Dengue II: the Cayman Islands trial Bill Indge

Dengue The lethal gene The gene is lethal to mosquito larvae but it does not affect adults The gene is switched off when tetracycline is added to the diet. The principles A lethal gene is inserted into the DNA of Aedes aegypti mosquito embryos. Mosquitoes carrying this gene are identified and used to rear large numbers of transgenic males. Transgenic male mosquitoes are released. When they mate with wild female mosquitoes, their offspring inherit the lethal gene and die. Transgenic male mosquitoes and the lethal gene

Dengue Marker gene that codes for a fluorescent protein Lethal gene Transposable element For the sake of simplicity, we will represent the plasmid like this Constructing the plasmid

Dengue 1 The recombinant DNA allows transgenic embryos to be identified rapidly. Explain how. (2 marks) 2 It is possible to rear large numbers of mosquitoes containing this lethal gene in a laboratory. Use the information provided to explain how. (2 marks) 3 Only transgenic male mosquitoes are released. Explain why transgenic female mosquitoes are not released. (2 marks)

Dengue Transgenic embryos will carry the marker gene. This codes for a (fluorescent) protein which will show up when illuminated appropriately. Tetracycline will be added to the diet of the mosquito larvae being reared in the laboratory. Switching off the lethal gene so the larvae will survive. 1 The recombinant DNA allows transgenic embryos to be identified rapidly. Explain how. (2 marks) 2 It is possible to rear large numbers of mosquitoes containing this lethal gene in a laboratory. Use the information provided to explain how. (2 marks)

Dengue The transgenic females will bite humans/require human blood. Adds to the pool of vectors/More mosquitoes present so greater probability of spreading dengue. 3 Only transgenic male mosquitoes are released. Explain why transgenic female mosquitoes are not released. (2 marks)

Dengue  Mosquito DNA cut with transposase enzyme Mosquito DNA The transposase cuts the mosquito DNA at the same base sequence. Use your knowledge of enzyme action to explain why. (2 marks)

Dengue  Mosquito DNA cut with transposase enzyme Mosquito DNA X Y The sequence of bases at X is TAGGT. What is the sequence of bases at Y? (1 mark)

Dengue  Mosquito DNA cut with transposase enzyme Mosquito DNA Insert DNA sequence with transposable element at either end

Dengue  Mosquito DNA cut with transposase enzyme Mosquito DNA Insert DNA sequence with transposable element at either end Add DNA polymerase Use the diagram to describe the role of DNA polymerase. (2 marks)

Dengue Only the specific base sequence concerned has the complementary shape To fit the active site of the transposase/enzyme ATCCA Polymerises/joins together the DNA nucleotides Filling in the gaps between the mosquito DNA and the transposable elements Use the diagram to describe the role of DNA polymerase. (2 marks) The sequence of bases at X is TAGGT. What is the sequence of bases at Y? (1 mark) The transposase cuts the mosquito DNA at the same base sequence. Use your knowledge of enzyme action to explain why. (2 marks)

Dengue 500 m Plot A Plot B Plot C The trial area The Cayman Islands trial

Dengue GM mosquitoes were released in the trial area (plot A on the map, coloured red) at the beginning of the rainy season. Plot C (coloured green) was the control area in which no mosquitoes were released. Ovitraps were then used to measure the mosquito population at weekly intervals. The percentage of ovitraps containing mosquito eggs each week was calculated for plot A and for plot C. 1 The yellow-shaded area (plot B on the map) was a buffer zone between the trial area and the control area. Explain why the buffer zone was necessary. (2 marks) 2 Must the area of plot A be the same as the area of plot C? Give a reason for your answer. (1 mark) The Cayman Islands trial

Dengue Keeps the GM mosquitoes from getting into the control area. The distance is too far for them to fly. No because the mosquito population is given as a percentage (of ovitraps containing eggs). This is independent of area. 1 The yellow-shaded area (plot B on the map) was a buffer zone between the trial area and the control area. Explain why the buffer zone was necessary. (2 marks) 2 Must the area of plot A be the same as the area of plot C? Give a reason for your answer. (1 mark)

Dengue 1 Apr 1 May 1 Jun 1 Jul 1 Aug 1 Sep 1 Oct Pe rcentage of traps containing eggs after 1 week Plot A Plot C The results

Dengue The results of the trial are shown in the graph. 3 Plot C was the control plot. The percentage of traps containing eggs in plot C rose in the early months of the trial. Explain why. (2 marks) 4 Plot A was the release plot. The percentage of traps in plot A containing eggs did not fall until August. Explain why the percentage took several months before it fell. (3 marks)

Dengue The trial started at the beginning of the rainy season. Mosquito eggs hatch/mosquitoes become active/start to breed. The population is measured by finding the number of ovitraps containing mosquito eggs. When GM mosquitoes are released they mate with females which will still lay eggs. Larvae from these eggs will die so there will be fewer mosquitoes in the next generation to lay eggs. The results of the trial are shown in the graph. 3 Plot C was the control plot. The percentage of traps containing eggs in plot C rose in the early months of the trial. Explain why. (2 marks) 4 Plot A was the release plot. The percentage of traps in plot A containing eggs did not fall until August. Explain why the percentage took several months before it fell. (3 marks) This resource is part of Biological Sciences Review, a magazine written for A-level students by subject experts. To subscribe to the full magazine go to