Practise recombinant DNA, gene transfer, PCR, electrophoresis and medical or agricultural applications while interpreting molecular methods and biological data.
The potato plant, Solanum tuberosum, is an important food crop. Crop yield is reduced if the leaves of the plant are eaten by the larvae (immature stages) of the Colorado beetle, Leptinotarsa decemlineata.
Crop scientists used recombinant DNA technology to create two genetically modified (GM) varieties of potato plant. These plants produce proteins that are poisonous to insects.
- GM potato variety A contains two new genes, S N and Bt.
- GM potato variety B contains two new genes, SN and OCII.
The new varieties were tested by having a constant number of Colorado beetle larvae introduced to the plants at time 0 hours. The number of larvae that were alive after 24,48 and 72 hours was recorded. The percentage of the larvae that had died in each time interval was calculated. This was repeated for potato plants that had not been genetically modified (non-GM).
Table 4.1 shows the percentage of Colorado beetle larvae that had died on the GM potato plant varieties and on non-GM potato plants.
Table 4.1
Suggest what is meant by recombinant DNA technology.
Discuss how the results in Table 4.1 provide information that could help to solve the global demand for food.
One cause of the genetic disease severe combined immunodeficiency (SCID) is a mutation in the ADA gene. This mutation results in a deficiency of the enzyme adenosine deaminase (ADA).
Although ADA is found throughout the body, it is especially active in lymphocytes. The absence of functional ADA causes the build-up of toxic metabolites that kill lymphocytes and damage organs.
Babies are often diagnosed with SCID by six months old. Treatment can greatly improve the life expectancy of children with SCID.
Some treatment options are available.
- Enzyme replacement therapy with recombinant human ADA made by genetically modified (GM) Escherichia coli. Weekly intra-muscular injections are given.
- Bone marrow transplant if a well-matched donor, such as a close relative, can be found.
- Gene therapy.
Suggest and explain why it may be more appropriate to use enzyme replacement therapy to treat SCID instead of a bone marrow transplant.
Outline the procedure used for gene therapy treatment of a person with SCID.
Suggest the social and ethical implications of gene therapy for SCID that need to be considered before treatment is carried out.
It has been hypothesised that the mutation rate of an animal species may affect how fast animals of that species age and how long they live (lifespan).
Table 7.1 compares the mutation rate and lifespan of five species of mammal.
Table 7.1
Genetic engineering could be used to improve farmed animals, such as dairy cattle. State two features of dairy cattle that could be improved by genetic engineering.
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 (haematopoietic) 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 (injection) 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.