2024 ACJC H2 Bio P3 MS

This document consists of 21 printed pages. [Turn over Anglo-Chinese Junior College JC2 Biology Preliminary Examination Higher 2 CANDIDATE NAME FORM CLASS TUTORIAL CLASS I N D E X NUMBER BIOLOGY Paper 3 Long Structured and Free-response Questions Additional Materials: Free-response Question Answer Booklet 9744/03 28 August 2024 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 q u e s t i o n i n t h e s p a c e s p r o v i d e d o n t h e s e p a r a t e F r e e - r e s p o n s e 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 / 10 3 / 10 4 / 5 / 25 Total / 7 5

2 Anglo-Chinese Junior College 2024 J2 H2 9744 Paper 3 Preliminary Examination For Examiner Use Section A Answer all the questions in this section. 1 Lactose is the main carbohydrate found in mammalian milk and is i m p o r t a n t f o r m e e t i n g t h e nutritional needs of an infant. Lactose is a disaccharide of glucose and galactose, and has been considered healthier than other disaccharides such as sucrose and maltose. Table 1.1 shows the glycaemic index of lactose, sucrose, maltose and glucose. The glycaemic index measures the rise in blood glucose levels two hours after i n g e s t i o n o f t h e m e a s u r e d substance. Table 1.1 carbohydrate glycaemic index lactose 46 sucrose 65 maltose 105 glucose 100 (a) Suggest explanations for the following two observations: ( i ) the glycaemic index of lactose is lower than glucose Time is needed for the hydrolysis of lactose after it is ingested; …………………………………………………………………………………………............ [1] ( i i ) the glycaemic index of lactose is lower than sucrose. 1. There are fewer types of enzymes / lower concentration of enzymes in the gastrointestinal tract able to hydrolyse lactose compared to sucrose; 2. More time / metabolic steps are needed to convert galactose to glucose compared to fructose to glucose; OR More time / metabolic steps are needed to hydrolyse lactose compared to sucrose; …………………………………………………………………………………………............ [2]

3 [Turn over Anglo-Chinese Junior College 2024 J2 H2 9744 Paper 3 Preliminary Examination For Examiner Use Lactose intolerance is a condition which occurs due to the lack of the enzyme lactase in the small intestines. In humans, lactase is encoded by the LCT gene on chromosome 2. In most of the human population, lactose intolerance occurs as the expression of the LCT gene declines past the infant stage due to a reduced reliance on milk. Adults with lactose intolerance will present with gastrointestinal symptoms when milk is consumed, and the condition worsens with age. It is hypothesised that methylation of the promoter of the LCT gene is involved in the age-related decrease in gene expression. ( b ) Explain how methylation of the promoter may lead to a decrease in LCT gene expression. 1. Methyl groups may be added to cytosine / CpG sites in the promoter; 2. Catalysed by the enzyme DNA methyltransferase; 3. Methylation changes the 3D conformation of DNA at the promoter; 4. Methylated DNA also attracts other proteins (methyl-CpG-binding proteins) which recruit histone deacetylase enzymes that remove acetyl groups from histones, increasing the affinity of positively-charged histone tails for the negatively-charged DNA and making the chromatin more compact; 5. Transcription is inhibited by preventing the binding of the transcription factors and RNA polymerase to form the transcription initiation complex; Max 3m ……………………………………………………………………………………………………….. [3]

4 Anglo-Chinese Junior College 2024 J2 H2 9744 Paper 3 Preliminary Examination For Examiner Use The extent of methylation of the LCT gene was analysed in 48 individuals between the ages of 8 to 23 and of a largely European descent. Fig. 1.1 records the percentage methylation of the LCT gene and the lactase activity measured in each individual. Fig. 1.1 (c) (i) Draw a line of best fit on Fig. 1.1. [1] Straight or curved line + extending from 74% to 95% on x-axis, without extrapolation beyond this range + good distribution of number of data points above and below the best fit line (A! between 20 to 40 au at starting point, and between 0 to 8 au at ending point); ( i i ) Discuss whether the results in Fig. 1.1 support the hypothesis that methylation is involved in the age-related decline in LCT gene expression. Yes 1. As percentage methylation of the LCT gene increases, there i s a general decreasing trend in the lactase activity; 2. Quote data based on best fit line drawn e.g. when percentage m e t h y l a t i o n increases from 74 to 95, lactase activity decreases from 39au to 0au; No 3. Data does not show whether increased percentage methylation is correlated with age; 4. Youngest individual is 8 years old, which is long past the infant stage (as the expression of the gene declines past the infant stage); 5. Data taken from individuals of largely European descent, hence may not be representative of other human populations/is biased; 6. Individual with low percentage methylation may still have low lactase activity; 7. Quote data point to support MP6: one individual with low per centage methylation of 74% has a relatively low lactase activity of 18au (A! 17 to 19au); At least 1 MP for both ‘supporting’ and ‘not supporting’ for full marks, max 4m …………………………………………………………………………………………............ [4] 0 10 20 30 40 50 60 70 80 70 75 80 85 90 95 100 lactase activity / arbitrary units percentage methylation

5 [Turn over Anglo-Chinese Junior College 2024 J2 H2 9744 Paper 3 Preliminary Examination For Examiner Use In some individuals, LCT gene expression continues throughout adulthood, allowing the digestion of lactose in milk. These adult individuals are said to be lactase persistent and can tolerate the consumption of milk. Genomic studies have identified the MCM6 gene to be involved in the regulation of the expression of the LCT gene. Scientists made these observations of the two genes, represented in Fig. 1.2:  MCM6 gene is located upstream of the LCT gene on the same chromosome  MCM6 gene contains 15 introns  m u t a t i o n s i n s p e c i f i c D N A s e q u e n c e s w i t h i n t w o o f t h e i n t r o n s have been found in individuals who are lactase persistent  r e g u l a t o r y p r o t e i n s c a n b i n d t o t h e s e s p e c i f i c D N A s e q u e n c e s that have undergone mutations  t h e s e r e g u l a t o r y p r o t e i n s c o n trol the transcription of the LCT gene. Fig. 1.2 (d) (i) Suggest how mutations to specific DNA sequences in the introns of the MCM6 gene may result in the lactase persistent condition. 1. *Mutation results in specific DNA sequences in introns acting as enhancers whic