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. [Figure 5.1]