ASRJC 2023 JC2 H2 Biology Prelims Paper 3 (QP)

ASRJC BIOLOGY DEPT 9744/2023/J2PRELIM/P3 [Turn over H2 ANDERSON SERANGOON JUNIOR COLLEGE HIGHER 2 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 25 printed pages and 3 blank pages. For Examiner’s Use 1 2 3 4 or 5 Total / 75

2 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 synthesised polypep tides 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 for degradati on. Therefore, protein folding needs to be completed quickly to prevent premature degradation. A recently discovered endoplasmic reticulum (E R) 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. ……………………………………………… …………………………………………… ……………………………………………… …………………………………………… ……………………………………………… …………………………………………… ……………………………………………… …………………………………………… ……………………………………………………………………………….………... [2] (ii) ER vesicle formation is not a random event but is carefully co-ordinated. Describe how ER vesicle formation is triggered. ……………………………………………… …………………………………………… ……………………………………………… …………………………………………… ……………………………………………… …………………………………………… ……………………………………………… …………………………………………… ………………………………………………………………………………………... [2] (iii) explain how unfolded polypeptides in the ER are degraded. ……………………………………………… …………………………………………… ……………………………………………… …………………………………………… ……………………………………………… …………………………………………… ……………………………………………… …………………………………………… ……………………………………………… …………………………………………… ……………………………………………… …………………………………………… ………………………………………………………………………………………... [3]

4 ASRJC BIOLOGY DEPT 9744/2023/J2PRELIM/P3 Misfolded proteins in the ER tend to spontaneously associate with one another to form an aggregate, causing cellular toxicity. 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 (b) (i) Explain how the amino acids in Fig. 1.2 contribute to the misfolded proteins forming an aggregate with one another. ……………………………………………… …………………………………………… ……………………………………………… …………………………………………… ……………………………………………… …………………………………………… ……………………………………………… ………………………………………... [2] (ii) Suggest how accumulation of protein aggregates in the ER can affect the function of the ER. ……………………………………………… …………………………………………… ………………………………………………………………………………………... [1]

5 ASRJC BIOLOGY DEPT 9744/2023/J2PRELIM/P3 [Turn over Accumulation of protein aggregates can also cause mitochondrial diseases (MD). Mitochondria are found in all nucleated eukaryoti c cells and are the principal generators of cellular ATP. The mitochondrial circular DNA genome comprises 37 genes, which code for 13 polypeptides for oxidative phosphorylation an d 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-using, non-photosynthetic prokaryotic cell. Explain two pieces of evidence for this hypothesis. ……………………………………………… …………………………………………… ……………………………………………… …………………………………………… ……………………………………………… …………………………………………… ……………………………………………… …………………………………………… ………………………………………………………………………………………… [2] 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 (tRNA leu).  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. ……………………………………………… …………………………………………….. ……………………………………………… …………………………………………….. ……………………………………………… …………………………………………….. ……………………………………………… …………………………………………….. ……………………………………………… …………………………………………….. ……………………………………………… …………………………………….……. [3]

6 ASRJC BIOLOGY DEPT 9744/2023/J2PRELIM/P3 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 couple B, had one or more children affected by a MD. The type of MD was different for each couple. None of the parents showed signs or symptoms of MD.  Couple A had four children who were all affected by a MD.  Couple B had four children and only one was affected by a MD. (iii) Using the information provi ded, suggest why all of couple A’s children had a MD and only one of couple B’s children had a MD. couple A ……………………………………………… …………………………………………….. ……………………………………………… …………………………………………….. …………………………………………………………………………………….…… [2] couple B ……………………………………………