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DNA is a polymer of nucleotides. Each contains one of four bases, A, T, G or C. Sugar and phosphate form the two sides of the double helix; the paired bases form the rungs. Pairing is complementary: A–T and C–G.

The double-helix structure was revealed in 1953 through the work of James Watson, Francis Crick, Maurice Wilkins and Rosalind Franklin.

The human genome has about 3 billion bases on 23 pairs of chromosomes and about 20,000 genes. Genes make up only about 1% of the sequence and range from a few hundred to more than 2 million bases.

Experiment: Extracting DNA from strawberries

1. Remove the green tops. Seal the strawberries in the bag and squash them gently for 2 minutes.
2. Add the extraction liquid, reseal and squash for 1 more minute without making lots of foam.
3. Filter the mixture into a clean cup or tube; gently squeeze the filter to collect the liquid.
4. Tilt the cup and slowly pour an equal volume of ice-cold ethanol down the side so it forms a layer on top. Do not stir.
5. Watch the boundary for 1–2 minutes. Wind the white strands that form onto the stirrer and describe them.

	What I observed
Appearance of the filtered liquid	Predict what DNA will look like when you can see it.
Where the DNA appeared	Predict what DNA will look like when you can see it.
Appearance of the DNA	Predict what DNA will look like when you can see it.
Rough amount (little / some / a lot)	Predict what DNA will look like when you can see it.

Questions

1. Write the complementary strand for A G C T T A G C and explain the rule you used.

2. Draw a Punnett square for Tt × tt (T = tall, dominant). Give the expected genotype and phenotype ratios.

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3. Explain the job of the detergent, the salt and the cold ethanol in the extraction.

4. A class counts 30 offspring from Yy × Yy crosses: 20 yellow, 10 green. Compare with the expected ratio and suggest why they differ.

Sources for every fact on this sheet: echoesof.science/school/dna/sources/

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