6.4 Extracting DNA from fruit
5 exam questions worth 8 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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Explain why washing-up liquid is used when extracting DNA from strawberries.
Mark scheme
- breaks down (cell and nuclear) membranes (1)
- to release the DNA (1)
Model answer
The detergent breaks down the cell and nuclear membranes, so the DNA is released.
What is the purpose of adding salt?
Mark scheme
makes the DNA clump together
Model answer
Salt makes the DNA strands clump together.
Ice-cold ethanol is poured slowly down the side of the test tube.
What would you see, and why does it happen?
Mark scheme
- white (stringy) strands / precipitate at the boundary (1)
- DNA is insoluble in cold ethanol (1)
Model answer
White strands of DNA appear at the boundary between the layers, because DNA is insoluble in cold ethanol.
Why is protease sometimes added to the filtered mixture?
Mark scheme
breaks down the proteins (attached to / around the DNA)
Model answer
Protease breaks down the proteins attached to the DNA.
Put these steps in the correct order:
A filter the mixture B pour in ice-cold ethanol C mash the fruit with detergent and salt D add protease
Mark scheme
C, A, D, B (2) allow 1 mark for one pair in the wrong order
Model answer
C, A, D, B.
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