ASRJC 2023 JC2 H2 Biology Prelims Paper 3 (Ans)

ASRJC BIOLOGY DEPT 9744//2023/J2PRELIM/P3 [Turn over H2 ANDERSON SERANGOON JUNIOR COLLEGE HIGHER 2 ANSWERS 2023 JC2 PRELIMINARY EXAMINATION CANDIDATE NAME CLASS INDEX NUMBER BIOLOGY 9744/03 PAPER 3 LONG STRUCTURED AND FREE RESPONSE QUESTIONS Candidates answer on the Question Paper. No Additional Materials are required. 15 SEPTEMBER 2023 FRIDAY 2 HOURS READ THESE INSTRUCTIONS FIRST Write your name and class on all the work you hand in. Write in dark blue or black pen. You may use an HB pencil for any diagrams or graph. Do not use paper clips, highlighters, glue or correction fluid. Section A Answer all questions in the spaces provided on the Question Paper. Section B Answer any one question in the spaces provided on the Question Paper. The use of an approved scientific calculator is expected, where appropriate. You may lose marks if you do not show your working or if you do not use appropriate units. At the end of the examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. This document consists of 19 printed pages and 1 blank page For Examiner’s Use 1 / 30 2 / 10 3 / 10 4 /5 / 25 Total / 75

ASRJC BIOLOGY DEPT 9744/2023/J2PRELIM/P3 Section A Answer all the questions in this section. 1 Proteins must fold into defined three-dimensional structures to gain functional activity. In the cellular environment, newly synthes ised polypeptides are at great risk of misfolding and aggregation. Cells hence engage proteins called chaperones to assist in protein folding. These chaperones have two roles: 1. They bind to proteins to promote folding. 2. They direct misfolded polypeptides for degradation in the cytosol. However, polypeptides that are in the midst of folding may be mistaken by chaperones as misfolded proteins and then directed fo r degradation. Therefore, protein folding needs to be completed quickly to prevent premature degradation. A recently discovered endoplasmic reticulum (ER) protein complex called S-E complex was found to delay premature degradation of polypeptides that are in the midst of folding. In its absence, approximately 30% of newly synthesised proteins that could otherwise fold correctly are degraded. Fig. 1.1 illustrates these processes. Fig. 1.1

3 ASRJC BIOLOGY DEPT 9744/2023/J2PRELIM/P3 [Turn over With reference to Fig. 1.1, (a) (i) suggest how the S-E complex allows polypeptides to complete their folding. 1. S-E complex (is a membrane-anchored protein that) binds to chaperone- polypeptide 2. Prevents polypeptide from being premat urely transported out of the ER for degradation OR Allows polypeptide to stay longer in the ER to complete folding [2] (ii) ER vesicle formation is not a random event but is carefully co-ordinated. Describe how ER vesicle formation is triggered. 1. Folded protein binds to the domain of COP-II protein facing the ER lumen 2. Such binding triggers ER membrane to undergo budding / pinching off 3. fusion of the two approaching ends of phospholipid bilayer forms a vesicle [Any 2] [2] (iii) explain how unfolded polypeptides in the ER are degraded. 1. Polypeptide transported through a transport protein embedded in the RER membrane 2. Attached to ubiquitin proteins (when in cytosol) 3. Targeted to the proteasome 4. For hydrolysis into individual amino acids / shorter peptides [Any 3] [3] Misfolded proteins in the ER tend to spontaneously associate with one another to form an aggregate, causing cellular toxicity. The misfolded proteins contained many amino acids in Fig. 1.2. Hence it is important that any misfolded protein is immediately degraded and does not remain in the ER for too long. The presence of the amino acids in Fig. 1.2 in the primary sequence contributes to the misfolded proteins forming an aggregate with one another. Fig. 1.2

ASRJC BIOLOGY DEPT 9744/2023/J2PRELIM/P3 (b) (i) Explain how the amino acids in Fig. 1.2 contribute to the misfolded proteins forming an aggregate with one another. 1. Misfolded proteins have their hydrophobic amino acid R groups/side chains exposed 2. The need to shield themselves away from the aqueous ER lumen 3. Hence, hydrophobic R groups from di fferent misfolded protein associate with one another via hydrophobic interaction [Any 2] [2] (ii) Suggest how accumulation of protein aggregates in the ER can affect the function of the ER. 1. Such aggregate may interact with enzymes / membrane proteins to interfere with their function 2. constrains the ER lumen space av ailable to metabolic enzymes / other proteins [1] Accumulation of protein aggregates can also cause mitochondrial diseases (MD). Mitochondria are found in all nucleated eukaryotic cells and are the principal generators of cellular ATP. The mito chondrial circular DNA genome comprises 37 genes, which code for 13 essential polypeptides for oxidative phosphorylation and the necessary RNA machinery for their translation within the mitochondria. (c) (i) It was hypothesised that mitochondria arose when an early ancestor of the eukaryotic cell engulfed an oxygen-usi ng, non-photosynthetic prokaryotic cell. Explain two pieces of evidence for this hypothesis.  Mitochondria is bound by a double membrane where the inner membrane is the original membrane of the prok aryotes while the outer one is the membrane of the phagocytic vesicle.  Mitochondria contains 70S ribosomes and circular DNA (not enclosed in nucleus).  Mitochondria is autonomous organelles that grow and reproduce within the cell. [2]

5 ASRJC BIOLOGY DEPT 9744/2023/J2PRELIM/P3 [Turn over One form of MD is caused by a mutation of a mitochondrial gene that codes for a tRNA.  The mutation involves substitution of guanine for adenine in the DNA base sequence.  This changes the anticodon on the am inoacyl-tRNA carrying leucine (tRNAleu).  This mutant tRNA leu also recognises the phenylalanine codon, resulting in the formation of a non-functional protein in the mitochondrion. (ii) Suggest how the change in the anticodon of a tRNA leads to mitochondrial diseases.  Change in the anticodon of the tRNA results in the incorporation of leucine instead of phenylalanine into the polypeptide chain during translation  .. the different R-group of amino acid results in different folding of the polypeptide chain, hence, change in the 3D conformation of the tertiary structure  Change in the protein/enzyme required for oxidative phosphorylation , hence, less/ no ATP synthesised. [3] While some MDs are caused by mutations of mitochondrial genes inside the mitochondria, most MDs are caused by mutations of genes in the cell nucleus that are involved in the functioning of mitochondria. MDs caused by nuclear DNA mutations are autosomal recessive. All of a person’s mitochondria are inherited from their mother via the egg cell. Two couples, couple A and co