2026 ACJC H2 Biology Prelim P3 MS

This document consists of 18 printed pages. [Turn over Anglo-Chinese Junior College JC2 Biology Preliminary Examination Higher 2 CANDIDATE NAME FORM CLASS TUTORIAL CLASS INDEX NUMBER BIOLOGY Paper 3 Long Structured and Free-response Questions Additional Materials: Free-response Question Answer Booklet 9744/03 02 September 2026 2 hours READ THESE INSTRUCTIONS FIRST Write your Name, Class and Index number in the spaces at the top of this page. Write in dark blue or black pen. You may use an HB pencil for any diagrams or graphs. Do not use staples, paper clips, 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 separate Free-response Question Answer Booklet. 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. The number of marks is given in brackets [ ] at the end of each question or part question. For Examiners’ use only 1 / 30 2 / 8 3 / 12 4 / 5 / 25 Total / 75

2 Anglo-Chinese Junior College 2026 J2 H2 9477 Paper 3 Preliminary Examination For Examiner Use Section A 1 Alzheimer’s disease (AD) is a progressive neurodegenerative disorder characteri sed by cognitive decline and memory loss. A major hallmark of AD is the accumulation of amyloid-beta (Aβ) plaques in the brain. Aβ peptides are produced when the Amyloid Precursor Protein (APP) is cleaved abnormally by an enzyme complex called γ-secretase. Presenilin-1, encoded by the PSEN1 gene, forms the catalytic subunit of γ-secretase. Mutations in the PSEN1 gene are the most common cause of early-onset Familial Alzheimer’s Disease (FAD). To date, more than 300 mutations have been described in the PSEN1 gene. (a) Fig. 1.1 shows a segment of the non-template DNA strand for both the wild-type PSEN1 allele and a patient's mutant allele. This segment codes for an essential hydrophobic transmembrane domain in the protein. Triplet base : 103 104 105 106 107 108 Wild-type allele : 5' - GGT ATC GTG ATT GTC ACC - 3' Mutant allele : 5' - GGT ATC GTG ATT GAC ACC - 3' Fig. 1.1 Table 1.1 shows a section of the genetic code. Table 1.1 mRNA codon amino acid GGU glycine GAC aspartic acid GUC, GUG valine AUC, AUU isoleucine ACC threonine (i) Identify the type of mutation that has occurred at the DNA level in the patient's PSEN1 gene and determine its effect on the resulting amino acid sequence. 1. Substitution; A! T is substituted by A at codon 107 2. Valine is substituted by aspartic acid; …………………………………………………………………………………………............ [2]

3 [Turn over Anglo-Chinese Junior College 2026 J2 H2 9744 Paper 3 Preliminary Examination For Examiner Use (ii) Suggest how this change in the primary structure of the Presenilin-1 protein could disrupt its function within the hydrophobic core of the phospholipid bilayer. 1. Valine has a non- polar / hydrophobic R- group while aspartic acid has a charged / hydrophilic R-group (R! polar); 2. The substitution disrupts the hydrophobic interactions (R! bonds) between R groups and phospholipids (A! the hydrophobic transmembrane domain is not formed), 3. [compulsory] no longer able to be embedded in the membrane; R! alter 3D conformation, hence non-functional (this generic answer does not use the context provided in the question) No ECF for students who wrote “aspartic acid is substituted by valine” in part (i) …………………………………………………………………………………………............ [2] (b) Fig. 1.2 shows a pedigree of a family affected by a rare, aggressive form of FAD caused by a missense mutation in the PSEN1 gene. Fig. 1.2

4 Anglo-Chinese Junior College 2026 J2 H2 9477 Paper 3 Preliminary Examination For Examiner Use (i) Deduce the likely mode of inheritance of this form of FAD. Justify your answer. 1. Autosomal dominant; R! Allele causing FAD is a dominant allele 2. (FAD is caused by dominant allele because) it did not skip any generations; R! when only 1 parent is affected, majority of the offspring is affected 3. (FAD cannot be X -linked dominant because) II-6 is unaffected and hence homozygous recessive and would not have an affected son, III-7. OR III-9 is affected, yet one of his daughters is unaffected. OR AD affected both males (7) and females (6); …………………………………………………………………………………………............ [3] To confirm the genetic status of the family members, gel electrophoresis coupled with Southern blotting was performed. The mutation in the PSEN1 gene eliminates a specific restriction site for the restriction enzyme, EcoRI. • The wild-type PSEN1 allele yields two fragments of 3 kb and 2 kb when digested with EcoRI. • The mutant allele yields a single fragment of 5 kb. Fig. 1.3 shows the positions of the restriction sites on the wild- type and the mutant PSEN1 alleles, denoted by arrows, and the position where the radioactive probe binds to the two alleles. Fig. 1.3

5 [Turn over Anglo-Chinese Junior College 2026 J2 H2 9744 Paper 3 Preliminary Examination For Examiner Use Fig. 1.4 shows the Southern blot results for selected family members. Fig. 1.4 (ii) Outline the steps required to perform the southern blotting technique after the DNA fragments have been separated by gel electrophoresis to produce the results shown in Fig. 1.4. 1. Use an alkaline transfer buffer to denature the double-stranded DNA fragments into single strands; 2. Transfer the single-stranded DNA from the gel onto a nitrocellulose membrane via capillary action; 3. Bake the membrane at 80 °C for 2 hours to permanently attach the DNA to the membrane; 4. Incubate the membrane with a single-stranded, radioactively- labelled DNA probe that has a complementary base sequence to the PSEN1 gene region; 5. Wash off unbound probes and expose the membrane to X -ray film to produce dark bands on light background; …………………………………………………………………………………………............ [4]

6 Anglo-Chinese Junior College 2026 J2 H2 9477 Paper 3 Preliminary Examination For Examiner Use (iii) Complete Fig. 1. 4 by drawing the expected DNA band (s) for Individual A, given that Individual A is homozygous dominant for FAD. 1. Correct band at 5 kb; 2. Correct thickness of band; (A! 3 – 4 mm) If bands are wrong (i.e. MP1 is incorrect), MP2 is automatically forfeit. …………………………………………………………………………………………............ [2]

7 [Turn over Anglo-Chinese Junior College 2026 J2 H2 9744 Paper 3 Preliminary Examination For Examiner Use (c) For many years, the "Amyloid Cascade Hypothesis" suggested that the accumulation of Aβ plaques in the brain is the primary driver of neurodegeneration and cognitive decline in AD. To investigate this relationship, clinical researchers monitored a cohort of patients over 15 years using brain scans to track Aβ deposition, and clinical tests to measure cognitive decline. Fig. 1.5 shows both Aβ plaque accumulation in the cerebral cortex and the rate of clinical cognitive decline over time. Year 0 represents the point at which clinical symptoms of cognitive impairment first become detectable. Fig. 1.5 (i) Describe the relationship between Aβ plaque accumulation and the development of cli