page 04 6. A newly identified drug designed to treat cancer by inhibiting cell cycle progression was tested on cancer cells in vitro. The distribution of the cancer cells across the different phases of the cell cycle was then investigated by measuring the DNA content of the cells. Results for untreated cells are shown in Figure 1 and for treated cells in Figure 2. DNA content is displayed with arbitrary units where 2n units represents the DNA content of a non‑dividing diploid cell. Figure 1 Figure 2 untreated cells treated cells (with drug) relative amount of DNA relative amount of DNA number of cells number of cells 2n 4n 2n 4n At which phase(s) of the cell cycle does this drug act? A G1 only B S only C G1 and G2 D G2 or M 7. The three Rs of replacement, reduction and refinement are used in animal studies to avoid, reduce or minimise the harm to animals. Which of the following is an example of replacement? A Use of most appropriate anaesthetic regimes for pain relief during procedures. B Smaller mammals such as mice or rats instead of larger mammals. C Substitution of in vivo studies with tissue culture to test the effect of a new drug. D Less invasive techniques instead of surgical techniques. 8. Active cyclin‑CDK complexes regulate the cell cycle by A phosphorylation of specific proteins B proteolytic cleavage of specific proteins C dephosphorylation of specific proteins D acting as transcription factors.