NYJC Prelim 9744 Paper 3 Answer

NANYANG JUNIOR COLLEGE JC 2 PRELIMINARY EXAMINATION Higher 2 CANDIDATE NAME ANSWERS CLASS BIOLOGY 9744/03 Paper 3 Long Structured and Free-response Questions 13 September 2024 Additional Materials: Insert, Answer booklet 2 hours READ THESE INSTRUCTIONS FIRST Write your name and CT on all the work you hand in. Write in dark blue or black pen. You may use an HB pencil for any diagrams or graphs. Do not use staples, paper clips, highlighters, glue or correction fluid. Section A Answer all questions in the spaces provided on the Question Paper. Section B Answer any one question on the separate Answer Paper. The use of an approved scientific calculator is expected, where appropriate. You may lose marks if you do not show your working or if you do no use appropriate units. At the end of the examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. For Examiner’s Use Section A 1 25 2 15 3 10 Section B 25 Total 75 This document consists of 13 printed pages and 1 blank page .

2 [Turn over 9744 / H2 Biology / 03

2 Section A Answer all the questions in this section. Information for Question 1 One way of classifying bacteria is by looking at the differences in their cell wall structure. The differences are shown by using the Gram stain. ● A Gram-positive bacterium has a cell wall mainly composed of a thick layer of peptidoglycan (murein). ● A Gram-negative bacterium has a more complex cell wall. This wall is composed of a much thinner layer of peptidoglycan and an outer layer known as the outer membrane. Escherichia coli is a Gram-negative bacterium. Fig. 1.1 is a diagram through the cell surface membrane and the cell wall of E. coli . Fig. 1.1 9744 / H2 Biology / 03

3 Bacteria reproduce asexually through binary fission. Due to their fast growth and simple genetics, bacteria such as E.coli are widely used in molecular biology. Fig. 1.2 shows a bacterial cell undergoing binary fission. Fig. 1.2 9744 / H2 Biology / 03

4 1 Part of the information required for Question 1 is presented in the insert. Cross references to Fig. 1.1 and Fig. 1.2 are included in the questions for which Fig. 1.1 and Fig. 1.2 are relevant. Bioinformatics is the collection, processing and analysis of biological data using computer software. The application of bioinformatics allows whole sequences of homologous genes to be compared across different bacterial species. (a) Explain why bioinformatics can used to compare gene sequences and suggest a conclusion that can be made from the percentage similarity data obtained. any one from: why 1 large amount of, data / DNA sequences ; 2 fast / accurate / efficient ; A description of fast / accurate / efficient / detects each small difference = accurate any one from: conclusions 3 more similarity the more recent a common ancestor / AW ; 4 molecular clock ; [2] (b) Humans use antibiotics to treat bacteria infections. Different types of antibiotics work in different ways to either kill or inhibit the reproduction of the bacteria. (i) Outline how penicillin acts on bacteria and use Fig. 1.1 to suggest why penicillin has little or no effect at treating diseases caused by Gram-negative bacteria. A antibiotic for penicillin throughout Max 2 1 weakens /AW, the cell wall¬ ; I punches holes / holes made 2 acts, on growing cells / when cell wall being synthesised; 3 inhibits / binds to / AW, enzymes / transpeptidases (for cross linkage formation) ; I ref. to synthesis of peptidoglycan 4 prevents formation of cross, links / linkages (between peptidoglycan / murein molecules) / AW ; 9744 / H2 Biology / 03

5 Max 2 Suggestions why antibiotic is less effective on gram negative 5 outer membrane prevents / interferes with / protects from / AW, entry (of penicillin) ; A idea of, more difficult / further, to reach peptidoglycan layer 6 proteins in outer membrane may pump out antibiotic ; A presence of efflux pumps 7 enzymes may be present (in periplasm) to degrade antibiotic / AW ; 8 suggestion that antibiotic cannot cross hydrophobic region of (outer) membrane ; 9 AVP ; e.g. proportionally, less / lower concentration of, penicillin reaches murein for, enzyme / transpeptidase, inhibition [3] (ii) The use of antibiotics has resulted in the emergence of antibiotic resistant bacteria. Antibiotics such as nalidixic acid acts as an inhibitor of an enzyme involved in DNA replication. Suggest how a substitution mutation in the gene coding for this enzyme could result in antibiotic resistance. Change in mRNA codon, therefore change in amino acid; Change in R group, resulting in change in bonds formed; Change in specific 3-D conformation / tertiary structure (A change in pp folding / coiling), change in binding site for antibiotic (R active site), therefore antibiotic cannot bind [4] 9744 / H2 Biology / 03

6 (c) The outer membrane of E.coli contains transport proteins called OmpF porins. These porins allow the passive movement of water, ions and small, polar molecules across the outer membrane. Each OmpF porin is formed from three identical polypeptides. (i) Suggest and explain the features of an OmpF porin as a membrane transport protein. four from: 1 channel protein ; A pore protein / has a pore / has a channel 2 channel / pore, can form from polypeptides or (protein) has quaternary structure ; 3 hydrophilic R-groups, on amino acids lining channel / face inwards (towards channel) ; A hydrophilic, lining / channel A water-filled, channel / AW A idea that passage is through hydrophilic region of protein 4 allows facilitated diffusion ; A diffusion alone if in context of through the protein but R if via phospholipid bilayer 5 increases permeability for, movement of water / osmosis ; I faster 6 no (specific) binding sites / (channel) not specific / not selective / allows more than one type of substance through / AW ; 7 globular ; 8 AVP ; e.g. ref. to hydrophobic part of protein, faces / interacts with, hydrophobic, region / core / fatty acid tails ref. to hydrophilic parts of, protein / polypeptide, extend into, external environment / periplasm / aqueous regions / AW [4] 9744 / H2 Biology / 03

7 (ii) E. coli can regulate the number of OmpF porins in the outer membrane to adapt to changing conditions. One control mechanism used by E. coli involves the production of a small mRNA molecule known as micF. micF binds to the part of the mRNA molecule containing the START codon for the OmpF polypeptide. Expl