Yeast cells are unicellular eukaryotes that alter which genes are switched on or off in response to whether different sugars are present. Fig. 5.1 summarises one example of this. When glucose is present, the following happens: yeast cells use constitutively expressed enzymes to metabolise glucose; the Mig1 transcription factor $A$ attaches to promoter $B$; transcription of gene $C$ is prevented; enzyme $D$ is no longer produced. When galactose is present and glucose is absent, a different sequence occurs: the Msn2 transcription factor $E$ attaches to promoter $B$; transcription of gene $C$ is activated; enzyme $D$ is made and assists in converting galactose to glucose. Gene $F$ encodes the Mig1 transcription factor, $A$. Gene $G$ encodes the Msn2 transcription factor, $E$.
(a(i))[3]
With reference to Fig. 5.1, identify one letter for each of the following: a structural gene; a control (regulatory) sequence; a repressor molecule.
(a(ii))[2]
Explain why enzyme $D$ is referred to as inducible.
(b(i))[2]
Calculate the percentage of Mig1 molecules in the nucleus when glucose is present. Show your working and give your answer to two significant figures.
(b(ii))[2]
When glucose is absent, $13\%$ of the available Mig1 molecules are found in the nucleus. Explain why this value differs from your answer to (i).
(c)[2]
Yeast cells cannot take up and metabolise the disaccharide lactose.
Some yeast strains have been genetically engineered to solve this by inserting two genes from the bacterium \textit{Escherichia coli} into the yeast cells.
Name the two bacterial genes that have been inserted into the yeast cells.
Worked solution & mark scheme
This 11-mark question has a full step-by-step worked solution and mark scheme. One marking point: “The structural gene is C” …