DNA and the genome — Edexcel GCSE Biology
Describe the structure of DNA, define the genome and a gene, and explain how DNA is extracted from fruit.
Revision notes
3 short notes, in the order of the specification. Each one in short:
DNA is a polymer made of repeating units called nucleotides. Each nucleotide has a sugar, a phosphate group and one of four bases. Two strands coil into a double helix and are linked by complementary base pairs, joined by weak hydrogen bonds: adenine (A) pairs with thymine (T), and cytosine (C) pairs with guanine (G).
The genome is the entire DNA of an organism, including all of its genes. A gene is a short section of a DNA molecule that codes for a specific protein. Each gene has a different sequence of bases, so each one codes for a different protein. The genome is all of the DNA, not just the genes.
DNA can be extracted from fruit such as strawberries. Mash the fruit, then add detergent, which breaks down cell and nuclear membranes to release the DNA, and salt, which helps the DNA clump together. Filter the mixture, then add ice-cold ethanol on top. DNA is not soluble in cold ethanol, so it appears as white strands.
Exam questions
5 exam-style questions (12 marks), each with its mark scheme.
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10 cards: flip them, mark what you knew, and practise the rest.
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The whole of genetics on one page, so you can see where this subtopic fits.
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Quick check questions
What are the two strands of DNA coiled into?
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A double helix.
Which bases pair together in DNA?
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A with T, and C with G.
What is a gene?
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A section of a DNA molecule that codes for a specific protein.
What is the purpose of the detergent when extracting DNA from fruit?
Show answer
It breaks down the cell membranes to release the DNA.
Frequently asked questions
What is the structure of DNA?
DNA is a polymer made of nucleotides, with two strands coiled to form a double helix. Each nucleotide has a sugar, a phosphate group and one of four bases. The strands are linked by complementary base pairs, adenine with thymine and cytosine with guanine, held together by weak hydrogen bonds.
What is the difference between a gene and a genome?
A gene is a short section of DNA that codes for a specific protein, while the genome is the entire DNA of an organism. The genome includes all of the organism's genes. Each gene has a different sequence of bases, so each one codes for a different protein.
Why is salt added when extracting DNA from fruit?
Salt is added when extracting DNA from fruit because it helps the DNA clump together. Detergent is added with it to break down the cell membranes and nuclear membranes, so the DNA is released. After filtering, ice-cold ethanol is added, and the DNA comes out of solution as white, stringy strands.
Why does DNA appear when cold ethanol is added?
DNA appears when ice-cold ethanol is added because DNA is not soluble in cold ethanol. It comes out of solution (precipitates) at the boundary between the ethanol and the filtrate as white, stringy strands. You are seeing thousands of DNA strands clumped together, not the double helix itself.
Written and checked against the Edexcel GCSE Biology (1BI0) specification · Updated October 2026