near the promoter switch the gene off (repressors) or on (activators).
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