SAJC 2023 H2 BIO P3 ANS

1 SAJC / H2 Biology 9744/3 JC2 Prelim 2023 Civics Group A Level Index Number Name (use BLOCK LETTERS) ST. ANDREW’S JUNIOR COLLEGE 2023 JC2 PRELIMINARY EXAMINATIONS H2 BIOLOGY 9744/03 Paper 3 Mark scheme Friday 15th September 2023 2 hours READ THESE INSTRUCTIONS FIRST Write your name, civics group and index number on all the work you hand in. Write in dark blue or black pen on both sides of the paper. You may use a soft pencil for any diagram, graph or rough working. Do not use staples, paper clips, highlighters, glue or correction fluid. Answer all questions. Write your answers in the spaces provided on the question paper. All working for numerical answers must be shown. Conceptual error (C) Data Quoting (D) Expression (E) Misreading the question (Q) For Examiners’ Use 1 /40 2 /10 3 or 4 /25 Total /75 This document consists of XX printed pages. [Turn over] H2

2 SAJC / H2 Biology 9744/3 JC2 Prelim 2023 Answer all questions. QUESTION 1 On October 14, 2021, the World Health Organization (WHO) released the Global Tuberculosis Report 2021, which showed that 9.87 million new cases of tuberculosis and 1.5 million people died of tuberculosis in 2020. Among them, the estimated number of new cases of tuberculosis in China in 2020 was 842,000 (833,000 in 2019), and the estimated incidence of tuberculosis was 59/100,000 (58/100,000 in 2019). Among the 30 countries with a high burden of tuberculosis, China ranks second in the number of estimated cases of tuberculosis and has over 30,000 deaths due to tuberculosis. (a)(i) In Fig. 1.1 below, label X and Y appropriately, to reflect the pathogenes is of tuberculosis. Fig. 1.1 [2] Ans: X: Mycobacterium tuberculosis [Reject: bacteria] [Accept: italics] Y: granuloma [Accept: Tubercle]

3 SAJC / H2 Biology 9744/3 JC2 Prelim 2023 (ii) With reference to a cellular organelle in the macrophages, describe how macrophages usually attempt to process engulfed bacteria. ……..………………………………………………………………..…………………..……....[2] 1. Lysosomes contain hydrolytic enzymes in macrophages, (e.g. proteases), which hydrolyzes bacteria; 2. after lysosomes fused with phagosomes/endocytic vesicle containing bacteria; Fig. 1.2 shows the cell wall of the bacteria causing tuberculosis and its unique make up of fatty acids which are slightly different from those used in the synthesis of triglycerides in eukaryotes. Fig. 1.2 (iii) With reference to Fig. 1.2, explain how the bacteria causing tuberculosis can evade the mechanisms of the macrophage host cells. ……..…………………………………………………………………...…………………….....[1] 1. Their mycolic acid wax coat on their cell walls also protects them against hydrolytic enzymes in lysosomes; (iv) Complete and annotate on the diagram below to show the production of a triglyceride from its constituents in eukaryotes. …………………………………………………………………..……………………..….….[2]

4 SAJC / H2 Biology 9744/3 JC2 Prelim 2023 Answer: 1. Correction drawing of triglyceride molecule; 2. Annotation: naming of glycerol, 3 fatty acids; removal of 3 water molecules; and label of ester bonds Ester bond

5 SAJC / H2 Biology 9744/3 JC2 Prelim 2023 Most infections are latent and do not have symptoms. Latent infections can progress to active form of the disease which, if left untreated, kills about half of those infected. The most common classic symptoms of active TB include chronic cough with blood - containing sputum, chest pain and shortness of breath. (v) Explain the cause of the shortness of breath symptom of TB in its active stage. ……..……………………………………………………………………..………..…………....[2] 1. Bacteria destroy alveoli (e.g. by releasing toxins or infecting alveolar epithelial cells) /causes other immune cells trying to contain the infection to release toxins which destroy alveoli tissue and form cavities in the lungs; ( these damaged areas may become infected with other bacteria and form pockets of pus) 2. Damage of alv eoli leads to less surface area for diffusion processes / increases diffusion distance in the lungs; (leading to wheezing for air) . [Accept: less efficient gaseous exchange] Bacteria causing tuberculosis is able to acquire genes coding for antibiotic resistance through various ways, leading to multi-drug resistant tuberculosis. An example is bacteria that are resistant to rifampicin, a key drug in the treatment of tuberculosis. The mode of action of rifampicin in bacteria is by binding to the beta (β)-subunit of the RNA polymerase, inhibiting the elongation of messenger RNA. In about 96% of bacteria resistant to rifampicin, there are mutations in the so-called “hot- spot region” of DNA transcribing into codons 507–533 of the rpoB gene, which codes for the beta (β)-subunit of bacterial RNA polymerase. [edited from Palomino JC, Martin A. Drug Resistance Mechanisms in Mycobacterium tuberculosis. Antibiotics (Basel). 2014 Jul 2;3(3):317-40] (vi) Calculate the length of the DNA in the “hot-spot region”. Represent your answer in bp. …………………. bp [2] 1. Number of codons = 533 – 507 +1 = 27;

6 SAJC / H2 Biology 9744/3 JC2 Prelim 2023 2. Each codon consists of 3 bases, length of DNA = 27 x 3 = 81 bp; [allow ecf: e.g. 26 x 3 = 78 bp] (vii) Explain why a mutation to the rpoB gene can lead to resistance to rifampicin. …………………………………………………………………………………………………...[3] 1. Ref. change to DNA sequence will lead to altered mRNA / codon sequence; 2. Ref. altered amino acid sequence; and 3D conformation of the beta subunit RNA polymerase; 3. Rifampicin is unable to recognize through 3D conformation / bind to (beta subunit of) RNA polymerase; unable to inhibit transcription in bacteria / cannot inhibit elongation of mRNA chain; (leading to resistance of rifampicin) [Reject: answers mentioning only on RNA polymerase unable to catalyse transcription without any reference to antibiotic rifampicin] (viii) Discuss how natural selection could have led to the evolution of antibiotic rifampicin resistance in bacteria causing tuberculosis. ………………………………………………………………..……………………...............…[4] 1. Genetic variation in susceptibility to antibiotics exists in the bacterial populations due to mutations; 2. Bacteria are subjected to selection pressure + e.g. introduction of antibiotics; 3. Bacteria with genes coding for antibiotic resistance are at a selective advantage + reason able to avoid binding of rifampicin to RNA polymerase; 4. Such bacteria can better survive and reproduce / by binary fission, passing down the advantages alleles [reject: traits/characteristics] to their offspring; 5. Ref. pass on resistant allele by horizontal gene transfer / transduction/ conjugation / transformation to other bacteria cells; (followed by passing on of such alleles to progeny cells by binary fission) 6. Over time, allele frequencies change and bacteria resistant to rifamcipin become predominant (in the population); [Max 4]

7 SAJC / H2 Biology 9744/3 JC2 Prelim 2023 (b) Table 1 below shows the effect of a newly discovered antibiotic X on the survival of bacteria causing tuberculosis. The same volume of bacteria causing tuberculosis was plated on nutrient medium plates and incubated at 37°C overnight. For control plates, there was no antibiotic added to the nutrient medium plates, while test plates contains the antibiotic. For each condition, 4 readings from 4 plates were measured to obtain replicate readings. At the end of incubation, the number of bacterial colonies are counted and presented in Table 1. Table 1: Effect of antibiotic X on the number of bacterial colonies after incubation Number of colonies Condition Count 1 Count 2 Count 3 Count 4 Average (whole number) Standard deviation (3 s.f.) Control 55 61 58 73 Test plates (ant