VJC 2023 H2 9744 P3 Ans

1 Victoria Junior College Biology Department 2023 H2 Preliminary Exams Paper 3 Proposed answers Marking abbreviations: A: Accept, R: Reject, BOD: benefit of doubt, AW: alternative wording, AVP: Any valid point, NAQ: not answering question, ECF: error carried forward Question 1 Cells can respond to their environment through regulating the expression of certain genes. Unicellular eukaryotes such as yeasts respond to changes in the glucose concentration in the environment. Glucose is the preferred carbon source for Saccharomyces cerevisiae (Baker’s yeasts) which metabolizes glucose by a purely glycolytic process (fermentation), producing ethanol even under aerobic conditions. Presence of glucose represses the uptake and metabolism of other carbon sources eg. galactose, maltose. When the glucose has been consumed, the cell switches to aerobic metabolism of the ethanol. (a) (i) State the difference in the number of ATP produced from one molecule of glucose between fermentation and aerobic respiration. [1] • 30 ATP; (ii) Suggest why yeast cells undergo fermentation even in the presence of oxygen. [2] • (idea of) Take advantage of the high glucose concentration quickly taking up glucose from the environment due to competition from other yeast cells; • (idea of ethanol as a reserve) when glucose is depleted, yeast can then fully oxidise ethanol to obtain ATP; (R: directly lift from text without further explanation) • So that repression on other carbon sources eg. galactose (monosaccharide) and maltose (disaccharide) is removed; • ATP produced via fermentation is faster as it involves fewer steps compared to complete oxidation; • AVP (b) Several components of the signalling pathway involved in glucose repression have been identified. These are shown in Fig.1.2. Snf1 is a protein kinase and binding of Mig1 to the promoter interferes with the binding of other proteins.

2 Fig 1.2 (i) With reference to the information given in Fig 1.2, explain how the presence of glucose in the environment is able to bring about the repression of GAL1 gene. [4] 1. Glucose taken up by facilitated diffusion via transport/channel proteins into cytoplasm; 2. Presence of glucose/ High glucose inactivates the Snf1 protein kinase; resulting in Mig1 not being phosphorylated (A: cytoplasmic Mig1 dephosphorylate) and; 3. Mig1 enters the nucleus, and binds to the promoter of GAL1; 4. Preventing the attachment of other transcription factors and RNA polymerase, prevents the assembly of transcription initiation complex and no transcription of GAL1gene; Mig1 binds to promoter sites with these features: ● 17 base pairs long ● Includes a region of five repeating adenine-thymine pairs ● Includes a region of six repeating cytosine-guanine pairs Promoter sites to which Mig1 binds are known as Mig1-binding promoter sites. (ii) Explain how Mig1 recognizes and binds to these sites. [2] • Mig1 ha