RI Nov 15 H2P2 ans

Raffles Institution Nov 2015 (H2 Biology) Paper 2 (for 9744 syllabus) 2017 Nov 2015 H2 Bio Paper 2 N15P2Q1 (a) Identify the structures A and B, as shown in Fig.1.1. [6] For each structure, state two features that can be seen in Fig.1.1. structure A: mitochondrion (R: mitochondria) feature 1: double membrane feature 2: highly folded inner membrane/cristae structure B: rough endoplasmic reticulum feature 1: flattened sacs called cisternae studded with ribosomes feature 2: continuous with outer membrane of nuclear envelope (b) Describe two functions of the Golgi body.[2] 1. To glycosylate proteins and lipids to form glycoproteins and glycolipids respectively; 2. To modify existing glycoproteins and glycolipids by modifying/cleaving the existing sugar chains; 3. To sort and package proteins into different vesicles and target the proteins to different parts of the cell or for secretion; 4. To form lysosomes; 5. To synthesise polysaccharides such as pectin which is transported in vesicles to the cell membrane; (c) Suggest two advantages to eukaryotic cells of having membrane bound organelles.[2] Membranes allows for compartmentalisation which allow 1. unique environments to be formed for highly specialised activities (e.g. acidic environment in lysosomes for hydrolytic enzymes to work); 2. spatial separation of biochemical processes & thus their sequential operation within a cell (e.g. protein modification in RER and further protein modification, sorting and packaging in the GA) ; 3. accumulation of ions to high concentrations (e.g.accumulation of a high concentration of H+ in the intermembrane space of the mitochondria enable a proton gradient to be established for chemiosmosis); 4. Membranes act as a surface for chemical reactions to occur in a sequential manner membranes may have functionally -related proteins grouped together so that sequential biochemical processes can occur (e.g. the thylakoid membranes of the chloroplast have electron carriers & ATP synthetase for chemiosmosis to occur); 5. Membranes increase surface area for chemical reactions (e.g. inner mitochondrial membrane is highly folded to hold more electron transport chains and ATP synthetase); (d) Explain the role of glycogen in animal cells.[2] 1. Glycogen is an large energy store found in the liver and muscles; 2. which can be hydrolysed to many glucose molecules that can be used as a respiratory substrate which oxidized during respiration to produce ATP. [Total :12]

Raffles Institution Nov 2015 (H2 Biology) Paper 2 (for 9744 syllabus) 2017 N15P2Q2 (a) Describe how replication of the lagging strand template occurs. [2] 1. The lagging strand is synthesised discontinuously in fragments known as Okazaki fragments. Each fragment is initiated by an RNA primer before the addition of DNA nucleotides; 2. A different DNA polymerase then excises the RNA primer and replaces it with deoxyribonucleotides and DNA ligase seals the nicks by forming phosphodiester bonds between adjacent nucleotides of the each of the DNA fragments on the new strand; Comments: Do note that in Fig. 2.1: DNA polymerase* works only in the 5’ to 3’ direction. So the DNA polymerases extending new strands in opposite directions with respect to the replication fork. i.e. The leading strand is being synthesized towards the replication fork while the lagging strand is being synthesized away from the replication fork due to the anti parallel nature of the 2 template strands. (b) State 2 ways in which DNA replication, (i) differs from transcription, DNA replication Transcription 1. Product Double-stranded DNA Single-stranded mRNA 2. Enzymes DNA polymerase links nucleoides RNA polymerase links nucleotides 3. Primer requirement RNA primer is required to initiate DNA replication RNA primer is NOT required to initiate DNA replication (ii) is similar to transcription. 1. Both use DNA as a template to synthesise the complementary strand; 2. Both processes occur in the nucleus; 3. Nucleotides in the nucleic acid that is synthesized are linked by phosphodiester bonds;

Raffles Institution Nov 2015 (H2 Biology) Paper 2 (for 9744 syllabus) 2017 (c) The symbols below represent the main components of RNA. In the space below, draw a short section of mRNA that is made up of two nucleotides, using these symbols to represent the main components. Add lines to show the positions of any bonds between the components. Label and name the components and the covalent bond that links the nucleotides.[3] 1. Phosphate group linked to carbon number 5 of ribose sugar and nitrogenous base linked to carbon number 1 of ribose sugar; 2. Correct labels for phosphate group, ribose sugar and nitrogenous base; 3. Phosphodiester bond correctly identified and labelled [Total : 10] N15P2Q3 (a) (i) Name structures S and T, as shown in Fig. 3.1. [2] 1. S: promoter*; 2. T: operator*; (ii) Identify a structural gene in Fig. 3.1 and explain what is meant by the term, structural gene. [1] 1. lacZ / lacy / lacA gene + Structural gene is any gene that codes for a protein product that has an enzymatic function in a metabolic pathway; (iii) Identify a regulatory gene in Fig. 3.1 and explain what is meant by the term, regulatory gene. [1] 1. lacI gene + Regulatory gene codes for a protein (e.g. repressor) involved regulating expression of structural genes; Phosphate group Phosphodiester bond Ribose sugar group Nitrogenous base

Raffles Institution Nov 2015 (H2 Biology) Paper 2 (for 9744 syllabus) 2017 (b) Using Fig. 3.1, describe how the presence of lactose induces a bacterium to use lactose as a respiratory substrate. [3] 1. Lactose is converted to its isomer allolactose which acts as an inducer to bind to the active repressor protein at its allosteric site; 2. This makes the repressor inactive as it alters the conformation of the DNA- binding site of the repressor and which then can no longer bind to the operator; 3. RNA polymerase is now free to bind to the promoter and can move downstream to transcribe* the structural genes to form -galactosidase, permease and transacetylase for metabolism of lactose; [Total: 7] N15P2Q4 Fig. 4.1 shows the development of a metastatic cancer in the colon over a period of ten or more years. Metastatic is a term used to describe cancer that is spreading from one organ to another. APC, ras and p53 are tumour suppressor genes. (error: ras is proto-oncogene!!)*** (a) State two environmental causes of cancer. [2] 1. ultraviolet light / radioactivity / ionizing radiations (mention of radiation alone not sufficient as visible light also emits radiation); 2. Carcinogens such as tar in cigarette smoke, asbestos, benzene, formaldehyde, ethidium bromide etc. (give named example); (b) With reference to Fig. 4.1, explain why cancer development is a multi-step process. [3] 1. The development of cancer requires the accumulation of mutations in the genes in a single cell; [1] Idea of accumulation of different mutations using Fig. 4.1 (1mark for any two points 2,3,4) 2. Loss-of- function mutation in 2 copies/alleles of APC tumour suppressor gene results in dysregulation of cell cycle to have excessive cell division; 3. Gain-in-function mutation in just one copy/allele