2018_Stem-Cells_STQ_MS

2018 Stem Cells STQ MS2018 / H2 / EJC PRELIM / P2 Q31Bone marrow contains stem cells that divide by mitosis to form blood cells. The fate of a stem cell was tracked and 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 and at 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 chromosomes would 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 in phenotypes;4. those with advantageous phenotypes are selected for, resulting in change in allele frequency in 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 student focused on an organelle which he described as having “an envelope surrounding genetic material containing both 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 cell undergoing differentiation.……………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………[3]1.The darker stained patches are heterochromatin while the lighter stained patches are euchromatin;2.The genes that are found within heterochromatin are transcriptionally inactive/not express