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/ synthe