RI Nov 13 H2P2 Ans

Raffles Institution Nov 2013 (H2 Biology) Paper 2 (for 9744 syllabus) 2017 Nov 2013 H2 Bio Paper 2 N13P2Q1 (a) Describe the processes that occur in the Golgi body. [3] 1. Glycosylation* where oligosaccharides/short sugar chains are added to proteins and lipids to form glycoproteins and glycolipids respectively; 2. Modification of existing glycoproteins and glycolipids made in the endoplasmic reticulum by modifying the existing oligosaccharide / cleaving sugar molecule(s) from the oligosaccharide; 3. In plant cells, Golgi body is the site for synthesis of polysaccharides such as pectin (a component in plant cell wall) and then transported in vesicles to the cell membrane; 4. Sorting* of proteins into different kinds of vesicles to target the proteins to different parts of the cell or for secretion; (b) Describe the role of vesicles that fuse with the forming face of the Golgi body. [3] 1. Vesicles that bud off from rough endoplasmic reticulum (RER) carry proteins; 2. and bud off from smooth endoplasmic reticulum (SER) carry lipids; 3. These vesicles are targeted to fuse to cis-face of Golgi by microtubules of the cytoskeleton/proteins; (c) Outline two roles for the vesicles that are formed at the maturing face of the Golgi body. [4] 1. Lysosome / lysosomal vesicles; 2. fuses with vesicles/vacuoles formed by endocytosis to digest the contents (e.g. foreign particles) within the vesicles/vacuoles; (pt 1 & 2 = 2 marks) 3. Exocytic vesicles; 4. fuses with plasma membrane to release enzymes outside of cell by exocytosis to breakdown extracellular content / fuses with plasma membrane to release contents outside of cell by exocytosis; (pt 3 & 4 = 2 marks) 5. Secretory vesicles; 6. fuses with plasma membrane to release contents upon receiving signal; (pt 5 & 6 = 2 marks) Any 2 pairs of points N13P2Q2 (a) State what happens if a cell loses control of the cell cycle. [1] 1. Uncontrolled cell proliferation; (b) With reference to Fig. 2.1, suggest how the dysregulation of checkpoints of cell division may occur. [3] 1. Point mutation of S-cyclin gene to cause change in amino acid sequence of S- cyclin protein, thus S-cyclin is resistant to degradation / Point mutation of M-cyclin gene to cause change in amino acid sequence of M-cyclin protein, thus M-cyclin is resistant to degradation; 2. Prolonged activation of M-CDK and S-CDK to cause more rounds of cell division thus excessive cell proliferation; 3. Gene amplification of CDK gene to increase number of copies of gene resulting in increase amount of CDK protein; 4. Increase number of binding sites for S-cyclin/M-cyclin to form increase amount of active CDK;

Raffles Institution Nov 2013 (H2 Biology) Paper 2 (for 9744 syllabus) 2017 (c) (i) Name one causative agent of cancer. [1] 1. Viruses e.g. HPV, avian sarcoma virus; 2. Ultraviolet light / Ionising radiation (mention of radiation along is insufficient); 3. Carcinogens e.g. tar in cigarette smoke, asbestos, benzene, formaldehyde, ethidium bromide; Any one (ii) Outline the development of cancer including the effects of this causative agent. [5] 1. The causative agent increase chances of DNA damage and mutations in the genes which control regulatory checkpoints of the cell cycle in a single cell; 2. Loss-of-function mutation of tumor suppressor genes will result in inability to inhibit cell cycle, repair damaged DNA and promote apoptosis; 3. Gain-in-function mutation of proto -oncogenes to form oncogenes will result in overexpression of proteins/growth factors OR production of hyperactive/degradation resistant proteins/growth factors; 4. leading to excessive cell proliferation/division to form tumour; 5. Loss of contact inhibition enables cells to grow into a tumour; 6. Activation of genes coding for telomerase so that cells can divide indefinitely; 7. Angiogenesis must occur within the tumour so that the blood vessels formed can transport oxygen and nutrients for its growth; 8. Resulting in the formation of a maglinant tumour capable of metastasizing to other parts of body to form secondary tumours; N13P2Q3 Fig.3.1 shows the mechanism of DNA replication originally proposed by Watson and Crick. (a) State the name given to this mechanism of DNA replication. [1] Semi-conservative DNA replication* (b) Describe how these results provided evidence for Watson and Crick’s proposed mechanism. [3] 1. In generation 0 only the heavy 15N-15N DNA molecules were present which appeared as the lowest band in the caesium chloride solution. 2. During semi -conservative replication, the original 15N-15N DNA strands unzipped* and served as templates* for the formation of the new strands . Since only 14N DNA was present in the medium, t he resulting DNA molecules in generation 1 were hybrid* DNA molecules where each molecule was made up of one original 15N strand and one new 14N strand . This hybrid DNA molecule thus appeared as the intermediate band in the caesium chloride solution. 3. Each hybrid DNA molecule from generation 1 unzipped and were used as templates for DNA replication . Hence 50% of the DNA in generation 2 was made up of hybrid DNA (i.e. 15N-14N) which appeared as the intermediate band and 50% was made up of light DNA (i.e. 14N-14N) which appeared as the uppermost band in the caesium chloride solution. Thus the appearance of t he various bands at the various generations is consistent with semi-conservative replication.

Raffles Institution Nov 2013 (H2 Biology) Paper 2 (for 9744 syllabus) 2017 (c) List three ways in which transcription is different from DNA replication.[3] Feature DNA replication Transcription Template In replication, both DNA strands serve as template. In transcription, only one strand acts as template. Product 2 double-stranded DNA molecules are synthesised. 1 single stranded RNA molecule is synthesised. Polymerase involved DNA polymerase* catalyses the reaction. DNA-dependent RNA polymerase* catalyses the reaction. Process associated with The process is associated with cell and nuclear division. The process is associated with protein synthesis. Base that pairs with Adenine Thymine is used during the process. Uracil is used during the process. Process Both DNA strands are used as template in the synthesis of DNA. One DNA strand is used as template for the synthesis of mRNA. Nucleotides used Deoxyribonucleotides and ribonucleotides (for RNA primer) are used Ribonucleotides are used. Comments: The main differences between transcription and replication were well known to most candidates and many provided full responses. (d) Explain how the information to synthesise polypeptides is coded for by DNA.[3] 1. A specific sequence of nucleotides on a template DNA strand is transcribed by RNA polymerase to form mRNA by complementary base pairing*; 2. The mRNA contains triplet base codes known as codons* that each codes for a specific amino acid; 3. When the mRNA containing a specific sequence of codons which code for a specific amino acid sequence,is translated at ribosomes, polypeptides are formed. N13P2Q4 Fig.4.1 shows an outline of the first three stages of aerobic respiration (a) Name the stages A, B and C and state precisely where in a eukaryotic cell these stages occur.[3] A: Glycolysis occurs in the cytosol B: Link reaction occurs in the matrix of the mitochondrion C: Krebs cycle occurs in the matrix of the mitochondrion Comments: Candidates demonstrated sound knowledge and understanding of