24 9 The human immunodeficiency virus (HIV), shown in the image, causes acquired immunodeficiency syndrome (AIDS). © Liya Graphics/Shutterstock (a) HIV particles contain (1) A DNA and DNA polymerase B DNA and reverse transcriptase C RNA and DNA polymerase D RNA and reverse transcriptase (b) HIV enters and destroys T helper cells. This can cause AIDS. (i) Describe how HIV particles are able to enter T helper cells. 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Turn over 25 (ii) Explain why the destruction of T helper cells causes the symptoms of AIDS. (4) ..................................................................................................................................................................................................................................................................................... ..................................................................................................................................................................................................................................................................................... ..................................................................................................................................................................................................................................................................................... ..................................................................................................................................................................................................................................................................................... ..................................................................................................................................................................................................................................................................................... ..................................................................................................................................................................................................................................................................................... ..................................................................................................................................................................................................................................................................................... ..................................................................................................................................................................................................................................................................................... ..................................................................................................................................................................................................................................................................................... ..................................................................................................................................................................................................................................................................................... ..................................................................................................................................................................................................................................................................................... ..................................................................................................................................................................................................................................................................................... ..................................................................................................................................................................................................................................................................................... 26 (c) A small number of people have been identified who are resistant to HIV. They have a mutation in a gene coding for a protein in the cell membrane. (i) Deduce why this mutation makes these people resistant to HIV infection. 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(ii) Stem cell therapy can be used to treat patients infected with HIV. The bone marrow of these patients can be destroyed using radiotherapy. The patients can then be given stem cells from the bone marrow of a donor who has this mutation. Explain why these stem cells may prevent HIV causing AIDS. 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(Total for Question 9 = 14 marks) Turn over 27 BLANK PAGE