EduNinjaGene expression in a cell is controlled. When a gene is expressed (switched on), the gene is transcribed. When a gene is not expressed (switched off), the gene is not transcribed.
Environmental changes can cause some genes to be switched on or switched off.
The lac operon codes for inducible enzymes. Repressible operons code for repressible enzymes.
Suggest and explain why it is an advantage to a prokaryote to have a repressible operon.
Gene I is located a short distance away from the lac operon.
The product of gene I, a repressor protein, is a constitutive protein.
State what is meant by the term constitutive protein.
Yeast cells are unicellular eukaryotes that respond to the presence and absence of different sugars by switching genes on or off. One example of this is summarised in Fig. 5.1.
If glucose is present, a sequence of events occurs.
- Yeast cells metabolise glucose using constitutively expressed enzymes.
- Mig1 transcription factor (A) binds to promoter B.
- This stops transcription of gene C.
- Production of enzyme D stops.
If galactose is present and glucose is absent, a different sequence of events occurs.
- The Msn2 transcription factor (E) binds to promoter B.
- This activates transcription of gene C.
- Enzyme D is produced and helps convert galactose to glucose.
Gene F codes for the Mig1 transcription factor, A.
Gene G codes for the Msn2 transcription factor, E.
Explain why enzyme D is described as inducible.
The lac operon of prokaryotes contains a group of structural genes that are under the same control and are transcribed together.
Another operon found in prokaryotes is the trp operon.
Fig. 6.1 summarises the structure and control of the trp operon.
Fig. 6.1
codes for the enzyme tryptophan synthase.
Tryptophan synthase catalyses the formation of the amino acid tryptophan.
Explain why tryptophan synthase is an example of a repressible enzyme.
If E. coli is put into a nutrient medium containing lactose, some new enzymes are synthesised. These are described as inducible enzymes.
Explain what is meant by an inducible enzyme.
The structural genes of the lac operon are not expressed when lactose is absent.
Suggest one reason why this is beneficial to E. coli.