Dengue II: the Cayman Islands trial Bill Indge

1
Dengue II the Cayman Islands trialBill Indge
2
Transgenic male mosquitoes and the lethal gene

• 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.

• 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.

3
Constructing the plasmid
Marker gene that codes for a fluorescent protein
Lethal gene
Transposable element
Transposable element
For the sake of simplicity, we will represent the
plasmid like this
4
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)
5
1 The recombinant DNA allows transgenic embryos
to be identified rapidly. Explain how. (2 marks)

• Transgenic embryos will carry the marker gene.
• This codes for a (fluorescent) protein which will
  show up when illuminated appropriately.

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)

• 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.

6
3 Only transgenic male mosquitoes are released.
Explain why transgenic female mosquitoes are not
released. (2 marks)

• The transgenic females will bite humans/require
  human blood.
• Adds to the pool of vectors/More mosquitoes
  present so greater probability of spreading
  dengue.

7
?
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)
8
?
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)
9
?
Mosquito DNA cut with transposase enzyme
Mosquito DNA
Insert DNA sequence with transposable element at
either end
10
?
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)
11
The transposase cuts the mosquito DNA at the same
base sequence. Use your knowledge of enzyme
action to explain why. (2 marks)

• Only the specific base sequence concerned has the
  complementary shape
• To fit the active site of the transposase/enzyme

The sequence of bases at X is TAGGT. What is the
sequence of bases at Y? (1 mark)

• ATCCA

Use the diagram to describe the role of DNA
polymerase. (2 marks)

• Polymerises/joins together the DNA nucleotides
• Filling in the gaps between the mosquito DNA and
  the transposable elements

12
The Cayman Islands trial
Plot B
Plot A
Plot C
500 m
The trial area
13
The Cayman Islands trial

• 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)
14
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)

• Keeps the GM mosquitoes from getting into the
  control area.
• The distance is too far for them to fly.

2 Must the area of plot A be the same as the area
of plot C? Give a reason for your answer. (1
mark)

• No because the mosquito population is given as a
  percentage (of ovitraps containing eggs).
• This is independent of area.

15
The results
Percentage of traps containing eggs after 1 week
80 60 40 20 0
Plot C
Plot A
1 Apr 1 May 1 Jun 1 Jul 1
Aug 1 Sep 1 Oct
16
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)
17
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)

• The trial started at the beginning of the rainy
  season.
• Mosquito eggs hatch/mosquitoes become
  active/start to breed.

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)

• 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.

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 www.hoddereducation.co.uk/biol
ogicalsciencesreview