N19 H2 P3 ans for uploading updated 2023

2019 H2 A level Paper 3 2019 H2 Biology A Level Paper 3 [Turn over Section A Answer all the questions in this section. 1 (a) (i) Name the term used to describe the organisation of species according to their evolutionary relationships, as shown in Fig. 1.1. [1] Phylogeny* (ii) State how the information needed to construct a diagram such as that shown in Fig. 1.1 can be obtained through molecular techniques. [4] 1. Amplify DNA sequences of a homologous* gene common to all the species of organisms being compared via polymerase chain reaction; 2. Sequence the DNA fragments obtained from the different organisms; 3. Align the DNA sequences and calculate/compare the differences between them; 4. Species that are closely related have more similar nucleotide sequences than do distantly related species and should be shown to diverge from a more recent common ancestor in the phylogenetic tree. (iii) The five species named in Fig. 1.1 all share the characteristic of making curcumin. These five species have never all been classified together in a single genus. Explain why the characteristic of making curcumin is not sufficient to place all five species in the same genus. [2] 1. Compared only 1 characteristic which is insufficient evidence to show that they belong to the same genus; 2. The 5 species could still be significantly different morphologically compared to those in the same genus. R: successful interbreeding to give fertile viable offspring. (iv) The five species named in Fig. 1.1 occur in just two of the 400 or more families of flowering plants that exist. Suggest why the ability to make curcumin is limited to these two families of plants. [2] 1. The two families must have diverged from a common ancestor which had the ability to make curcumin; 2. The trait for making curcumin first emerged in this common ancestor. (b) (i) With reference to Fig. 1.2, describe the effect of curcumin on the percentage of cancer cells in each phase of interphase nine hours after entering G1 phase. [3] 1. The higher the concentration of curcumin the cells were exposed to, the greater the percentage of cancer cells still in G1 phase* nine hours later; 2. cite data from graph comparing 2 curcumin concentrations and the percentage of cancer cells at G1 phase. E.g. 76% of cells at G1 phase in 40 μmol dm-3 curcumin higher than 20% of cells at G1 phase in 0 μmol dm-3 curcumin. 3. The higher the concentration of curcumin the cells were exposed to, the lower the percentage of cancer cells in S phase* nine hours later;

2 2019 H2 Biology A Level Paper 3 4. cite data from graph comparing 2 curcumin concentrations and the percentage of cancer cells at S phase. E.g. 17% of cells at S phase in 40 μmol dm-3 curcumin lower than 74% of cells at S phase in 0 μmol dm-3 curcumin. 5. No significant effect of curcumin on the percentage of cancer cells at G2 phase*; 6. Around 6%-9.5% cancer cells across all four different curcumin concentrations in the culture media. (ii) With reference to Fig. 1.2, suggest how curcumin affects the mitotic cell cycle and how this could help in the treatment of cancer. [4] 1. Curcumin halts the mitotic cell cycle at the G1 checkpoint; 2. Eg. of possible reasons to halt cell cycle: cause DNA damage, signal absence of growth factors / nutrients; 3. Cancer cells will be unable to progress in the cell cycle / proceed to S phase; 4. Prevent uncontrolled cell division in cancer cells; (iii) CDK2 affects the multiplication of cancer cells. Identify one other molecule in Fig. 1.3 that affects the multiplication of cancer cells and explain how curcumin’s effect on it could help to treat or prevent cancer. [3] 1. p53*; 2. Curcumin could increase the expression / frequency of transcription of the p53 gene, resulting in production of more p53 proteins; 3. which will result in increased ability to inhibit cell cycle / repair damaged DNA / promote apoptosis; OR 4. growth factors (FGF*/TGF1*)/ transcription factor NFB* 5. bind to growth factors to prevent their binding to receptors; 6. prevent stimulation/progression of the cell cycle leading to reduced cell division; R: CDK2* Note: Using the information provided in Fig. 1.4, PhK and TNFα are possible answers to this question too. However, points 4-6 would still be necessary in order to link PhK and TNFα to the treatment of cancer. (c) (i) With reference to Fig. 1.3 and Fig. 1.4, explain why curcumin is useful in the treatment of burns to prevent changes in the appearance of skin. [4] 1. Curcumin reduces the activity of PhK* and TNFα* which are released by damaged/burned skin; 2. Lower levels of PhK activity will decrease activation of NFκB; 3. which upregulates the expression of genes involved in scar tissue formation; 4. Lower levels of TNFα activity will decrease attraction of white blood cells and the release of FGF and TGFβ1 by these cells; 5. reducing fibroblast proliferation and differentiation into scar tissue;

3 2019 H2 Biology A level Paper 3 [Turn over (ii) Curcumin is a non-competitive inhibitor of the enzyme phosphorylase kinase, PhK. Explain how curcumin interacts with the PhK molecule to decrease its activity. [3] 1. Curcumin binds to site other than active site on PhK; 2. Alters the shape/conformation of the specific* Phk active site* 3. so that active site is no longer complementary* in shape and charge to substrate, hence substrate cannot bind so the rate of reaction is reduced; (iii) Explain the roles of kinases (such as PhK) and phosphatases in cell signalling. [4] 1. Kinases catalyse the addition of phosphate* groups from ATP* to a protein; 2. Kinases activate a large number of molecules resulting in signal amplification; 3. Phosphatases catalyse the removal of phosphate* groups from proteins by hydrolysis. 4. Phosphatases inactivate relay molecules so that propagation of the signal will be inhibited/signal will be terminated. [Total: 30] 2 Lymphoid stem cells are a subset of blood stem cells that give rise to the B lymphocytes and T lymphocytes of the immune system. (a) (i) State the level of potency of lymphoid stem cells. [1] Multipotent* (ii) Outline two defining features of stem cells. [2] 1. A stem cell is an undifferentiated/unspecialized* cell capable of undergoing proliferation* and self-renewal*; 2. and retains potential to differentiate* to produce specialized cells upon receiving appropriate molecular signals*; (b) Name precisely the genetic process occurring at: (i) stage 1, to give naïve B lymphocytes capable of recognizing different antigens. [1] Somatic recombination* (ii) stage 2, to give an activated B lymphocyte that now produces IgG antibodies instead of the membrane-bound IgM antibodies. [1] Class switching* (iii) Stage 3, to give sub-clones that vary slightly in their ability to bind antigen X. [1] Somatic hypermutation* (c) (i) List the ways in which genetic variation is generated during meiosis, identifying all the stages of meiosis at which each process occurs. [4] 1. Independent assortment involving the random orientation of homologous chromosomes* at the equator/metaphase plate* during metaphase I*; 2. And separation of homologous chromosomes at anaphase I* ultimately results in different combinations of parental chromosomes in the gametes; 3. Crossing over * between non-sister chromatids of homologous chromosomes* occurs at prophase I* at points called chiasmata;

4 2019 H2 Biology A Level Paper 3 4. Where equivalent portions of these chromatids break and rejoin, resulting in exchange of geneti