TJC 2023 JC1 H2 EOY ANS

1 JC1 2023 H2 PROMOTIONAL EXAMINATION ANSWERS SECTION A: MCQ 1 C 6 A 11 A 16 B 2 A 7 A 12 D 17 B 3 D 8 C 13 D 18 C 4 B 9 B 14 B 19 A 5 C 10 A 15 D 20 B SECTION B: STRUCTURED QUESTIONS 1 (a) Fig. 1.1 shows the behaviour of eight chromosomes in one stage of meiosis of a diploid plant cell. Fig. 1.1 Additional info. A pair of homologous chromosomes with multiple chiasmata. (i) Name the stage of the nuclear division. [1] Prophase I (ii) Based on the observation of the chromosomes in Fig. 1.1, explain why it cannot be a stage in mitosis. [1] During prophase of mitosis, homologous chromosomes do not pair up / no chiasmata formation / no crossing over . (Note the phrasing of the answer is with respect to mitosis.) Accept opposite: There is pairing of homologous chromosomes / chiasmata formation / crossing over between homologous chromosomes which only happen during prophase I of meiosis. (Note the phrasing of the answer to show that it cannot be a stage of mitosis.) Two chiasmata formed between homologous chromosomes. Centromere c.

2 Feedback / comment 1. Candidates are reminded to address the question fully instead of just describing the feature of prophase I. (See above answer) 2. [Phrasing error]: “There are pairs of homologous chromosomes.” [Conceptual error]: “There are no homologous chromosomes during mitosis”. Both answers are rejected as there are homologous chromosomes in cell s undergoing mitosis, but they do not pair up. (iii) The diploid number of the flowering plant is 8. Complete Table 1.1 to show the number of chromosomes found in the cell at the end of each stage of the meiotic cell cycle. Table 1.1 [2] Feedback / comment 1. Candidates generally found this question challenging and did not get full credit. However, some candidates sketched the chromosomes in empty spaces to help visualize the number and could get full credit. stage no. of chromosomes per cell G1 8 S phase After DNA replication, 2 DNA molecules (as 2 sister chromatids are joined together at centromere) → each is 1 chromosome 8 Anaphase I Although homologous chromosome separates, cytokinesis has not taken place yet. 8 Telophase I Although homologous chromosome reach opp. poles of the cell, cytokinesis has not taken place yet. 8 Cytokinesis I 4 Cytokinesis II Chromatids separated and reach opp. poles of the cell + cell has divided into 2 4

3 There are 2 chromosomes in Fig. 20, Cell cycle notes page 33. For STQ 1a(iii) No. of chromosomes per cell = 8 (given) No. of chromosomes per cell = 8 No. of chromosomes per cell = 8 No. of chromosomes per cell = 8 No. of chromosomes per cell = 4 (given) No. of chromosomes per cell = 4 No. of chromosomes per