2018 VJC H2 Biology Prelims Paper 2 Answers

Victoria Junior College Prelim Paper 2 Answers 1 Victoria Junior College 2018 H2 Biology Prelim Paper 2 Answer 1 In eukaryotic cells, the degradation of mRNA is an essential part of the regulation of gene expression. It can be controlled in re sponse to developmental, environmental, and metabolic signals. mRNA hydrolysis is catalysed by numerous types of nucleases, such as the endonuclease Ribonuclease A (RNAse A), shown in Fig. 1.1. (a) Using a labelled and annotated diagram, illustrate the hydrolysis of the bond catalysed by RNAase. [3] (A monomer has been drawn for you.) • Accurate drawing of mRNA strand, at least 2 nucleotides (using symbols);; • Accurate drawing of phos phodiester linkage + label;; • Water; • Hydrolysis ; • Accurate drawing of correct number of nucleotides after hydrolysis; Fig 1.1B shows two important catalytic resi dues within the active site of RNAse A, which are His12 and His119. (b) Explain how these two histidines, wh ich are in position 12 and 119 of the 124 amino acid sequence, are brought together in the active site of the enzyme. [3] • Primary structure (number, type and sequence of amino acid )determines how the polypeptide chain folds upon itself;; • interactions between R groups of amino acids not located close to one another on the primary structure ; • To form the tertiary structure wit h a compact globular 3D structure; • Bringing faraway amino acids t ogether within the active site; (c) Predict how the catalytic activity of RNAse would be affected if both histidines were replaced by phenylalanines. [2] • Histidine has an R-group that is polar whereas phenylalanine has an R-group that is non-polar;; • This causes the change in the interaction between the catalytic residues and the substrate at the active si te; therefore; RNAase cata lytic activity will be greatly reduced / lost;; 2 (a) Based on your understanding of penicillin and with reference to Fig. 2.1, (i) deduce whether penicillin is more effect ive against gram-positive or gram-negative bacteria. [1] Gram positive;; (ii) suggest a reason for your answer in (a)(i). [3] • Penicillin binds irreversibly to the enzyme DD-transpeptidase;; • which is responsible for the catalysis of cross-link formation within the peptidoglycan cell wall;; •

Victoria Junior College Prelim Paper 2 Answers 2 • Gram positive bacteria have thicker pept idoglycan cell wall hence more affected than Gram negative bacteria;; • Penicillin is a hydrophilic molecule; • Unable to pass through the hydr ophobic core of outer membrane; • Gram positive bacteria has no outer me mbrane hence allowing penicillin easier access to the peptidoglycan cell wall;; (bi) Outline how the bacterium produces an efflux pump from a gene on a plasmid. [4] • RNA polymerase bind s to the promoter; • catalyses phosphodiester bonds between ribonucleotides • transcribes mRNA from DNA template strand; • mRNA binds to s