2017 HCI H2 Biology Prelims Paper 2 Answers

1 HWA CHONG INSTITUTION / 2016-2017 H2 BIOLOGY / Preliminary Examinations / Paper 2 / MS 1 HWA CHONG INSTITUTION (COLLEGE SECTION) 2017 JC2 9744 H2 BIOLOGY PRELIMINARY EXAMINATIONS PAPER 2 MARK SCHEME STRUCTURED QUESTIONS Question 1 (a)(i) Outline the cell theory. [max 2] any two: 1. living organisms composed of cells 2. cells form most basic unit of life 3. cells arise from other cells (a)(ii) With reference to Figure 1.1, explain how Pasteur’s experiment supports the cell theory. [2] 1. description of what happens to the nutrient broth (ref to stimulus) 2. shows that cells must come from pre-existing cells (a)(iii) Suggest a reason for the universal acceptance of the cell theory in our world today. [1] any one: 1. tested / scrutinized by other scientists 2. reproducible 3. overwhelmingly supported by scientific community (b)(i) State the name and chemical composition of the stuctures labelled A to C. [3] name chemical composition A 1a. cell wall 1b. peptidoglycan B 2a. cell membrane 2b. phospholipids and proteins C 3a. nucleoid 3b. DNA

2 HWA CHONG INSTITUTION / 2016-2017 H2 BIOLOGY / Preliminary Examinations / Paper 2 / MS 2 (c) P. syringae can cause disease in the leaves of its host plant by secreting toxins and cell wall degrading enzymes, without causing harm to itself. Explain why this is so. [2] 1. ref to toxins affect the functioning of membranous organelles 2. ref to different composition of cell wall 3. ref to specific 3D conformation of enzymes (d) State and explain which of the three types of interactions best describes the relationship involved in the endosymbiotic theory. [2] 1. mutualism 2. prokaryote produces ATP, host cell provides shelter / nutrients [Total: 12] Question 2 (a) With reference to Fig. 3.1, (i) name the stage of mitosis shown. [1] (late) anaphase (ii) describe what is happening during this stage of mitosis. [2] 1. centromere divide, forming daughter chromosomes 2. migrate to poles, centromere leading 3. pulled by kinetochore microtubules (b) Distinguish between multipotent cells and totipotent cells. [max 2] 1. multipotent more specialized 2. multipotent cells cannot give rise to organs but totipotent cells can 3. correct example of multipotent cell and totipotent cell (c) Suggest why there might be a connection between the use of stem cells in treatment and c a n c e r . [ m a x 3 ] 1. stem cells may undergo uncontrolled cell division 2. telomerase active 3. risk of developing tumour where stem cells implanted 4. ref to exposure to carcinogens 5. appropriate ref in context to cancer critical genes (d) State whether you agree or disagree that this is unethical and explain why you reached this decision. [3] A1 agree A2 human animal hybrid would not happen in nature A3 ref to disrespect for human life A4 few examples of success in medical applications A5 may lead to abuse in future A6 possibility of unforeseen consequences A7 unnecessary / there are alternative techniques

3 HWA CHONG INSTITUTION / 2016-2017 H2 BIOLOGY / Preliminary Examinations / Paper 2 / MS 3 D1 disagree D2 the amount of non-human DNA is negligible D3 protocol limits keeping ‘embryo’ to 14 days D4 more ethical alternative to ESC D5 provides more stem cells than possible from ESC D6 can relieve human suffering D7 idea of rejection at implantation [Total: 11] Question 3 (a)(i) State the identity of ligand X. [1] glucagon (a)(ii) Explain why ligand X cannot diffuse directly into the liver cell to trigger a cellular response. [2] 1. Ref to: glucagon as a large and hydrophilic molecule 2. Ref to: hydrophobic core of cell membrane (b) With reference to Fig. 3.1, describe how the structure of GPLR enables it to function as a membrane-bound receptor. [3] 1. Hydrophilic amino acid residues on inter-helical loops are soluble in aqueous medium 2. Hydrophobic amino acid residues on transmembrane helices enable embedding of GPLR within cell membrane 3. Ref. to specific 3D conformation of binding sites (c) Explain how the structure of glycogen is adapted to its function as an efficient storage biomolecule. [3] Any three: 1. Ref. to (1,4) glycosidic bonds that result in coiling 2. Ref. to (1,6) glycosidic bonds that result in branching 3. Ref. to glycogen having several hundreds to thousands of glucose monomers 4. Ref. to anomeric carbon being involved in glycosidic bond formation 5. Ref. to glycosidic bonds being easily hydrolysed to release glucose monomers [Total: 9]

4 HWA CHONG INSTITUTION / 2016-2017 H2 BIOLOGY / Preliminary Examinations / Paper 2 / MS 4 Question 4 (a) Describe two structural differences between helices A and B. [2] 1. Ref to: double-stranded vs single-stranded 2. Ref to: deoxyribonucleotides vs made of amino acids 3. Ref to: phosphodiester bonds vs peptide bonds 4. Ref to: Hydrogen bonds formed between comp lementary bases vs hydrogen bonds formed between NH and CO groups (b)(i) Describe how a primer strand is synthesised. [3] 1. Ref to: DNA template 2. Ribonucleotides form complementar y base pairs with the DNA template 3. Ref to: formation of phosphodiester bonds. 4. Ref to: synthesis in the 5’ to 3’ direction (b)(ii) With reference to Fig. 4.1, explain if the primer is priming the synthesis of the leading strand or lagging strand. [2] Leading strand is synthesised towards the replication fork (c) Explain the role of splicing in the structure and function of the two forms of IR. [3] 1. Ref to. different combinations of exons, 2. Ref to. different amino acid sequences and different specific 3D conformation of binding sites 3. Ref to. specific ligands IR-A and IR-B can bind, to different degrees / with different efficacy [Total: 10]

5 HWA CHONG INSTITUTION / 2016-2017 H2 BIOLOGY / Preliminary Examinations / Paper 2 / MS 5 Question 5 (a) With reference to Fig. 5.1, predict and explain the most likely mode of inheritance of G6PD deficiency. [3] 1. sex-linked recessive 2. all affected individuals in the family are males, indicating that males display disease phenotype more often than females as males are hemizygous OR approximately half of the sons of carrier females are affected, as every son has a 50% chance of receiving the X chromosome with recessive allele 3. unaffected parents can produce affected offspring such as II-1 / III-4 / IV-3 / IV-6, indicating that the mothers must have a dominant allele to mask the effect of the recessive allele OR if fathers are not affected, daughters will not be affected but may be carrier, as they will receive the X chromosome with dominant allele from their father (b) Using suitable symbols, draw a genetic diagram to show the expected phenotypic ratio of the ABO blood group and G6PD production in offspring of II-3 and II-4. [6] Let D be the dominant allele for production of G6PD d be recessive allele for no production of G6PD I A be the (codominant) allele for production of A antigen IB be the (codominant) allele for production of B antigen IO be the recessive allele for no production of antigen II-4 II-3 Parental phenotypes: No G6PD deficiency, No G6PD deficiency