2022 HCI JC2 H2 Biology Prelims Paper 2 MS

HWA CHONG INSTITUTION / 2022 H2 BIOLOGY / Preliminary Examination Paper 2 / MS HWA CHONG INSTITUTION (COLLEGE SECTION) 2022 JC2 9744 H2 BIOLOGY PRELIMINARY EXAMINATIONS PAPER 2 MARK SCHEME QUESTION 1 (a) Describe the main features of amylose visible in Fig. 1.1 [2] 1 Ref to α (1, 4) glycosidic bonds between glucose units ; 2 Ref to helical structure, with 6 glucose units per turn ; (b) Explain how the structure of amylose makes it suitable for its function. [2] 1 Ref to large amylose, thus insoluble in water, does not exert osmotic influence in the cell ; 2 Ref to hundreds to thousands of glucose, thus a large source of carbon ; 3 Ref to α (1, 4) glycosidic bonds, being easily hydrolysed ; 4 Ref to amylose being helical and thus compact ; 5 Ref to anomeric carbon involved in glycosidic bond formation, thus unreactive / chemically-stable ; (c) Suggest how double helix structure of amylose is formed. [1] Ref to via hydrogen bonds between adjacent amylose molecules ; (d) (i) State two way in which the structure of collagen differs from the structure of starch. [2] Collagen Starch 1 a Made up of amino acids 1 b Made up of α-glucose ; 2 a Linked by peptide bonds 2 b Linked by α(1,4) glycosidic bonds ; 3 a Triple helix tropocollagen 3 b Helical amylose, highly branched amylopectin ; 4 a ref to unbranched 4 b Ref to branched ; 5 a Ref to extensive hydrogen bonds / cross-links / staggered arrangement 5 b absent ; (ii) Explain how one of the differences stated in (d)(i) allows collagen to perform its function. [2] 1 Ref to glycine-X-Y, glycine with small R group can fit in / packed tightly together ; A: hydroxylproline having OH group for hydrogen bonding 2 Ref to providing high tensile strength ; 3 Ref to extensive hydrogen bonds within tropocollagen, between NH group of Gly in one α-chain and CO group of another amino acid in next α-chain ; 4 Ref to providing high tensile strength ;

HWA CHONG INSTITUTION / 2022 H2 BIOLOGY / Preliminary Examination Paper 2 / MS 5 Ref to covalent cross-links present within/between tropocollagen, between CO end of one molecule and NH end of another or ref to staggered arrangement stabilised by hydrophobic interactions ; 6 Ref to conferring greater tensile strength ; [Total: 9] QUESTION 2 With reference to Fig. 2.1, (a) (i) explain why calcium ions do not pass through the phospholipid bilayer. [2] 1 Calcium ions are water soluble/ hydrophilic; 2 unable to pass through hydrophobic core of the phospholipid bilayer which consists of hydrocarbon fatty acids chains; (ii) state and describe the process by which calcium ions are moved across the membrane. [4] 1 Active transport; Any 3 2 Intracellular Ca2+ binds to the binding sites of the calcium pump/ carrier protein; R: active site R: channel protein 3 Upon binding of Ca2+, carrier protein undergoes conformational change; 4 Ca2+ is transported against the concentration gradient from the cytoplasm (a region of lower Ca 2+ concentration) to outside the cell/ extracellular (a region of higher Ca 2+ concentration; 5 Ref. to usage of ATP as an energy source; *MP1 compulsory for full marks (b) With respect to the fluid mosaic model, discuss how high temperature affects membrane permeability. [3] Phospholipids 1 At high temperature, kinetic energy of the hydrocarbon chains of the phospholipids increases, and increase thermal agitation/ lateral movements/ AW; 2 Overcoming/ weaker hydrophobic interactions between phospholipids, increase transient gaps/ pores between adjacent phospholipids; Membrane proteins 3 Ref. to correct explanation following denaturation of membrane proteins at high temperature: 4 Ref. to increase membrane permeability; *Students have to mention effects on both phospholipids and membrane proteins to obtain full marks [Total: 9]

HWA CHONG INSTITUTION / 2022 H2 BIOLOGY / Preliminary Examination Paper 2 / MS QUESTION 3 (a) State the Central Dogma of Molecular Biology. [3] 1 ref. to unidirectional flow of genetic information from DNA to RNA to polypeptide ; A: genetic information being encoded in DNA R: statements wrt cell theory or genetic code R: discussion of semiconservative replication, i.e. how DNA acts as template in semi - conservative replication 2 ref. to genetic information being transferred to mRNA by transcription ; A: RNA 3 ref. to information on mRNA being used to synthesise polypeptides by translation ; R: protein (b) Explain how a molecule of telomerase synthesises additional lengths of DNA. [4] any four from 1 template RNA binds to (part of) region X / 3’ overhang ; R: telomerase – it is the RNA template component R: template DNA – it is the 3’ overhang! R: mere citation of DNA sequence (e.g. TTAGGG) without reference to 3’ overhang – these repeats are found all over the place in the telomere 2 template RNA and 3’ overhang bind to active site of, telomerase / TERT ; 3 DNA / free, nucleotides, pair with / bind to, RNA template via hydrogen bonds ; R: synthesis of primer 4 ref. to complementary, bases / base pairs ; 5 (all or none) ref. to A–T and C–G and U–A ; 6 phosphodiester bonds form, between (DNA) nucleotides / AW ; treat as neutral any refs. to likely enzymes, e.g. ligase, etc R: telomerase catalysing formation of hydrogen bonds between complementary base pairs 7 telomerase moves, in the direction of the arrow / to the right ; R: telomerase moving in the 5’ to 3’ direction – this is the direction of addition of dNTP and you need to contextualise to Fig. 3.2 (c) Explain why the action of telomerase challenges the Central Dogma of Molecular Biology. [2] 1 (template) RNA acts as a template rather than DNA ; 2 (direction) reverse flow of genetic information from RNA to DNA ; I: RNA is not translated to form proteins. Work on the fact at hand about what telomerase does - say explicitly that RNA is used to form DNA and this in opposite to central dogma! R: ref. to cell theory / ref. to cell being able to overcome Hayflick limit / ref. to preventing telomeres from shortening or sticking together (d) Suggest why prokaryotes do not have telomerase. [2] 1 prokaryotes have, circular DNA ; A: ‘loop of DNA’ I: plasmid(s) 2 prokaryotes do not have telomeres ; A: prokaryotic DNA has no ends I: ref. to prokaryotes having a smaller genome – it is not relevant to telomeres

HWA CHONG INSTITUTION / 2022 H2 BIOLOGY / Preliminary Examination Paper 2 / MS R: ref. to prokaryotes not having non-coding regions – they do. Telomere is just one type of non-coding region! R: ref. to lagging strand – it is not relevant to telomeres. R: ref. to ssDNA / use host cell polymerase – prokaryotes are living organisms and not viruses R: simultaneous transcription and translation – we are looking at DNA synthesis! [Total: 11] QUESTION 4 (a) Compare the structures of SARS-CoV-2 and influenza virus. [3] Any 3 1 Both contain RNA ; 2 Both have viral envelope with glycoproteins on its surface ; 3 There are 8 segments of RNA in influenza but 1 RNA for SARS-CoV-2 ; 4 HA glycoprotein is embedded on viral envelope of SARS-CoV-2 whereas S glycoprotein is embedded in the viral envelope of COVID-19 ; R: one type of glycoprotein vs two types of glycoproteins 5 AVP; (b) Describe how the SARS-CoV-2 virus enters a host cell. [3] 1 *S glycoprotein on the viral membrane binds to ACE2 and PRS2 receptors on the host cell membrane; 2 The virus enters the host cell by receptor-mediated endocytosis ; 3 forming an endosome/endocytic vesicle; 4 Ref. to subsequent viral envelope fusing with vesicle membrane and releasin