2015 HCI H2 Biology Prelims Paper 2 Answers

HWA CHONG INSTITUTION / 2015 JC2 H2 BIOLOGY / PRELIMINARY EXAMINATION / MARK SCHEME 1 HWA CHONG INSTITUTION 2015 JC2 H2 BIOLOGY PRELIMINARY EXAMINATION PAPER 2 MARK SCHEME STRUCTURED QUESTIONS Question 1 (a) Describe the role of haem group in haemoglobin. [2] 1. Haem group in haemoglobin consists of an iron ion held in a porphyrin ring structure 2. Ref to iron ion bind with oxygen (b) Discuss the advantages of having four subunits in haemoglobin. [3] 1. Binding of four oxygen molecules per haemoglobin results in increased oxygen carrying capacity 2. Ref to cooperative binding 3. Change in 3D conformation in one subunit results in changes in 3D conformation of the other subunits (c) Explain how the differences in haemoglobin structure of the Greylag and Andean geese contribute to their different oxygen affinities. [4] 1. Ref to different amino acid sequence in the for Greylag goose and Andean goose haemoglobin 2. Different R-groups interactions results in the different specific 3D conformation 3. Haem groups are more exposed to bind to oxygen in Andean goose 4. Ref to different subunit interactions and different extend of cooperativity [Total: 9] Question 2 (a) Distinguish between the helical structures shown in regions X and Y. [2] 1. The helix in region X is single-stranded but the helix in region Y is double-stranded 2. Hydrogen bonds are formed between amine (-NH) and carbonyl (-CO) groups along the polypeptide backbone in region X but hydrogen bonds are formed between complementary bases on the two DNA strands in region Y 3. Helix in region X makes one complete turn every 3.6 amino acids but helix in region Y makes one complete turn every 10 base-pairs (b) Describe the roles of DNA polymerase in the S phase of the cell cycle. [2] 1. DNA polymerase holds the DNA template and deoxyribonucleoside triphosphates / deoxyribonucleotides close together in the correct orientation 2. DNA polymerase catalyses the formation of phosphodiester bond between 5’ phosphate group of an incoming dNTP and the 3’ hydroxyl group of the growing daughter strand 3. DNA polymerase catalyses the formation of a daughter DNA strand that is complementary to the parental DNA strand

HWA CHONG INSTITUTION / 2015 JC2 H2 BIOLOGY / PRELIMINARY EXAMINATION / MARK SCHEME 2 4. DNA polymerase removes the RNA primer and replaces it with dNTPs. 5. DNA polymerase proofreads and removes mismatched bases / nucleotides on the newly-synthesised daughter DNA strands (c) ddCTP was added to a DNA replication reaction in large excess over the concentration of deoxycytidine triphosphates (dCTP). Explain how the addition of ddCTP would affect DNA replication. [4] 1. Daughter strands would be synthesised until the first guanine base (G) encountered in parental / template strand 2. ddCTP is incorporated into daughter strands i