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 cell = 8 Interphase (G1, S, G2) No. of chromosomes per cell = 2 Prophase I No. of chromosomes per cell = 2 Metaphase I No. of chromosomes per cell = 2 Anaphase I No. of chromosomes per cell = 2 Cytokinesis I No. of chromosomes per cell = 1 Metaphase II No. of chromosomes per cell = 1 Anaphase II No. of chromosomes per cell = 2 Cytokinesis II No. of chromosomes per cell = 1 No. of chromosomes per cell = 4

4 (iv) With reference to Table 1.1, explain the change in the number of chromosomes per cell by the end of cytokinesis I. [1] 1. During anaphase I, homologous chromosomes are separated to opposite poles. 2. Cytokinesis I resulted in two daughter cells . Each daughter cell has half the number of chromosomes from the original/parent cell / 4 chromosomes / 1 set of chromosomes. Note: “Daughter cell” should be mentioned at least once. Feedback / comment 1. [Incorrect phrasing] “Homologous chromosomes are split into the daughter cells during cytokinesis I.” (incorrect to say “split”) [Correct phrasing] Homologous chromosomes are separated equally into each of the 2 daughter cells during cytokinesis I. Fig. 2.1 shows the change in the amount of DNA per nucleus during a meiotic cell cycle. Fig. 1.2 (b) With reference to Fig. 1.2, account for the changes in amount of DNA per nucleus from the start of the cell cycle to the end of meiosis I. [2] (Answer does not need to include meiosis II.) 1. [QF] The amount of DNA per nucleus in the parent cell doubles/increase from 2 to 4 au from S to G2. 2. as the cell undergone DNA replication during S phase of interphase. 3. By the end of telophase I, there are two daughter nuclei. (R: daughter cells) 4. [QF] The amount of DNA per nucleus is back to its original amount 2au. T2 Time Amount of DNA per nucleus / au G2 T1 S 4 2 1 Interphase Legend T1: Telophase I T2: Telophase II G1

5 Feedback / comment 1. Many candidates were not able to appreciate the difference between end of meiosis I and cytokinesis and so did not differentiate daughter nuclei and daughter cells in their answers. Refer to the definition of meiosis (Cell cycle notes, page 3). At the end of meiosis I, two daughter nuclei are formed from one parental nucleus. At the end of cytokinesis, two daughter cells are formed, each daughter cell containing one daughter nucleus. Interphase Amount of DNA per nucleus = 2 au → 4 au Reason: DNA replication during S phase. T1: Amount of DNA per nucleus = 2 au START: Meiosis I END: Meiosis I Amount of DNA per nucleus = 4 au START: Meiosis II END: Meiosis II T2: Amount of DNA per nucleus = 1 au Note! Amount of DNA per nucleus = 2 au Amount of DNA per nucleus in each cell = 1 au Amount of DNA per nucleus in each cell = 2 au

6 (c) Describe one difference between a pair of homologous chromosomes and sister chromatids of a chromosome. [1] Feature A pair of homologous chromosomes Sister chromatids 1) Genetic similarity Not genetically identical to each other [1/2] Genetically identical to each other before crossing over [1/2] 2) Physical connection / attachment Not connected to each other Connected to each other at the centromere 3) Anaphase I Separate Do not separate 4) Alleles at corresponding gene loci. Same number and type of genes but may have different alleles for the same genes. Max ½ mark if “may” is missing from answer. Same number and type of genes and have same alleles for the same genes. Explanation: It is possible for the homologous chromosomes to have the same alleles for the same genes. In Fig 17 of Cell cycle notes (page 20), the pair of homologous chromosomes have different alleles for gene A/a but the same alleles for gene B/b. 5) Crossing over Crossing over occurs during prophase I. Max ½ mark if “prophase I” is missing. No crossing over. Feedback / comment 1. Generally, well done although many candidates were unable to gain full credit for point 4 and point 5.

7 (d) State the challenges of using embryonic stem (ES) cells for research or medical treatment and explain how induced pluripotent stem cells (iPSCs) may overcome each of these challenges. [3] 1. Obtaining ES cells destroy human embryos which may be regarded as living human beings. Obtaining iPSCs do not involve killing human embryos so there is no ethical issue. Note: Accept both kill OR destruction of embryos Refer to the destruction of embryos during the process of extracting ES cells + idea that embryos might be considered as living human beings. R: Destruction of ES cells. 2. ES cells may cause immune rejection in the patients as they