GCSE-readyDNA structure and extracting DNA from fruit
DNA is a long polymer found in every cell. You can extract it from fruit such as strawberries or kiwi using simple kitchen chemicals.
DNA is a polymer
Sugars and phosphates form the backbone of each strand. Two strands twist into a double helix, joined by complementary base pairs: A with T, C with G.
The bases are held together by weak hydrogen bonds.
Because A pairs with T, there is always the same amount of A as T, and of C as G.
Extracting DNA from fruit
2. Mix with washing-up liquid (detergent) and salt. The detergent breaks down the cell and nuclear membranes, releasing the DNA. The salt helps the DNA strands clump together.
3. Filter to remove the solid bits.
4. Gently pour ice-cold ethanol down the side of the tube so it forms a layer on top.
5. DNA does not dissolve in cold ethanol, so it comes out as white strands where the layers meet. Wind it onto a glass rod.
Why ice-cold ethanol?
The cold makes the DNA even less soluble and slows down enzymes that could break the DNA up.
In a DNA sample, 18% of the bases are adenine (A). What percentage are cytosine (C)?
- A pairs with T, so T = 18%.
- A + T = 36%, so C + G = 100 − 36 = 64%.
- C = G, so C = 64 ÷ 2 = 32%.
Answer: 32%
DNA is a polymer of nucleotides in a double helix, with A paired to T and C paired to G. To extract it: mash the fruit, add detergent and salt, filter, then add ice-cold ethanol so white strands of DNA appear.
Try two quick questions
See if it has sunk in. Answers are marked straight away.
2 more questions
Try each one, then tap to check.
In a DNA sample, 30% of the bases are thymine (T). What percentage are cytosine (C)?
Show the answer
Answer: 20 %
T pairs with A, so 60% are T or A. That leaves 40%, shared equally between C and G: 20%.
In DNA extraction, which membranes does the detergent break down?
- The cell wall only
- Only the vacuole membrane
- The DNA strands
- The cell membrane and the nuclear membrane
Show the answer
Answer: The cell membrane and the nuclear membrane
Detergent dissolves the lipids in membranes, releasing DNA.
Check you have got it
Answer 6 quick questions with instant marking. If you get one wrong, GCSE-ready shows you why and gives you another go. It is free, and you do not need an account.
Checked by the GCSE-ready editorial team, 17 October 2026. Last updated 2 October 2026. Spotted a mistake? Tell us and we will fix it.
What do exam command words like explain and evaluate mean? →