2025 TMJC H2Bio PE P3 (A)

1 Tampines Meridian Junior College 2025 JC2 Preliminary Examinations H2 Biology CANDIDATE NAME ( ) CIVICS GROUP H2 BIOLOGY 9744/03 Paper 3 Long Structured and Free-response Questions 22 September 2025 2 hours Candidates answer on the Question Paper. No additional materials are required. _________________________________________________________________________ This document consists of 27 printed pages and 3 blank pages. TAMPINES MERIDIAN JUNIOR COLLEGE JC2 PRELIMINARY EXAMINATION SUGGESTED ANSWERS

2 Tampines Meridian Junior College 2025 JC2 Preliminary Examinations H2 Biology Section A Answer all questions. 1 Adult stem cells are undifferentiated cells that are found in most animal tissues. Adult stem cells can divide throughout their lifespan to form identical stem cells (self-renewal) or to form cells that can differentiate into the functioning cells of that tissue. (a) Uncontrolled cell division is a characteristic feature of tumour formation from a differentiated cell. (i) Describe two other features of tumour formation from a fully differentiated cell. [2] 1. Gain of function m utations of proto -oncogenes to oncogene and loss of function mutations of tumour suppressor genes. 2. Accumulation of these mutations in a single cell. 3. No contact-inhibition, where cells continue to divide even after contact with other cells. 4. Shorter/Faster G1 and G2 phase during cell cycle. 5. Activation of telomerase gene, to code for telomerase such that shortened telomeres are pelongated after each round of replication / shorten less quickly. 6. Normal cell cycle checkpoints are not functioning / proteins needed at the checkpoints not produced. 7. AVP Telomeres are non-coding DNA found at the end of the chromosomes. Adult stem cells have chromosomes with long telomeres. (ii) Explain why having long telomeres is an advantage to cells that are undergoing many rounds of cell cycles. [2] 1. After each round of DNA replication, telomeres shorten due to the end replication problem occurs. 2. Presence of telomeres ensure that no coding sequences/genes are eroded. 3. Long telomeres ensure telomeres do not reach / takes longer time to reach critical length and the cells do not undergo apoptosis.

3 Tampines Meridian Junior College 2025 JC2 Preliminary Examinations H2 Biology (b) Haematopoietic stem cells (HSCs), also called blood stem cells, are adult stem cells that are located in the bone marrow. HSCs have a role in the formation of blood cells. Fig. 1.1 is an outline showing the formation of some of the different types of blood cell that can be formed from HSCs. The first stage is the division of HSCs to produce myeloid and lymphoid stem cells. Fig. 1.1 With reference to Fig 1.1, explain why GMP cells cannot be described as haematopoietic stem cells (HSCs). [2] • Though GMPs can undergo mitosis, they cannot divide indefinitely while HSCs can self- renew indefinitely. • GMP cells are restricted to differentiate into neutrophils or monocytes / has limited differentiation potential, while HSCs can differentiate into all blood cell types. • Differentiation has already started in GMP cells / GMP cells are differentiated while HSCs are undifferentiated. • In GMP cells, (some) genes coding for proteins to carry out specialised functions are already being expressed / genes coding for multipotency are switched off.

4 Tampines Meridian Junior College 2025 JC2 Preliminary Examinations H2 Biology (c) The differentiation of T ‑lymphocytes begins in the bone marrow and continues in an organ known as the thymus to produce fully differentiated T‑helper and T-cytotoxic cells. In the thymus, T‑lymphocytes that bind to self-antigens are destroyed. (i) Suggest why T ‑lymphocytes that bind to self -antigens need to be destroyed in the thymus. [3] 1. T-lymphocytes that bind to self -antigens need to be destroyed to prevent them from being released into the blood circulation / bloodstream. If self-reactive T cells were released into the bloodstream, 2. these self -reactive T -cells will then release cytokines to trigger immune response against own body’s cells. 3. This would result in destruction/killing of own cells (by cytotoxic T-cells)…. 4. …. potentially causing autoimmune diseases. 5. Furthermore, if these T cells were not eliminated, they could lead to the formation of memory T cells that recognises self-antigens that would perpetuate an autoimmune response over time. 6. Will not activate self-reacting B cells (and T-cytotoxic cells).

5 Tampines Meridian Junior College 2025 JC2 Preliminary Examinations H2 Biology Mature T-lymphocytes leave the thymus gland to travel throughout the body. They remain inactive inside organs, such as the spleen and lymph nodes, until activated by the presence of foreign antigens. Fig. 1.2 shows what happens to two mature T-lymphocytes, U1 and V1, in the presence of an antigen from a virus. Fig. 1.2

6 Tampines Meridian Junior College 2025 JC2 Preliminary Examinations H2 Biology (ii) U4 and V4 are two types of mature T-lymphocyte. Identify the names given to these types of T-lymphocytes by putting a (✓) beside the correct box. [1] Types of mature T- lymphocytes U4 V4 T-cytotoxic Cells ✓ T-helper cells ✓ (iii) Describe the roles of cells V4 in an adaptive immune response. [2] 1. T- cytotoxic cells recognise and bind to infected cells foreign antigens presented on class I MHC. 2. Once bound, T-cytotoxic cells release perforin and granzymes. 3. Perforins form pores in the infected cell surface membrane and disrupts membrane integrity / form pores to allow granzyme to enter the cells. 4. Granzymes induce apoptosis (programmed cell death) of the infected cell. [max 1 if T-helper cells is described] (d) T-helper cells play a crucial role in orchestrating the immune response against various pathogens. However, they are also the primary target of the human immunodeficiency virus (HIV), a retrovirus, which contains the enzyme reverse transcriptase. HIV infects and destroys T-helper cells, gradually reducing their numbers. This weakens the immune system, making individuals susceptible to opportunistic infections and cancers. Explain why HIV is described as a retrovirus. [2] 1. HIV carries single-stranded (+) RNA as its genetic material/genome. [Reject: HIV has RNA] 2. During its reproductive cycle, HIV uses its single stranded RNA as a template and is reverse transcribed to form single stranded DNA , then into double stranded DNA , by reverse transcriptase. 3. Double stranded DNA then integrates into the host’s DNA/chromosome as a provirus.

7 Tampines Meridian Junior College 2025 JC2 Preliminary Examinations H2 Biology (e) In people with HIV/AIDs, a serious lung disease known as pneumocystis pneumonia can result from infection by an opportunistic pathogen known as Pneumocystis jirovecii. Although the cells of many species of bacteria are the same size as those of P. jirovecii, research concluded that the organism is a eukaryote and is not a bacterium. In 1988, analysis of ribosomal RNA (rRNA) resulted in P . jirovecii being classified as a fungus. Studies o n the structure of P. jirovecii have identified that the cell wall is made of polysaccharides such as chitin