2018 Cell-Nuclear-Division STQ MS

2018 Cell and Nuclear Division STQ MS2018 / H2 / ACJC PRELIM / P2 Q6 (Mutations)1Many diseases arise due to gene mutations and/or chromosomal aberrations.(a)Distinguish between gene mutations and chromosomal aberrations.Gene mutationChromosomal aberration1. Definition1a. Change in nucleotide sequence1b. Change in chromosomal structure or number;2. No. of genes affected2a. Affects a single gene2b. Usually affects multiple genes (because involves a segment or an entire chromosome hence);3. Mechanisms3a. Substitution, deletion or addition of nucleotides (any 2)3b. Translocation, deletion or inversion of chromosomal structures or non-disjunction events during nuclear division (any 2);Any 2@1m each[2]Fig. 6.1 shows an error occurring during meiosis II. Fig. 6.1(b)(i)Complete Fig. 6.1 with the correct chromosome structures and chromosome number (in terms of n) in the gametes. [2] Meiosis IMeiosis IIGametesChromosome number (in terms of n) Chromosome number=2n n+1n-1n

(ii)Explain the error shown in Fig. 6.1.1.Non-disjunction caused by the failure of sister chromatids to separate during anaphase II;2.due to problems in the shortening of the spindle fibres/ centromeres fail to divide;[2](iii)Predict the chromosomal number (in terms of n) in each of the four gametes if a similar error had occurred in meiosis I instead of meiosis II.n+1, n+1, n-1, n-1[1](c)Such errors can occur during mitosis too. Comment on whether this would lead to changes in the gene pool of a population.1.If this occurs in somatic cells only / does not affect the germ line cells, it only affects the individual / not passed down to offspring, so no effect on overall gene pool in population; OR2.If this occurs in germ line cells, it will be passed to gametes, and possibly to offspring, so gene pool of a population will be affected;3.If this occurs in zygotic / embryonic stem cells, the whole individual will be affected, so gene pool of a population will be affected;[1] [Total: 8]

2018 / H2 / EJC PRELIM / P2 Q3 (Stem Cells)3Bone marrow contains stem cells that divide by mitosis to form blood cells. The fate of a stem cell was trackand it was recorded that during the observed duration the stem cell divided asymmetrically each time. Fig. 3.1 shows changes in the mass of DNA in a human stem cell from bone marrow during three cell cycles Fig. 3.1(a)With reference to the information provided above, (i)Describe what happens to bring about the changes in the mass of DNA per cell at time period K anat time period L. K …………………………………………………………………………………………………… ……………………………………………………………………………………………………… L …………………………………………………………………………………………………… …………………………………………………………………………………………………...[2]1.K: DNA replication/ synthesis during S phase of interphase;2.L : cytoplasmic division during cytokinesis;(ii)State one function of these stem cells undergoing the above type of cell division. ……………………………………………………………………………………………………… …………………………………………………………………………………………………...[1]1.To replace blood cells that die due to injury/ wear and tear/ disease;(iii)The process of meiosis is significant to natural selection in evolution.Explain this significance. ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… …………………………………………………………………………………………………...[2]1.Meiosis gives rise to genetically different haploid gametes;2.During Prophase I of meiosis, crossing over between chromatids of homologous chromosomwould give rise to different combinations of alleles

ORDuring Metaphase I and Anaphase I, independent assortment and separation of homologous chromosomes would give rise to different combinations of maternal and paternal chromosomes;3.Resulting in genetic variation in diploid organisms leading to different phenotypes/ variation phenotypes;4. those with advantageous phenotypes are selected for, resulting in change in allele frequencyin the population over time;(1 and 2 + 3 / 4 OR 1+ 3 and 4)A bone marrow cell was extracted and observed under the electron micrograph shown in Fig. 3.2. The studfocused on an organelle which he described as having “an envelope surrounding genetic material containboth darker and lighter stained patches, distinct from the site where ribosomal subunits were assembled”. Fig. 3.2(b)Explain the significance of the “darker and lighter stained patches” that the student referred to, in a cundergoing differentiation.……………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………[3]1.The darker stained patches are heterochromatin while the lighter stained patches are euchromat2.The genes that are found within heterochromatin are transcriptionally inactive/not expressed whthe genes found in euchromatin are transcriptionally active/are expressed;3.Thus producing specific proteins that allow the differentiated cell to perform specific functions;The use of embryonic stem cells (ESCs) for stem cell therapy and research is controversial and considered

many people as unethical. Scientists have circumvented this issue through the use of induced pluripotent stem cells (iPSCs) as an alternative to ESCs. Fig. 3.3 summarises the procedure for obtaining iPSCs and its use.Skin fibroblastsAddition of reprogramming genesInduced pluripotent stem cells Heart cellsFat cellsBlood cellsBrain cellsDifferentiationFig. 3.3 (c)Explain why the use of iPSCs is preferred over ESCs.………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………...…………………………………………………………………………[2]1.Since iPSCs can be derived directly from adult tissues/specialised somatic cell/ skin cells, it donot destroy any human embryos unlike ESCs;2.iPSCs from adult/specialised somatic cell/ skin cell can be easily obtained without risk to the donor, whereas obtaining the embryo to isolate ESCs is more invasive; (R: easy vs difficult because no elaboration)3.In contrast to ES cells extracted from human embryos, iPSCs derived from a patient’s own cewould open the possibility of generating patient-specific cells, which will not be rejected by timmune system upon transplantation; (A: idea of iPSCs obtained from self vs embryos not froself thus contain antigens that will be rejected by immune system);4.Sources of obtaining iPSCs is more easily accessible compared to the source of obtaining ESCobtaining the embryo;(any one, answer must make comparison with ESCs)[Total:

2018 / H2 / JJC PRELIM / P2 Q73Fig. 7.1 shows an electron micrograph of a chromosome in prophase II. Fig. 7.1(a)Name the structures A and B. [2]A: (a pair of recombinant) chromatids ; [R: sister chromatids / chromosomes]B: (kinetochore) spindle fibre ;(b)Explain why the chromosome occurs as a double structure. [2]1.Semi-conservative DNA replication occurred during S phase of interphase, producing two identical DNA molecules ;2.which coil and condense during prophase I of meiosis I to form a chromosome consisting of 2 (identical) sister chromatids held together at the centromere before crossing over occurred. ; BA

The risk of mis-segregation of chromosomes increases with age among women. This can lead to aneuploid embryos. Fig. 7.2 shows an oocyte undergoing nuclear division with mis-segregated and lagging chromosomes. Fig. 7.2(a)Identify the stage in meiosis as shown in Fig. 7.2. [1]Anaphase II ;(b)With reference to Fig. 7.2, suggest how aneuploid embryos are formed. [3]1.Mis-segregation of chromosomes arose when centromeres did not divide prop