Topic 3 – Genetics: 3.6 Explain how DNA can be extracted from fruit

The idea

DNA extraction from fruit relies on breaking cells open, separating DNA from protein and other cell material, and making the DNA insoluble so it clumps together. Mash the fruit to break cell walls, then add detergent, which breaks down cell membranes and releases the DNA into solution. Add salt, which helps the DNA clump together and separates it from protein. Filter the mixture to remove solid fruit debris, keeping the liquid. Then add cold ethanol carefully down the side so it forms a layer; DNA is insoluble in ethanol, so it precipitates as a white stringy layer at the boundary. Each step has a reason, and an explanation question wants the step plus its purpose.

What earns the marks

• Mashing or blending the fruit breaks down cell walls and separates cells.
• Detergent breaks down cell membranes so DNA is released into the mixture.
• Salt helps the DNA clump together and separates DNA from protein.
• Filtering removes solid fruit debris, leaving DNA in the liquid.
• Cold ethanol is added so DNA precipitates as a visible white layer because DNA is insoluble in ethanol.

Common mistakes and how to put them right

• Thinking detergent destroys the DNA.

  Put it right by explaining that detergent breaks down membranes to release DNA, not to break the DNA itself.

• Believing ethanol dissolves the DNA.

  Put it right by stating that DNA is insoluble in cold ethanol and so precipitates.

• Omitting the purpose of a step, such as adding salt.

  Put it right by giving both the step and its function in the explanation.

Exam tips

• For each step, write the action and then the reason, for example 'add detergent to break down cell membranes'.
• Link the cold ethanol step to the word precipitate and to DNA being insoluble in ethanol.
• Keep the order logical: mash, add detergent and salt, filter, then add cold ethanol.