VJC H3 BIO Answers 2017

1    VICTORIA JUNIOR COLLEGE BIOLOGY DEPARTMENT JC2 PRELIMINARY EXAMINATIONS 2017 Higher 3 VICTORIA JUNIOR COLLEGE VICTORIA JUNIOR COLLEGE VICTORIA JUNIOR COLLEGE VICTORIA JUNIOR COLLEGE VICTORIA JUNIOR COLLEGE VICTORIA JUNIOR COLLEGE VICTORIA JUNIOR COLLEGE VICTORIA JUNIOR COLLEGE VICTORIA JUNIOR COLLEGE VICTORIA JUNIOR COLLEGE VICTORIA JUNIOR COLLEGE VICTORIA JUNIOR COLLEGE VICTORIA JUNIOR COLLEGE VICTORIA JUNIOR COLLEGE VICTORIA JUNIOR COLLEGE VICTORIA JUNIOR COLLEGE VICTORIA JUNIOR COLLEGE VICTORIA JUNIOR COLLEGE VICTORIA JUNIOR COLLEGE VICTORIA JUNIOR COLLEGE CANDIDATE NAME CLASS INDEX NUMBER BIOLOGY 9815/01 25 September 2017 Additional Materials: Answer Paper 2 hours 30 minutes READ THESE INSTRUCTIONS FIRST Write your CLASS/ INDEX no. and name on all the work you hand in. Write in dark blue or blue pen. You may use a soft pencil for any diagrams, graphs or rough working. Do not use any staples, paper clips, highlighters, glue or correction fluid. Section A Answer all questions. Section B Answer three out of four questions. Section C Answer the question. At the end of the examinations, 1. Fasten all your work securely; 2. Circle the number of the section B question you have answered in the grid opposite. The number of marks is given in brackets [ ] at the end of each question or part question. This paper consists of 10 printed pages, including the cover page. For Examiner’s Use Section A 1 2 3 4 5 Section B 6 7 8 9 Section C 10 Total

2    Section A Answer all questions in this section. 1 (a) A genetic study of coat ty pe in purebred dogs has identified a mutation in the keratin gene KRT71, associated with the curly fur phenotype in some breeds. Keratin is known to be abundant in cystei ne. Suggest how a single mutation in KRT71 can lead to this curly fur phenotype. [2]  Mutation is likely to be a missense mutation that leads to a frameshift in the nucleotide sequence;  Leading to an unusually high number of codons specifying for the amino acid cysteine in the primary structure of altered keratin;  Cysteine has a sulfur-containing functional group/ thiol group;  The abundant number of cysteines (in altered hair keratin) allows numerous disulphide bridges to form between cysteines, resulting in curly hair; (b) Mandy decided that she wanted to loosen t he curls in her poodle’s fur. She first hosed her poodle down with water and then applied an ammonium thioglycolate solution (a reducing agent) as she roll ed her poodle’s fur around larger-diameter rollers. Next, she applied hydrogen peroxide (an oxidising agent) on the fur to set the curls. She was pleased with the results. Explain how this works at the molecular level. [2]  When fur keratin is exposed to the reducing agent, hydrogen lead to breaking of pre- existing disulfide bonds between cysteine amino acids;  New disulfide bonds form when the fur set in rollers is exposed to the oxidising hydrogen peroxide;  Diameter of curlers determine the number of new disulphide bonds that will form;  Fur wound around larger-diameter curlers allow only a few disulfide bonds to form; / if the curling rod has a small diameter, lots of disulfide bonds will form for curlier fur; (A! reverse statement) (c) Mandy was warned by the vet never to feed her poodle chocolates. Dogs cannot metabolize theobromine found in chocolates as well as humans can. For example, it will take a dog 17 ½ hours to metabolize and excrete just half of the theobromine it has ingested. It can thus accumulate in the animal’s body to a level that can be toxic or even fatal. Theobromine is a natural compound found in the cocoa bean and other plants. Genes of the cytochrome P450 family co de for enzymes that are involved in detoxification of drugs and compounds su ch as theobromine. Human enzymes work twice as fast as canine enzymes. Suggest the advantage of possessing this enzyme and discuss where this discovery places humans and non-canine mammals on the phylogenetic tree with dogs. [4]  Mammals have the enzyme to break down theobromine;  suggesting that there must have been a common ancestor of mammals that had the ability to break down theobromine;

3     This ability to digest theobromine, found naturally in cocoa plants helped our human ancestors to feed on another food source considered toxic to other mammals (ancestors of modern day dogs such as wolves) competing for food in the forest;;  Human enzymes are more efficient at detoxifying theobromine than canine enzymes possibly resulting from altered aa sequence that confers a configuration that has a higher affinity and hence higher reaction rate of theobromine detoxification / break down / removal;;  Suggests human mammals and non-canine mammals share a derived trait that groups them into a clade ( through descent with modification) with dogs as an outgroup;; (d) Mandy discovered that her poodle is suffering from biotin deficiency. This is a rare nutritional disorder which can become seri ous, even fatal, if allowed to progress untreated. Biotin is part of the B vitamin family. Mandy realised that her poodle’s biotin deficiency can be caused by cons uming raw egg whites over a period of time. Egg whites contain high levels of avidin, a protein that binds biotin strongly. In fact, this strong avidin-biotin interaction has been exploited in antigen detection in medical research. State the advantage and outline how such an immunoassay works. [2]  Primary antibody recognises specific antigen through antigen-binding site;  Secondary antibody, which binds primary antibody, is biotinylated;  Avidin molecule can be attached to a fluorophore or enzyme for detection step;  Attached biotin will form complex with avidin-conjugated enzyme or fluorophore to amplify signal / increase sensitivity of detection (enzymatic conversion of chromogenic substrate to a coloured product or detection by fluorescence); [Total: 10] 2 Human tau is encoded by the MAPT gene, located on chromosome 17. Tau is the major microtubule associated protein (MAP) of a normal mature neur on. It is found naturally as six molecular isoforms in the human brai n. An established function of MAP is their interaction with tubulin and promotion of its assembly into microtubules and stabilization of the microtubule network. (a) (i) Suggest how these different six isoforms of the Tau protein can be formed from a single gene. [1]  Alternative splicing of pre-mRNA to form different types of mature mRNA, which can then be translated into the various isoforms of Tau protein;; ( i i ) Explain the biological significance of the mechanism in (i) in normal physiology. [1] Any 1:  Developmentally regulated expression of certain isoforms to perform specific functions at specific times of embryonic development;;  Different isoforms are expressed in different cells and tissues to serve different functions;;  Different isoforms are expressed in the same cell at different times in appropriate response to different types of stimuli;;

4    Alzheimer’s disease and related neurodegenerative diseases are collectively known as tauopathies as they are characterized by the presence of aggregates of Tau. Tau protein is abnormally hyperphosphorylated and aggregated into bundles of filaments. (b) Explain how hyperphosphorylat ion may play a role in the