AQA A-Level Biology Paper 3 Predicted Paper 2023 with marking scheme

AQA A-Level Biology Paper 3 Predicted Paper 2023 with marking scheme Name …………………………………………………………………… Date ………………… 2 hours allowed. You may use a scientific calculator and a ruler with millimetre measurements. Answer all questions in section A. Answer one question from section B. Grade boundaries These are VERY rough guesses! Getting a B on this paper does not guarantee you the same mark in the exam. • A* 65% • A 55% • B 45% • C 40% • D 35% • E 30% SECTION A Answer all questions in this section. 01 The Hawaiian Islands have the most diverse collection of Drosophilid flies in the world with approximately 800 different species having been identified. All of the species have been traced back to a single common ancestor from mainland USA that colonised the islands millions of years ago. It is hypothesised that this could have been a single fertilised female blown across in a storm or trapped in a piece of fruit stuck to a bird. a) What is a gene pool? [1 mark] b) Explain how the Hawaiian species of flies could have evolved. [6 marks] c) Drosophila are one group of insects which contain polytene chromosomes. These are extra-large chromosomes found in the nuclei of salivary gland cells. They can be removed and stained so their banding patterns can be viewed under a light microscope. Explain how scientists could have used the banding pattern on the chromosomes to establish the evolutionary relationships between flies. [2 marks] d) About 100 species of the Hawaiian Drosophila have large, highly patterned wings and are known as the picture-wing Drosophila. In some species, e.g D. truncipenna the males have more detail patterning on their wings than the females. Suggest how the variety of wing patterns displayed by these flies helps to maintain them as separate species. [2 marks] 02 Some organisms are able to fix atmospheric nitrogen using a nitrogenase enzyme. Nitrogenase enzymes can be inhibited by high oxygen concentrations. Cyanobacteria are one of the few organisms that photosynthesise but also fix nitrogen. a) Explain how each of the following adaptations allow cyanobacteria to carry out both processes: i. Some colonies of cyanobacteria develop heterocysts; specialised cells which only contain photosystem I. [2 marks] ii. Some only fix nitrogen at night. [2 marks] Some plants make associations with nitrogen fixing bacteria in specialised structures known as root nodules. Genes are activated during root nodule development which code for the protein leghaemoglobin. This is a type of haemoglobin that is very similar to animal haemoglobin, it also binds to oxygen. b) To investigate its action in nodules scientists used siRNA to silence the leghaemoglobin genes in Lotus japonicus plants. They observed that these plants exhibited less than normal growth. Only after addition of nitrogen fertiliser were the plants able to grow normally. Suggest why. [3 marks] c) A company making meat free burgers is using genetically modified yeast to produce large quantities of leghaemoglobin to create more realistic plant-based burgers. Suggest why adding leghaemoglobin makes the burgers taste more like ones containing meat. [1 mark] 03 Taxol is a drug used to treat cancer; it was extracted from the bark of the Pacific yew tree. Scientists initially tested this drug on mice that had visible cancerous tumours in their lungs. Some results are shown in the table below: Days of Treatment Mean volume of tumour (mm3) Control mice Mice injected with Taxol in saline 1 1 1 10 8 2 20 23 10 30 52 22 40 117 52 50 342 85 60 652 120 70 744 207 a) Suggest and explain two factors which should be considered when deciding on the number of mice to use in a trial such as this. [2 marks] b) The scientists concluded that Taxol did not kill cancer cells. Does the table support this conclusion? Explain your answer. [2 marks] c) Taxol works by disrupting spindle activity. Explain how Taxol affects cancer cells. [3 marks] d) A more targeted drug needs to be developed. Aurora A is an enzyme that phosphorylates the protein TACC3. TACC3 then binds to microtubules which hold spindle fibres together. Aurora A is found to be amplified in tumour cells. Suggest how scientists could develop a cancer drug which targets Aurora A. [2 marks] 04 RT-PCR or reverse transcriptase PCR is used to test for RNA viral infection in patients, as it can accurately measure the amount of RNA in a sample. The RT-PCR uses the same reaction mixture as standard PCR except that reverse transcriptase enzymes and a fluorescent dye that binds to DNA are included. a) State two other ingredients which must be present in the RT-PCR reaction mixture. [2 marks] b) Explain the role of the reverse transcriptase in the RT-PCR. [1 mark] RT-PCR can be used to test if a patient is infected with a virus. A test is considered positive when the fluorescent signal reaches a certain threshold. The Ct value is the number of PCR cycles required for the fluorescent signal to cross a certain threshold. RT-PCR was used to test if patients were positive for the SARS-CoV-2 virus. The results of one RT-PCR are shown in the graph below: c) Suggest one explanation for the levelling out of the curve from cycle 50. [2 marks] d) How many cDNA molecules will have been produced from one molecule of DNA at the threshold value of this sample? Your answer should be in standard form. [2 marks] Because RNA degrades slowly over time it may still be detected many weeks after a patient is no longer infectious. In order to confirm if the RT- PCR was detecting RNA from the active SARS-CoV-2 virus, the lab would also carry out a viral culture test on the same sample. The method is detailed below: e) Suggest why penicillin and streptomycin were added to the cell culture. [1mark] f) Cytopathic effect (CPE) refers to physical changes in host cells caused by attacking viruses. When viewed under the microscope suggest what visible effect viral infection would have had on the cells. [1 mark]