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London Examinations GCE
Ordinary Level Paper 2
Friday 18 January 2008 – Afternoon Time: 2 hours
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In the boxes above, write your centre number, candidate number, your surname, initial(s) and signature. Check that you have the correct question paper. The paper is arranged in THREE sections, A, B and C. In Section A, answer ALL questions. In Section B, answer any TWO questions. In Section C, answer any TWO questions. Write your answers in the spaces provided in this question paper. Indicate which question you are answering by marking the box ( ). If you change your mind, put a line through the box ( ) and then indicate your new question with a cross ( ).
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Calculators may be used. The total mark for this paper is 100. The marks for parts of questions are shown in round brackets: e.g. (2). This paper has 11 questions. Any blank pages are indicated.
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SECTION A Answer ALL questions in this section.
Read the passage below. Use the information in the passage and your own knowledge to answer the questions that follow. Applications of gene therapy Gene therapy has been used to relieve the symptoms of some inherited conditions, such as cystic fibrosis. This condition is caused by a recessive allele, which produces abnormally thick mucus in the small tubes of the lungs. Microorganisms live and grow in the mucus and lung infections may occur. Antibiotics can be taken to reduce the infections.
Gene therapy for this condition involves using a spray containing special viruses. These viruses have been genetically modified to deliver the normal allele to the cells that produce mucus in the lungs. The viruses act as vectors for recombinant DNA. This viral DNA contains the normal human allele and, once it is inside the lung cells, 10 normal mucus is made. Unfortunately the allele stops working after a few weeks and the symptoms of the condition may return. Children with another condition called severe combined immunodeficiency (SCID) have also been helped by gene therapy. Children with SCID have inherited an allele that produces a toxin that destroys white blood cells. The children are unable to 15 defend themselves against pathogenic microorganisms. Such children have to be protected by living in a ‘bubble’, a transparent container designed to keep harmful microorganisms out. Gene therapy for SCID children involves several steps. First, a restriction enzyme is used to isolate the normal allele from healthy cell DNA. The normal allele is then put 20 into a virus, which then injects the normal allele into young white blood cells taken from the child. The genetically modified young white blood cells are put back into the child. Here they divide to produce a supply of cells that can protect the child.
(a) Using the letter G for the dominant allele and g for the recessive allele, explain how two unaffected parents can produce a child with cystic fibrosis. ..........................................................................................................................................