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 small ribosomal unit of 70S ribosome; • and undergoes simultaneous translation; • Idea of triplet code; • peptidyl transferase; catalyse s formation of peptide bonds; • between amino acids carried by tRNA on ribosomal A and P sites; • Formation of efflux pump polypeptide; • folds to form tertiary structure; (ii) Describe one similarity between the bacteria effl ux pump and the glucagon receptor that is important to their function. [1] • Both are transmembrane protei ns that span the membrane;; • The transmembrane sections of both proteins have hydrophobic amino acid residues that can form hydrophobic interactions with the hydrophobic core of the phospholipid bilayer;; (iii) Suggest two ways the struct ure of the bacterial efflux pump is different from an insulin receptor involved in blood glucose regulation. [4] RTK Efflux pump Number of transmembrane regions • One transmembrane section;; • 2 transmembrane section;; Important binding domains • Extracellular domain binds to insulin hormone;; • Intracellular region binds to antibiotic to pump it out of the cell;; Shape of active protein • Active receptor is made of 2 subunits that have dimerised;; • Efflux pump is a channel protein with a central hydrophilic core that allows hydrophilic molecules like antibiotics to pass though;; Presence of enzyme • Contain tyrosine kinase for cross phosphorylation of tyrosine residues on cytoplasmic domains • No tyrosine kinase in cytoplasm domain

Victoria Junior College Prelim Paper 2 Answers 3 3 Telomeres have a nucleotide sequence that is repeated as many as 2000 times. This repetition is shown in Fig. 3.1. Attached to the DNA of the telomere are protein units. (a) (i) What sequence of bases is repeated in the complementary polynucleotide shown in Fig. 3.1? [1] • AATCCC / adenine adenine thymine cytosin e cytosine cytosine;; (first 6) (ii) Suggest one reason for the presence of protein units in the telomere. [1] • Protect the DNA from degradation;; • Prevent binding of transcription fa ctors and RNA polymerase to the DNA;; • Enables homologous chromoso mes to pair during meiosis;; • AVP;; (b) In the past, repeating sequences were refe rred to as “junk DNA”. Explain why the term “junk DNA” is misleading in the context of telomere. [2] • “Junk” implies no, function / purpose;; ora • Repeating sequences of telomeres serve to protect genes from being eroded via successive rounds of replicat ion, maintain the integrit y of chromosomal end, and limit the lifespan of cells;; (c) The repetitive base sequence of telomere DNA is an exampl e of a non-coding base sequence. Explain what is meant by non-coding. [1] • Not transcribed to form a product (prote in / polypeptide / amino acid sequence);;

Victoria Junior College Prelim Paper 2 Answers 4 (d) A study of individual telomere lengths and its correlation with age is shown in Fig. 3.2. Account for the trend line shown in Fig. 3.2. [4] 1. Increase in age from 20 to 70, decrease in telomere length from 7.8 kb to 6.5 kb; 2. More, cell division / generations of cells / mitosis / replication; 3. Loss of, telomere / DNA / nucleotides / part of chromosome, at each replication; 4. Due to end replication problem; 5. During DNA replication, when the last RNA primer is removed / excised; 6. At the 3’ end of parental template strand / 5’end of daughter strand, it is not replaced by corresponding DNA sequence; 7. As DNA polymerase cannot add new nucleotides; without an existing 3’OH end; 8. Idea of resulting daughter DNA strand being shorter than the parental DNA strand; [Total: 9]

Victoria Junior College Prelim Paper 2 Answers 5 4 (a) Explain why ATP is regarded as the unive rsal energy currency in organisms. [2] • Found in all organisms;; • Loss of phosphate / hydrolysis, leads to, en ergy release / release of 30.5 kJ (per mole);; • ADP + Pi  ATP / reversible reaction;; • Small / water soluble, so can move around cell;; • Link between energy yielding and energy requiring reactions / AW;; • Example of use e.g. active transport / mu scle contraction / Calvin cycle / protein synthesis;; (b) Studies on cancer cells found that fast -growing cancer cells require much more energy than normal cells, which explains t he much higher rate of glucose uptake into cancer cells. However, it is also found that, unlike normal cells, the higher glucose uptake reduces oxygen uptake into cancer cells. This respiratory inhibition is known the Crabtree effect. It is proposed that this is due to more mitochondrial damages in cancer cells. (i) Besides the need for more energy for cell di vision, explain the process how cancer cells utilise glucose at a much higher rate than normal cells to produce energy. [3] • Ref. to anaerobic respiration;; • Ref. to glycolysis producing 2 net ATP;; • Pyruvate acting as the alternativ e hydrogen acceptor to reg