19. Genetic technology
A section of Biology, 9700
Listing 10 of 107 questions
In humans, the gene RPE65 encodes a protein responsible for regenerating visual pigment in rod and cone cells after they have been exposed to light. A recessive allele of this gene causes impaired vision from birth, progressing to complete blindness in early adulthood. This condition is called LCA. In 2008, trials were carried out into the possibility and safety of treating LCA using gene therapy. Suggest and explain why LCA is suitable for treatment using gene therapy. Six adults with this condition were used in the study. Genetically modified adenoviruses (a type of virus that can cause respiratory infections) were used as vectors. The vectors were injected beneath the retina of one eye of each of the participants. Suggest two ways in which the genome of the adenoviruses used as vectors would differ from that of normal adenoviruses. 1. 2. Improvements were found in the vision of all the participants, but the small number in the trials made most of these improvements not statistically significant. Suggest why these trials were designed to include such a small number of participants.
9700_w11_qp_42
THEORY
2011
Paper 4, Variant 2
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Haemophilia A and haemophilia B are common hereditary disorders of blood clotting. Haemophilia A is a sex-linked genetic disorder that affects approximately 1 in 20 000 males worldwide. It is caused by a recessive allele of a gene coding for a clotting factor and results in excessive bleeding. There is currently no cure, but symptoms of haemophilia can be treated with a transfusion of a clotting factor to slow down the bleeding. State how genetic screening could reduce the number of cases of haemophilia. Some genetic disorders can be treated with gene therapy. Outline the aims of gene therapy. Suggest why haemophilia A is a suitable disorder for treatment with gene therapy. Haemophilia A and haemophilia B are caused by mutations in different blood clotting genes, F8 and F9 respectively. Both disorders have been treated with gene therapy involving the use of a vector. Table 4.1 shows the lengths, in kilobases , of the F8 and F9 genes. Table 4.1 haemophilia gene gene length / kb A F8 >8 B F9 1.4 With reference to Table 4.1, suggest why gene therapy using the F9 gene has been more successful than using the F8 gene. Two frequently used vectors in gene therapy are compared in Table 4.2. Table 4.2 feature vector adenovirus retrovirus genetic material of virus double-stranded DNA single-stranded RNA expression of inserted gene high gene expression gene expression in dividing cells only host immune response to virus high low With reference to Table 4.2, explain the advantages and disadvantages of using adenovirus rather than retrovirus as a vector.
9700_w15_qp_43
THEORY
2015
Paper 4, Variant 3
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Leber’s congenital amaurosis (LCA) is an autosomal recessive eye disease. LCA results in eye disorders, including severe loss of vision, at birth. LCA has been successfully treated by gene therapy, using a virus instead of a plasmid as the vector. Adeno‑associated virus (AAvectors containing the therapeutic allele were injected directly into the retina, the layer at the back of the eye containing the photoreceptor cells. People who had been blind from a young age were able to see again. There is a risk associated with the injection method used to deliver the vectors, as it might cause the retina to detach, damaging vision. This method of delivery was first used for LCA before being trialled on other retinal diseases that gradually reduce the vision of people as they get older. Suggest the main steps involved in creating recombinant DNA for this example of gene therapy. Explain why the fact that LCA is an autosomal recessive genetic disease makes it suitable for treatment with gene therapy. Suggest why the retinal injection method of gene therapy was used for LCA before it was trialled on other retinal diseases that gradually reduce the vision of people as they get older. Scientists tried to create an improved virus vector for gene therapy. step 1 – The scientists used a special form of the polymerase chain reaction (PCR). This form of PCR causes mutations in the DNA sequence of AAV by base substitution. step 2 – The viruses containing different base substitutions were tested. This was done by using the different viruses to deliver a new gene, the gene for green fluorescent protein (GFP), into the photoreceptor cells of mice, using the retinal injection method. step 3 – The best virus, known as 7m8, caused the photoreceptor cells in the retina of the mouse to fluoresce brightly, even when the recombinant virus was injected into the fluid inside the eye instead of into the retina itself. step 4 – The 7m8 virus was used to cure a mouse with LCA by injecting this virus containing the therapeutic allele into the fluid inside the eye of the mouse. Suggest how errors occurring during PCR can cause base substitution mutations in the DNA sequence of AAV. Explain why the photoreceptor cells of the mouse fluoresced in step 3. Predict the impact of the 7m8 AAV on treatment for age‑related retinal diseases.
9700_w18_qp_41
THEORY
2018
Paper 4, Variant 1
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Leber’s congenital amaurosis (LCA) is an autosomal recessive eye disease. LCA results in eye disorders, including severe loss of vision, at birth. LCA has been successfully treated by gene therapy, using a virus instead of a plasmid as the vector. Adeno‑associated virus (AAvectors containing the therapeutic allele were injected directly into the retina, the layer at the back of the eye containing the photoreceptor cells. People who had been blind from a young age were able to see again. There is a risk associated with the injection method used to deliver the vectors, as it might cause the retina to detach, damaging vision. This method of delivery was first used for LCA before being trialled on other retinal diseases that gradually reduce the vision of people as they get older. Suggest the main steps involved in creating recombinant DNA for this example of gene therapy. Explain why the fact that LCA is an autosomal recessive genetic disease makes it suitable for treatment with gene therapy. Suggest why the retinal injection method of gene therapy was used for LCA before it was trialled on other retinal diseases that gradually reduce the vision of people as they get older. Scientists tried to create an improved virus vector for gene therapy. step 1 – The scientists used a special form of the polymerase chain reaction (PCR). This form of PCR causes mutations in the DNA sequence of AAV by base substitution. step 2 – The viruses containing different base substitutions were tested. This was done by using the different viruses to deliver a new gene, the gene for green fluorescent protein (GFP), into the photoreceptor cells of mice, using the retinal injection method. step 3 – The best virus, known as 7m8, caused the photoreceptor cells in the retina of the mouse to fluoresce brightly, even when the recombinant virus was injected into the fluid inside the eye instead of into the retina itself. step 4 – The 7m8 virus was used to cure a mouse with LCA by injecting this virus containing the therapeutic allele into the fluid inside the eye of the mouse. Suggest how errors occurring during PCR can cause base substitution mutations in the DNA sequence of AAV. Explain why the photoreceptor cells of the mouse fluoresced in step 3. Predict the impact of the 7m8 AAV on treatment for age‑related retinal diseases.
9700_w18_qp_43
THEORY
2018
Paper 4, Variant 3
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Adenosine deaminase (ADA) deficiency is an immune system disorder caused by a recessive autosomal mutation. Severe combined immunodeficiency caused by a lack of ADA is called ADA-SCID. Genetic engineering is used to make a recombinant human protein to treat people with ADA-SCID. Outline the principles of genetic engineering. In 2016 gene therapy to cure ADA-SCID was approved in Europe. The gene therapy involves three main steps. • Blood stem cells are taken from the bone marrow of the person with ADA-SCID. • The functional gene and its promoter are inserted into the blood stem cells. • A single infusion of the gene-corrected cells is given to the patient. Explain why a single infusion of gene-corrected stem cells is enough to cure the disease. Explain why a promoter has to be transferred as well as the desired gene. A modified retrovirus is used to insert the new gene into the DNA of the blood stem cells. State two ethical considerations of using a retrovirus for gene therapy. The gene therapy technique used to cure ADA-SCID is not suitable for treating the genetic disease called Huntington’s disease. A newer technique called gene editing could potentially be used instead to cure Huntington’s disease. Explain why gene editing is more suitable as a potential cure for Huntington’s disease.
9700_w22_qp_41
THEORY
2022
Paper 4, Variant 1
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Adenosine deaminase (ADA) deficiency is an immune system disorder caused by a recessive autosomal mutation. Severe combined immunodeficiency caused by a lack of ADA is called ADA-SCID. Genetic engineering is used to make a recombinant human protein to treat people with ADA-SCID. Outline the principles of genetic engineering. In 2016 gene therapy to cure ADA-SCID was approved in Europe. The gene therapy involves three main steps. • Blood stem cells are taken from the bone marrow of the person with ADA-SCID. • The functional gene and its promoter are inserted into the blood stem cells. • A single infusion of the gene-corrected cells is given to the patient. Explain why a single infusion of gene-corrected stem cells is enough to cure the disease. Explain why a promoter has to be transferred as well as the desired gene. A modified retrovirus is used to insert the new gene into the DNA of the blood stem cells. State two ethical considerations of using a retrovirus for gene therapy. The gene therapy technique used to cure ADA-SCID is not suitable for treating the genetic disease called Huntington’s disease. A newer technique called gene editing could potentially be used instead to cure Huntington’s disease. Explain why gene editing is more suitable as a potential cure for Huntington’s disease.
9700_w22_qp_43
THEORY
2022
Paper 4, Variant 3
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Variable number tandem repeats (VNTRs) are repetitive, non-coding sections of DNA. A particular VNTR is located at the same locus in different individuals, but the number of repeats in that VNTR varies between individuals. Explain how, in the process of genetic fingerprinting, gel electrophoresis is able to distinguish between the VNTRs that occur at the same loci of different individuals. Gel electrophoresis is also used in genetic screening. The mutation of the β-globin gene which gives rise to sickle cell anaemia removes a recognition site of a restriction enzyme, R, as shown in . R cuts DNA at the sites indicated by arrows ( ). The lengths of the resulting fragments are shown in kilobases . 1.1 kb normal allele of ȕ-globin gene, HbA sickle cell allele, HbS 0.2 kb 1.3 kb non-coding DNA shows an electrophoresis gel with a stained band of DNA from an individual who was homozygous for the normal allele for β-globin, HbA HbA. This band is the 1.1 kb fragment shown in . The 0.2 kb fragment is not shown. Complete by drawing the stained DNA that would result from an individual who is heterozygous for the sickle cell allele, HbA HbS. Put your answer on to . stained 1.1 kb DNA fragment HbA HbA individual HbA HbS individual direction of movement of DNA fragments electrophoresis gel Describe the different circumstances in which this genetic screening for the sickle cell allele, HbS, might be used.
9700_s12_qp_41
THEORY
2012
Paper 4, Variant 1
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Variable number tandem repeats (VNTRs) are repetitive, non-coding sections of DNA. A particular VNTR is located at the same locus in different individuals, but the number of repeats in that VNTR varies between individuals. Explain how, in the process of genetic fingerprinting, gel electrophoresis is able to distinguish between the VNTRs that occur at the same loci of different individuals. Gel electrophoresis is also used in genetic screening. The mutation of the β-globin gene which gives rise to sickle cell anaemia removes a recognition site of a restriction enzyme, R, as shown in . R cuts DNA at the sites indicated by arrows ( ). The lengths of the resulting fragments are shown in kilobases . 1.1 kb normal allele of ȕ-globin gene, HbA sickle cell allele, HbS 0.2 kb 1.3 kb non-coding DNA shows an electrophoresis gel with a stained band of DNA from an individual who was homozygous for the normal allele for β-globin, HbA HbA. This band is the 1.1 kb fragment shown in . The 0.2 kb fragment is not shown. Complete by drawing the stained DNA that would result from an individual who is heterozygous for the sickle cell allele, HbA HbS. Put your answer on to . stained 1.1 kb DNA fragment HbA HbA individual HbA HbS individual direction of movement of DNA fragments electrophoresis gel Describe the different circumstances in which this genetic screening for the sickle cell allele, HbS, might be used.
9700_s12_qp_43
THEORY
2012
Paper 4, Variant 3
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Bacteria were grown in a medium containing 15N. After several generations, all of the DNA contained 15N. Some of these bacteria were transferred to a medium containing the common isotope of nitrogen, 14N. The bacteria were allowed to divide once. The DNA of some of these bacteria was extracted and analysed. This DNA was all hybrid DNA containing equal amounts of 14N and 15N. Some bacteria from the medium with 15N were transferred into a medium of 14N. The bacteria were allowed to divide twice. The graph shows the percentages of 14N and 15N in the DNA of these bacteria. 14N 14N / 15N percentage of DNA Some bacteria from the medium with 15N were transferred into a medium of 14N. The bacteria were allowed to divide three times. What would be the percentages of 14N and 15N in the DNA extracted from these bacteria?
9700_w11_qp_12
MCQ
2011
Paper 1, Variant 2
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In 1984, the geneticist Alec Jeffreys invented a DNA testing technique, known as DNA profiling, that produces a DNA banding pattern on a gel. The DNA banding pattern is unique to each individual. DNA profiling can be used in police forensic work to catch criminals. Since 1987, police in many countries have collected and stored DNA from crime scenes to create DNA profiles, which they try to match with the DNA profiles of criminal suspects. DNA at a crime scene may be obtained from hairs and traces of blood, semen and saliva. Explain why PCR may be needed before DNA from a crime scene can be profiled. Explain why electrophoresis produces a DNA banding pattern on a gel. GEDmatch is described as ‘an open data personal genomics website’. It can be used by people who want to upload their DNA data to trace their ancestors and other relatives. In 2018, police in the USA solved a large number of serious crimes. Some of these crimes had been unsolved for over thirty years. The police used GEDmatch to profile DNA taken from crime scenes and to look for matching DNA profiles. In many cases the police found partial matches to the relatives of criminals. This allowed the criminals to be identified and then charged on the basis of a complete DNA profile match. Suggest why the police strategy of comparing crime scene DNA with the GEDmatch database was so successful. Explain why GEDmatch is an example of bioinformatics. The first successful conviction resulting from the use of GEDmatch by the police was widely reported. Some journalists and broadcasters thought that the GEDmatch website should not have been used by the police in this way. In the days following the news, the number of citizens choosing to upload their DNA data to GEDmatch increased from 1500 to 5000 a day. Comment on the social and ethical issues raised by this first successful conviction.
9700_w21_qp_41
THEORY
2021
Paper 4, Variant 1
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Questions Discovered
107