TMJC 2020 Bio Prelim Paper 3 (Q)

Tampines Meridian Junior College 2020 JC2 Preliminary Examinations H2 Biology CANDIDATE NAME CIVICS GROUP _____________________________________________________________________________ H2 BIOLOGY 9744 Paper 3 Long Structured and Free Response Questions 18 September 2020 2 hours _____________________________________________________________________________ READ THESE INSTRUCTIONS FIRST Write your name and Civics Group in the spaces at the top of the page. Write in dark blue or black pen on both sides of the paper. You may use an HB pencil for any diagrams, graphs or rough working. Do not use staples, paper clips, glue or correction fluid. The use of an approved scientific calculator is expected, where appropriate. Section A Answer all questions. Section B Answer ONE question. You may lose marks if you do not show your working or if you do not use appropriate units. At the end of the examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. ______________________________________________________________________________________ This document consists of 17 printed pages and 1 blank page. For examiner’s Use Section A 1 / 32 2 / 18 Section B 3 or 4 / 25 Total / 75 TAMPINES MERIDIAN JUNIOR COLLEGE JC2 PRELIMINARY EXAMINATION

2 Tampines Meridian Junior College 2020 JC2 Preliminary Examinations H2 Biology Section A Answer all questions in this section. QUESTION 1 The cystic fibrosis transmembrane conductance regulator (CFTR) protein is an ion channel. In the lung, the CFTR ion channel moves chloride ions from inside the cell to outside the cell. Cystic fibrosis (CF) is a serious genetic condition caused by recessive mutations in the gene for the CFTR protein. One of the most common mutated alleles of this gene is known as ΔF508, a deletion of the amino acid phenylalanine at the 508 th amino acid position of the polypeptide. This mutation results in a CFTR protein that does not conduct chloride ions. Features of CF, which are also observed in heterozygotes, include: • a reduction in water loss through epithelial cell membranes • a reduction in sweating. When there is less water outside the cells, the mucus in the airways becomes dehydrated and thickens, as shown in Fig. 1.1 . The cilia becomes unable to sweep properly when the mucus is thick as sticky mucus weighs them down. Because the cilia are unable to move properly, mucus gets stuck in the airways, making it difficult to breathe. In addition, germs caught in the mucus are no longer expelled from the airway, allowing them to multiply and cause infections. Thick mucus in the lungs and frequent airway infections are some of the most common problems that people with CF face. Fig. 1.1

3 Tampines Meridian Junior College 2020 JC2 Preliminary Examinations H2 Biology a) Describe how the ΔF508 mutant CFTR protein may bring about a reduction in water loss through epithelial cell membranes. [3] ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………. b) Explain why inherited diseases, in general, have low occurrences in the general population.[3] ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………. c) CF is one of the most common inherited diseases. This unusually high occurrence has given rise to the suggestion that carriers of the ΔF508 allele may have a selective advantage over non-carriers. Approximately 1 in 30 people in the European and North American populations are carriers of the defective allele ΔF508. Within the European and North American populations, calculate the probability that a newly- born baby will be homozygous for this allele. You should show your working. [2] Probability = …………………….

4 Tampines Meridian Junior College 2020 JC2 Preliminary Examinations H2 Biology d) There are at least 1500 different mutated alleles of the CFTR gene. These alleles have been classified into six classes according to the mechanism by which they disrupt the synthesis, trafficking and function of CFTR protein. The six classes are described in Table 1.1. Table 1.1 class of mutation effect of CFTR mutation class I complete lack CFTR protein class II misfolded CFTR protein not transported to the cell surface class III reduced or lack of CFTR channel opening (‘gating’ defect) class IV a ‘misshaped’ CFTR pore that restricts the movement of chloride ions through the channel (‘conductance’ defect) class V splicing defect with a great reduction in normal CFTR proteins class VI decreased CFTR protein stability in the cell surface that leads to its removal and degradation i) State the most likely class of mutation to which ΔF508 belongs. [1] ……………………………………………………………………………………………………….. ii) With reference to Table 1.1, explain why in the population of individuals who carry two recessive alleles, the symptoms of CF may range from mild to severe. [3] ……………………………………………………………………………………………………….. ……………………………………………………………………………………………………….. ……………………………………………………………………………………………………….. ……………………………………………………………………………………………………….. ……………………………………………………………………………………………………….. ……………………………………………………………………………………………………….. iii) Suggest how, in the class VI mutation, the unstable CFTR protein is removed from the cell surface membrane and subsequently degraded. [4] ……………………………………………………………………………………………………….. ……………………………………………………………………………………………………….. ……………………………………………………………………………………………………….. ……………………………………………………………………………………………………….. ……………………………………………………………………………………………………….. ……………………………………………………………………………………………………….. ………………………………………………………………………………………………………..

5 Tampines Meridian Junior College 2020 JC2 Preliminary Examinations H2 Biology Cholera is caused by a toxin secreted by the bacterium Vibrio cholerae in the gut. This toxin enters the epithelial cells and activates an intracellular G -protein that leads to an excessive amount of cyclic AMP (cAMP). cAMP binds to CFTR to facilitate abnormally high movement of chloride out of the cell, increasing water loss from the gut epithelial cells. This results in severe diarrhea and may lead to death if untreated. In an experiment carried out in 1994, it was shown that when mice that were heterozygous for ΔF508 were exposed to cholera toxin, they lost 50% less water than homozygous dominant mice also exposed to cholera toxin. This supported a suggestion that the selective advantage of carrying the ΔF508 allele may be protection from the effects of cholera. e) Suggest how the ΔF508 allele might be expected to convey a selective advantage in areas of the world where cholera is common. [4] ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………. ……………………………………………………………………………………………………………. …………………………………………………………………………………………………………….

6 Tampines Meridian Junior College 2020 JC2 Preliminary Examinations H2 Biology f) In 2000, a further experiment to investigate the possible link between t