TMJC 2020 Bio Prelim Paper 3 (A)

1 Tampines Meridian Junior College 2020 JC2 Preliminary Examinations H2 Biology _____________________________________________________________________________ H2 BIOLOGY 9744 Paper 3 Long Structured and Free Response Questions 18 September 2020 2 hours _____________________________________________________________________________ No Where did I go wrong?  How can I improve? ☺ 1 I don’t understand what the question wants from me. • Identify topic(s) related to the question. • Analyse the preamble and/or diagram carefully. 2 I don’t know / can’t remember the conceptual facts. • Review your study techniques to find out what works best for you. 3 I did not give the essential keywords / wrong keyword. • You must go back to your notes and learn why you missed the essential points in addressing the questions. 4 My answers are incomplete / not of enough depth. 5 I misinterpreted the questions / data, hence wrote the wrong answer. • Read all the directions (identify keywords or phrases and register what they mean). • Take the time to paraphrase the question. 6 I did not contextualize my answers to the question. That is, I did not make use of the information in the preamble / stimulus / figure. • When the question revolves around a specific example, use the contextual information to craft your answers. 7 I did not cite data / I did not include the units for data / did not cite meaningful data for both axes. • Cite complete data: both x-axis and y-axis, with units. • Examine the trend of the graph. If necessary, divide the graph into  2 parts for meaningful citation of data. 8 I did not organize my answers properly, especially for comparison questions / essay questions. • For essay, paragraph your answers for each major idea. • For comparison, number your points. Use comparative words (e.g. but, whereas, while etc.) 9 I did not manage to attempt the question due to insufficient time. • Look through the whole paper and attempt questions you are confident in. • Be concise. Do not write excessively. • When you are stuck at a question, move on. 10 I was not able to apply the conceptual facts to this kind of ‘suggest’ questions. • Identify the topic(s) that the question is related to, draw links to the specific section or concept • Write down keywords or phrases. • Finally see how these would match the question TAMPINES MERIDIAN JUNIOR COLLEGE JC2 PRELIMINARY EXAMINATIONS

2 Tampines Meridian Junior College 2020 JC2 Preliminary Examinations H2 Biology Where did I go wrong….? (Tick in the appropriate box) Structured Qn / No CHECKLIST NUMBER 1 2 3 4 5 6 7 8 9 10 Total Count What was my major weakness(es)…?

3 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 a) Describe how the ΔF508 mutant CFTR protein may bring about a reduction in water loss through epithelial cell membranes. [3] [TRANSPORT] 1. A deletion of phenylalanine changes the conformation of the CFTR protein 2. Unable to transport chloride ions, leading to an increase of ion concentration inside the cell 3. Decreases the water potential inside the cell 4. Less water movement out of cell by osmosis / reverse argument [Any 3]

4 Tampines Meridian Junior College 2020 JC2 Preliminary Examinations H2 Biology b) Explain why inherited diseases, in general, have low occurrences in the general population. [3] [EVOLUTION] 1. Ref. to affected individuals at selective disadvantage / reduced chance of survival 2. Do not survive to reproductive age to pass on the mutant allele to offspring 3. Most inherited diseases are caused by recessive alleles 4. Idea of Low chance of two carriers mating 5. Allele is expressed only in individuals who are homozygous recessive [Any 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] [INHERITANCE] Probability of two carriers being the parents of a newly-born child = 1/30 × 1/30 = 1/900 [1] Probability of such a child being homozygous recessive = 1/4 Therefore, overall probability = 1/4 × 1/900 = 1/3600 [1] [Accept: 0.00028 / 0.028%]

5 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] [MUTATION] • Class III / class IV 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] [INHERITANCE] 1. Idea that Different mutations results in different versions/variants of CFTR proteins that functions to different extents 2. Ref. to class I/II/III being severe mutations, and class IV/V/VI being mild mutation OR Ref. to class I/II being severe, III/IV being intermediate, class IV/V/VI being mild 3. Ref. to a mild mutation results in a CFTR protein that can still transport som