2013 HCI H2 Biology Prelims Paper 3 Answers

HWA CHONG INSTITUTION / 2013 JC2 H2 BIOLOGY / PRELIMINARY EXAMINATION / MARK SCHEME 1 HWA CHONG INSTITUTION 2013 JC2 H2 BIOLOGY PRELIMINARY EXAMINATION MARK SCHEME QUESTION 1 (a) Outline how the isolated viral RNA can be convert ed into a double-stranded cDNA in step 1. [2] 1. Reverse transcriptase catalyses synthesis of the single-stranded c DNA using the isolated viral RNA template 2. DNA polymerase converts the single-st randed cDNA into a double-stranded cDNA (b) (i) Explain how the recombinant Ti plasmid in step 2 can be formed. [3] 1. The same restriction enzyme is used to cleave both the viral gene coding for TMV coat protein from the cDNA formed in Step 1 and the Ti plasmid 2. to produce complementary sticky ends which fa cilitate annealing of / formation of hydrogen bonds between the viral gene and linearized Ti plasmid 3. DNA ligase is added to covalently link / form phos phodiester bonds between the viral gene and linearized Ti plasmid to produce a recombinant Ti plasmid (ii) Explain the purpose of having a leaf-specific prom oter in the recombinant Ti plasmid formed in step 2. [2] 1. The leaf-specific promoter will ens ure that the viral gene coding for TMV coat protein is only expressed in the leaves of transgenic plants 2. to ensure production of TMV coat protein (c) Suggest another important role of Agrobacterium tumefaciens in the process of creating transgenic plants which are resistant to TMV. [1] 1. Agrobacterium tumefaciens allows for in vivo amplification / production of numerous / multiple copies of the recombinant Ti plasmid (d) (i) Describe the functions of two other named ingredients in the growth media in the development of plant tissues. [4] Ingredient Use in developing tissue 1a. Carbon source, e.g. glucose / sucrose 1b. As respiratory substrate due to lack in autotrophic ability for proliferation of cells and regeneration of shoots 2a. Plant growth regulators, auxins 2b. Stimulates root growth by promoting cell growth & differentiation 3a. Plant growth regulators, cytokinins 3b. Stimulates shoot growth by promoting cell growth & differentiation 4a. Amino acids 4b. Precursors for prot ein synthesis to stimulate cell growth 5a. Vitamins 5b. As catalysts / coenzymes / enz yme activators in various metabolic processes 6a. Antibiotics 6b. Prevent growth of systemic mi crobes / effectively control systemic infection (ii) Explain the significance of the change of environment for the newly form ed plantlets in the final stage of tissue c u l t u r e . [ 2 ] 1. The change of environment represents the shift from heterotrophic / sugar-requiring to autotrophic / free- living condition 2. and the acclimatization of the plantlets to the outdoor environment before they develop into whole transgenic plants [Total: 14 marks] PAPER 3

HWA CHONG INSTITUTION / 2013 JC2 H2 BIOLOGY / PRELIMINARY EXAMINATION / MARK SCHEME 2 QUESTION 2 (a) State what is meant by genetic testing. [1] 1. identification of changes in chromosomes/ genes/ alleles/ DNA 2. to detect suspected genetic disease (b) Using the information given in Fig. 2.1, explain why (i) the three-day-old embryo will develop into a child affected by cystic fibrosis. [2] 1. reference to cystic fibrosis as a recessive condition 2. requiring two copies of mutant allele/ allele 2 3. Three-day-old embryo has same banding pattern as affected child (ii) the position of allele 2 on the agarose gel indicates that it contains a deletion in comparison with allele 1. [2] 1. allele 2 is shorter / lighter / has a lower molecular mass than allele 1 2. allele 2 is able to migrate a longer distance / move faster through agarose gel (c) Suggest why the child cannot pass on the normal CFTR allele to his offspring. [1] 1. tracheal epithelial cells are somatic cells / are not germ cells (d) State one limitation of liposome-mediated gene therap y and suggest one improvement to this limitation. [2] limitation: 1. transient expression of CFTR allele 2. CFTR allele delivered is not integrated into chromosome 3. short-lived/ transient natur e/ repeated treatment needed 4. not specific in delivering CFTR allele into target cells improvement: 1. Use retrovirus-m ediated gene therapy 2. design liposomes carrying glycoproteins/glycolipids in their membranes that are recognised by cell receptors (e) Describe how RFLP analysis facilitated linkage mapping of the CFTR gene on chromosome 7. [4] 1. restriction sites occur more than once outside the CFTR locus, i.e. Taq I 2. point mutations that create / destroy the restriction sites 3. restriction digestion / cutting with Taq I produce unique patterns of restriction fragments respectively 4. each pattern / set of restrict ion fragment is an RFLP marker 5. determine the COV between each RFLP marker and the CF condition 6. the longer the distance between an RFLP marker and a particular CFTR allele, the higher the frequency of crossing over 7. Application: to determine order of CFTR gene in relation to RFLP markers and relative distances between the loci [Total: 12 marks]

HWA CHONG INSTITUTION / 2013 JC2 H2 BIOLOGY / PRELIMINARY EXAMINATION / MARK SCHEME 3 QUESTION 3 (a) Describe how the expression of the miRNA gene controls the expression of the structural gene. [3] 1. Transcription of the miRNA gene gives a double-st randed miRNA precursor/ RNA folding onto itself/ forming a hairpin loop 2. Drosha cleaves/cut the free ends of the double strand ed miRNA precursor and this is exported out of the nucleus OR Dicer then cleaves/ cut the loop and gives a double stranded miRNA Reject: Non-specific terms (e.g processed/ modify miRNA) 3. RISC binds to one of the miRNA strands 4. and becomes complementary to the mRNA of t he structural gene, prev enting its translation/ downregulating the expression of structural gene (b) Researchers believe that cancer cells may have mutations in some of the miRNA genes. With reference to proto-oncogenes, suggest how these mutations may have contributed to the progression of cancer. [2] 1. (i) Mutation of miRNA gene results in miRNA that are no longer complementary to target mRNA and thus cannot bind to mRNA to down-regulate expression OR (ii) Mutation of miRNA gene results in miRNA that cannot form double stranded miRNA thus Drosha/Dicer/RISC cannot bind to it OR (iii) Mutation of miRNA gene results in miRNA promot er of the miRNA is muta ted resulting in no miRNA being produced Reject: Vague description (inactive/ non -functional miRNA etc) or reference to nonsense mutation resulting in truncated miRNA. 2. hence leading to overexpression of the proto-oncogene products resulting in excessive proliferation Reject: Ref to disruption of tumour-supressor gene or lack of ref to proto-oncogene. (c) State a role of stem cells in an adult organism. [1] 1. maintain and repair the tissue in which they are found 2. replace cells that die because of injury or disease (d) Describe how the knockout of DGCR8 affects RNA interference [2] 1. Drosha is inactivated 2. miRNA/precursor is not processed / not cleaved 3. RISC can’t bind miRNA 4. RNA interference is reduced / translation is left on (e)(i) Describe how self-renewal in stem cells allows for their proliferation and subsequent differentiation. [2] 1. By process of mitosis / symmetric divisions 2. to produce a pool of undifferentiated cells 3. Through assymmetric division 4. to generate committed precursors that then become committed to specific lineages 5. Differentiation occurs when p