2018 VJC H2 Biology Prelims Paper 3 Answers

Victoria Junior College Prelim Paper 3 answers Section A Answer all the questions in this section. 1 (a) Evaluate the effectiveness of vaccines agains t the rise of antibiotic resistant strains of pnemococcal bacteria. [4] [Max 4] • Highly effective/effect ive to a large extent; • Antibiotic is administered when a bacterial infection has already occurred; • Ref. to the idea that bacteria hav e reached a population large enough to cause disease / symptoms; • Ref. to the idea that bacteri a have multiplied many times; • Ref. to the idea that each time the bacteria divide, their DNA is copied and mistakes in this process can create variations within the population; • Antibiotic resistance can be one of the variations; • Presence of antibiotics can act as a selective pressure; • Select for more resistant bacteria; • Resistant bacteria survive and reproduce; • Pass down their antibiotic resistance alleles; • Increase the frequency of the antibiotic resistance alleles within the bacterial population; • Vaccines are administered prior to an infection; • Ref. to the idea that infection c an be brought under control before bacteria has the chance to multiply; • Antibiotics tend to target one spec ific bacterial protein / mechanism; • Vaccines can expose the immune syst em to a huge number of bacterial proteins / antigens; • Promotes the development of a vast r epertoire of antibodies that can fight against the bacterial infection in various ways; • Ref. to action of antibodies: e.g. ops onisation, agglutination, neutralisation • Ref. to idea that chances of bacteri a simultaneously evolving resistance to counter every type of antibody produced is slim; • Vaccines can curb overuse of antibiotics; (b) Describe how a typical T4 bacteriophage can work against an antibiotic- resistant bacterium. [3] • T4 bacteriophage binds to specific rec eptors on antibiotic-resistant bacterium; • The T4 phage penetrates the bacterium cell wall by contracting its contractile sheath of the tail which drives the hollow tube of the tail into the host bacterium; facilitating the entry of viral DNA; • The phage then replicates its genome and uses the bacterium’s protein synthesis machinery to synthesise phage proteins and structural components; • The T4 phage components assemble around the viral genome to form new mature phage particles;

Victoria Junior College Prelim Paper 3 answers • A phage-coded lysozyme breaks down the peptidoglycan cell wall of the host cell, causing cell lysis and release of the new T4 bacteriophages; • Resulting in the deat h of the bacterium; (c) (i) Using the information given, describe how a bacterium appears to have acquired antibiotic resistance due to the release of such plasmid DNA. [2] • Competent bacteria cells within the population will take up the plasmid DNA via transformation;; • Plasmid DNA may contain genes for antibiotic resistance;; (ii) Suggest how this could lead to an increase in the proportion of bacteria with antibiotic resistance in the population. [3] • The transformed bacteria cells can then undergo conjugation with the bacteria near them; or binary fission • Within the population, there are now antibiotic resistant bacteria and those that are not; • Use of antibiotics act as a selection pressure; • Antibiotic resistant bacteria have a se lective advantage and they are selected for; • These selected bacteria will survive, reproduce and pass on their alleles to the next generation; • increasing frequency of antibiotic-resi stant bacteria within the population; (d) Briefly comment on the validity of the hypothesis based on the results shown in Fig. 1.2. [3] • The hypothesis seems valid;; • T4 bacteriophage results in less intact plasmid and chromosomal DNA within the bacteria whereas bacteria that have interacted with SUSP1 and SUSP2 has more intact plasmid and chromosomal DNA;; • Endonucleases present in the T4 bacteriophages allows for the digestion of the bacteria’s plasmid and chromosomal DNA whereas these endonucleases are absent in SUSP1 and SUSP2;; (e) As a good Biology student, name and describe a procedure that the researchers can undertake to determine the presence or absence of the hydrolytic endonucleases in the “superspreaders”. [4] • Southern Blotting; • Place the gel in a mixture of al kali and salt to denature the double stranded DNA fragments into single stranded DNA; • The gel is then covered with a nitrocellulose filter; • Additional absorbent papers are added on top of the nitrocellulose filter to draw up the single stranded DNA from the gel by capillary action and transferred them onto the nitrocellulose filter; • Nitrocellulose filter is baked at 80 oC so that the DNA is permanently bound to the filter; • The filter is then exposed to a so lution containing radioactively labeled single-stranded DNA probes;

Victoria Junior College Prelim Paper 3 answers • ref to nucleic acid • which are complementary to the DNA sequence of the endonucleases; • Excess probes are then washed off and the bands are visualised under autoradioraphy; (f) Besides mutation, suggest how the environmental scientists’ prediction miht come true. [1] • As temperature rises, pe rmafrost melting occurs;; • releasing new bacteria and viruses, which were once trapped in the frozen soil, into the environment;; OR • As sea level rises, flooding of low lying lands occurs, less land available;; • different species of organisms may fi nd themselves in closer proximity, allow for greater antigenic shift in viruses to occur, resulting in new strains of viruses formed;; • AVP [Total: 20] 2 (a) (i) Label on Fig. 2.1, two critical posit ions where a cell can be regulated in the mitotic cell cycle. [2] • label - meataphase checkpoint; DNA damage checkpoint; G2 checkpoint (ii) Explain the significance of the critical positions where a cell can be regulated in the mitotic cell cycle. [2] • metaphase checkpoint – ensure all chromosomes are attached to mitotic spindle • G2 checkpoint – check no DNA damage and chromosomes all replicated • G1 checkpoint – cell size is large enough, nutrients available, growth factors present. 3 Identify the two areas labelled A in the di viding cells in Fig. 2.2 and outline their function. [3] • spindle apparatus / spindle fibres; Acc ept spindle / microtubules / tubulin / centrioles / microtubule organising centres / MTOCs [1] function to max 2 • attach to chromosomes / kinetochores ; • detail of, elongation / structure / shortening, of microtubules ; • for movement of chromosomes ; • during mitosis ; Accept if centrioles given as identity • forms poles of the cell ; • organises the spindle ;

Victoria Junior College Prelim Paper 3 answers 4 (i) Explain why it is nece ssary to increase the permeabi lity of the cell surface membranes before staining cells using the technique of immunofluorescence. [2] • Membrane has as hydrophobic hydrocarbon core; • Antibodies are hydrophilic; • Antibody molecules too large to fit between phospholipid molecules in the bilayer;; (ii) Suggest one advantage of using immunofluorescence in studying the changes that occur in cells during cell division. [1] • locate position of specific, proteins / structures ; • antibody molecules have complementary sh ape to target, proteins / structures ; • can see distribution of, proteins / structures, in light microscope ; • do not need to prepare sections for the electron microscope ; • easier to look at a large number of cells than in EM ; • higher degree of specificity than us ing other staining techniques ; • idea of variable regions of antibodi es giving greater specificity ; 5 (i) Explain why embryonic stem cells do not give rise to tumour. [3] 1. They are able to divide continuous ly but not excessively/uncontrollably; Cell division of stem cells can