VJC 2023 H2 9744 P3 QP

1 VICTORIA JUNIOR COLLEGE JC 2 PRELIMINARY EXAMINATION HIGHER 2 CANDIDATE NAME: ……………………………………………………….………………. CT GROUP: ………………..……… BIOLOGY Paper 3 Long Structured and Free-response Questions 9744 /03 20 September 2023 2 hours Candidates answer on the Question Paper. No Additional Materials are required. This document consists of 22 printed pages and 2 blank pages. [Turn over] READ THESE INSTRUCTIONS FIRST Write your name and CT group in the spaces at the top of this page. Write in dark blue or black pen. You may use an HB pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. Section A Answer all questions in the spaces provided on the Question Paper. Section B Answer any one question in the space provided on the Question Paper. The use of an approved scientific calculator is expected, where appropriate. You may lose marks if you do not show your working or if you do not use appropriate units. The number of marks is given in bracket [ ] at the end of each question or part question. For Examiner’s Use Section A 1 /28 2 /12 3 /10 Section B /25 Total

2 Section A Answer all the questions in this section. 1 Cells respond to changes in their internal as well as external environment. Unicellular eukaryotes such as yeasts respond to changes in the glucose concentration in the environment. Glucose is the preferred carbon source for Baker’s yeasts (Saccharomyces cerevisiae) which metabolises 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 such as galactose and 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] (ii) Suggest why yeast cells undergo fermentation even in the presence of oxygen. [2]

3 (b) Several components of the signalling pathway involved in glucose repression in yeast have been identified. These are shown in Fig.1.1 . Snf1 is a protein kinase and binding of Mig1 to the promoter interferes with the binding of other proteins. Fig 1.1 (i) With reference to the information given in Fig 1.1, explain how the presence of glucose in the environment is able to bring about the repression of GAL1 gene. [4]

4 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 recognises and binds to these sites. [2] Scientists analysed the yeast genome to look for DNA that matches the features shown by Mig1-binding promoter site. Analysis of four chromosomes (A-D) revealed the presence of 26 Mig1-binding promoter sites. yeast chromosome number of Mig1-binding promoter sites A 1 B 9 C 2 D 14 Table 1.1 Since five different enzymes coded by five different genes are required for galactose metabolism, the expected number of Mig1-binding promoter sites for an individual diploid yeast is 10. (iii) Explain why the expected number of Mig1-binding sites is 10. [2]

5 (iv) Suggest reasons for the difference in the number of Mig1-binding promoter sites seen in Table 1.1 and the expected number. [2] (c) Changes in gene regulation was also observed in Aedes mosquitoes infected with dengue virus. Scientists carried out experiments to study how the Aedes mosquitoes respond to infection by the dengue virus (DENV) after taking a blood meal infected with the virus. Two groups of pre-mated females, 7– 10 days old, were used for the experiment. One group was fed the DENV -2 infected blood while the control group was fed with uninfected blood. Engorged females were then sorted into tubes and kept in the incubator for 3 days with a 10% sucrose solution provided. Eggs were counted daily, from 3 to 7 days after receiving the blood meal. At the end of 7 days, the ovary of the mosquito was removed and the remaining mosquito homogenised. Analysis of the gene expression in the ovary was carried out. Fig 1.2 shows the experimental setup. Fig 1.2 Experimental setup Source: https://pubmed.ncbi.nlm.nih.gov/35814655/ (i) Explain the importance of the control group in this experiment. [2]

6 Fig 1.3 shows the fecundity (number of eggs laid per mosquito) for the two groups of female mosquitoes. Fig 1.3 (**) – indicates that the p < 0.01 (ii) Suggest two reasons for the variation seen in the fecundity of the individual mosquitoes within each group. [2]

7 A study of the ovari an tissues obtained from the infected mosquitoes showed the cells infected with the dengue virus exhibit reduced ● cytochrome c oxidase activity ● synthesis of ribosomal proteins ● RNA binding to proteins (iii) Based on the information provided, explain the difference in the fecundity between the two groups of mosquitoes shown in Fig. 1.3. [3]

8 (d) Two genes were found to be upregulated in the infected mosquitoes. To investigate the role of these two genes (Oatp, amd), scientists carried out further studies using mosquito cell culture. Three cell cultures were set up, one as a control and two experimental. In each experimental group, small interfering RNA (siRNA) specific to the mRNA produced by the gene was introduced into the cell culture. Table 1.2 summarises the treatment for the control and experimental groups. Setup siRNA added Control None Experiment 1 (E1) specific for mRNA of Oatp Experiment 2 (E2) specific for mRNA of amd Table 1.2 Fig 1.4 shows the sequence of events occurring in the cells after the introduction of siRNA. Fig 1.4

9 (i) With reference to Fig. 1.4, explain how siRNA can affect the expression of a gene. [2] The cells in both the control and experimental groups were then infected with the dengue virus. The number of copies of dengue RNA was then determined 24 hours post infection. The results are shown in Fig 1.5. Fig. 1.5 (**) – indicates that the p < 0.01 (ii) With reference to Fig 1.4 and Fig 1.5, state a possible function of the two genes (Oatp, amd). Explain your answer. [3]

10 (e) Diseases caused by viruses such as dengue fever, Covid-19, have caused significant concerns to humans. Medication such as antibiotics are not effective against viruses. Viruses are so unique that they are not even grouped into any of the three domains – Eukarya, Bacteria and Archaea which encompasses all life on earth. Members of these three domains are either unicellular or multicellular. Justify why dengue virus should not be classified into any of these domains. [3] [Total: 28]