TJC 2023 H2 BIO P3 ANS

1 [TURN OVER READ THESE INSTRUCTIONS FIRST Write your Center number, index number and name 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. DO NOT WRITE IN ANY BARCODES. Section A Answer all questions in the spaces provided on the Question Paper. Section B Answer any one question in spaces 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 any working or if you do not use appropriate units. The number of marks is given in the brackets [ ] at the end of each question or part question. TEMASEK JUNIOR COLLEGE 2023 JC2 PRELIMINARY EXAMINATION Higher 2 CANDIDATE NAME CENTRE NUMBER S INDEX NUMBER BIOLOGY 9744/03 Paper 3 Long Structured and Free Response Questions PART I 12 SEPTEMBER 2023 2 hours Candidates answer on the Question Paper. No Additional Materials are required. For Examiner’s Use 1 / 29 2 / 11 3 / 10 4 / 5 / 25 Section A This document consists of 9 printed pages and 3 blank pages.

2 Answer all questions in this section. 1 There are a variety of infectious diseases that can affect humans. (a) Explain what is meant by an infectious disease. [2] Infectious Diseases Part III Lecture Notes p.2 1. [Infectious] Infectious diseases are caused by pathogens (A: bacteria, viruses, fungi, protozoa) that can spread from one organism / host to another (i.e. transmissible, communicable). [1] 2. [Disease] The pathogens cause damage or injury to the host that impairs the normal function of the body. [1] (b) The human immunodeficiency virus (HIV) causes the infectious disease known as autoimmune deficiency syndrome (AIDS). (i) Compare the structures of HIV and a typical bacterium. [2] Similarities: 1. Both have genetic material. 2. Both have phospholipid bilayer (envelope for HIV, cell membrane of bacteria) 3. Both have glycoproteins. 4. Both do not have membrane-bound organelles. Differences: Feature HIV Bacteria 1. Genetic material Linear, single stranded (+) RNA Circular, double- stranded DNA 2. Presence of peptidoglycan cell wall No Yes 3. Presence of reverse transcriptase / HIV integrase / HIV protease (Any 1) Yes No 4. Presence of ribosomes No Yes 5. Presence of plasmid No Yes 6. Presence of flagellum No Yes HIV can integrate its genome into the host cell chromosome, thus forming a provirus. The provirus may remain transcriptionally silent for decades. (ii) Suggest and explain one way in which the provirus can remain transcriptionally silent. [3] Any one modification 1. DNA methylation [1] 2. Causes DNA to bind more tightly to histones and nucleosomes to pack tightly [1] 3. Transcription factors and RNA polymerase cannot access promoter and HIV genes are not expressed [1] 4. Histone deacetylation [1] 5. Causes DNA to bind more tightly to histones and nucleosomes to pack tightly [1] 6. Transcription factors and RNA polymerase cannot access promoter and HIV genes are not expressed [1] 7. Binding of repressor to silencers / one of the viral gene codes for a repressor [1] 8. Slow down / prevent formation of transcription initiation complex [1]

3 [TURN OVER 9. Block activator from binding to enhancer / bind to activator to prevent activator from binding to enhancer / interact with proteins at TIC to make it more difficult for TIC assembly / recruit histone deacetylase for DNA to bind more tightly to histone [1] 10. Integrates itself into heterochromatin [1] The HIV/AIDS pandemic has had a very large impact on life expectancy in many African countries. Table 1.1 shows the average life expectancy of individuals without and with HIV/AIDS and the percentage of the population tested positive for HIV in five African countries. Table 1.1 country average life expectancy / years percentage of population tested positive for HIV / % without HIV/AIDS with HIV/AIDS Botswana 72.4 33.9 35.8 Côte d’Ivoire 55.6 42.8 10.8 Kenya 65.6 45.5 14.0 South Africa 66.3 48.8 19.9 Zimbabwe 69.0 40.2 25.1 (iii) Using the data shown in Table 1.1, calculate the percentage decrease in average life expectancy for patients with HIV/AIDS in Zimbabwe. Show your working and give your answer to the nearest whole number. 1. Correct calculation of decrease in average life expectancy: 69.0 – 40.2 = 28.8 [1/2] 2. Correct substitution of values into formula [1/2] 3. Correct answer: [1] 𝟐𝟐𝟐𝟐. 𝟐𝟐 𝟔𝟔𝟔𝟔. 𝟎𝟎 × 𝟏𝟏 𝟎𝟎𝟎𝟎 % = 𝟒𝟒 𝟐𝟐% percentage decrease: 42 % [2] (-1/2 if not to nearest whole number) (iv) After studying the data in Table 1.1, a student concluded that: “The higher the percentage of population tested positive for HIV, the greater the decrease in life expectancy with HIV / AIDS”. With reference to Table 1.1, discuss the validity of the student’s conclusion. [2] Valid (any 1): 1. Botswana has the highest percentage of population tested positive for HIV (35.8%) and has the greatest decrease in life expectancy with HIV/AIDS, from 72.4 years to 33.9 years, a decrease of 38.5 years. 2. Cote d’lvoire has the lowest percentage of population tested positive for HIV (10.8%) and has the least decrease in life expectancy with HIV/AIDS, from 55.6 years to 42.8 years, a decrease of 12.8 years.

4 Not valid: 3. While South Africa has a higher percentage of population with HIV (19.9%) compared to Kenya (14.0%), the decrease in life expectancy is lower in South Africa, 66.3 years to 48.8 years (decrease of 17.5 years) than Kenya, 65.6 years to 45.5 years (decrease of 20.1 years). (c) Another common infectious disease in developing nations is cholera. Cholera is caused by consuming food or water contaminated with a bacterium called Vibrio cholerae. Fig. 1.1 shows a transmission electron micrograph of Vibrio cholerae. ×30 000 Fig. 1.1 The symptoms of cholera are caused by the protein, cholaregen. Choleragen is a protein made up of six polypeptides: • a single copy of a polypeptide known as the A subunit that includes an extended alpha helix • five polypeptides that together make the B subunit. The B subunit of choleragen binds to a cell surface membrane component, known as GM1, of an intestinal epithelial cell. The complete choleragen protein then enters the cell by endocytosis. Once inside the cell, the A subunit of the protein acts as an enzyme, disrupting normal functions of the cell. (i) List the levels of protein structure present in choleragen. [1] Primary, secondary, tertiary, quaternary [all for 1] Fig. 1.2 shows the ctxAB operon which contains the structural genes that code for choleragen. Fig. 1.2 H-NS and ToxT are proteins that are coded for by separate regulatory genes found upstream of the operon. It is found that: • high levels of H-NS would reduce the expression of the operon, • high levels of ToxT would increase the expression of the operon, • the operon has similar regulation as lac operon.

5 [TURN OVER (ii) State the role of H-NS and ToxT proteins in the regulation of ctxAB operon. [2] H-NS: repressor protein ToxT: activator protein (iii) In the laboratory, it is possible to produce a form of choleragen consisting of only B subunit as a vaccine against cholera. Suggest why B subunit, rather than A subunit, is used in the production of the vaccine. [1] Any 1 1. B subunit is the portion that binds to cell, thus antibodies that target B subunit will prevents binding of choleragen to cell thus prevent entry to cell 2. B subunit is safer as it does not disrupt the normal functioning of the cell. 3. B subunit is larger, s