DNA: structure, extraction and Punnett squares

Lesson page: echoesof.science/school/dna/ages-15-16/ · Pupil worksheet: echoesof.science/school/dna/ages-15-16/print/

Learning aims

Describe the structure of DNA and complementary base pairing.
Recall key numbers of the human genome and how DNA is copied.
Extract DNA and explain each step.
Use Punnett squares and genetic vocabulary to predict ratios, and discuss chance variation.

Lesson plan

Time	Activity
5 min	Starter: show the 1953 double-helix model; what does the structure explain?
10 min	Read the text; practise complementary strands.
20 min	Strawberry DNA extraction in groups.
10 min	Punnett squares on the board; game.
5 min	Quiz; worksheet for homework.

Materials for the experiment

2 strawberries (fresh or frozen) per group
Resealable plastic bag
Extraction liquid: 2 teaspoons washing-up liquid, 1 teaspoon salt, 1/2 cup (about 120 ml) water
Coffee filter or filter paper and funnel, two clear plastic cups or a boiling tube
Ice-cold ethanol (dispensed by the teacher)
Wooden stirrer or glass rod
Eye protection

Safety: Ethanol is highly flammable: no flames or heaters in the room; the teacher dispenses it and keeps the bottle closed. Wear eye protection. Do not eat or taste anything used in the practical. Wipe up spills and wash hands afterwards.

Common misconceptions

Pupils may think…	Actually…
Dominant alleles are more common in the population.	Dominant only means the allele shows in a heterozygote; it says nothing about how common it is.
A 3:1 ratio means exactly three of every four children show the trait.	It is a probability for each child; small numbers vary a lot.
The white strands are a single DNA molecule.	They are many DNA molecules clumped together; the salt helps them clump.

Worksheet answers

T C G A A T C G (A always pairs with T and C with G).
Gametes T, t × t, t: 2 Tt : 2 tt, so genotype 1 Tt : 1 tt and phenotype 1 tall : 1 short.
Detergent breaks up the lipid membranes so DNA is released; salt neutralises DNA's charge and helps it clump; cold ethanol makes the DNA precipitate so it can be seen and collected.
Expected 3:1 is 22.5 yellow : 7.5 green; 20 : 10 is close. The difference is due to chance in a small sample; pooling more results should bring it closer.

Quiz answers

The complementary strand to A T T G C is… T A A C G. A pairs with T and G pairs with C.[1]
Who helped reveal the double helix in 1953? Watson, Crick, Wilkins and Franklin. NHGRI credits all four.[1]
In Yy × Yy, what fraction of offspring is expected to be yy? 1/4. The genotype ratio is 1 YY : 2 Yy : 1 yy.[5]
An individual with alleles Yy is… heterozygous. Heterozygous means two different alleles.[5]
Why is cold ethanol added in the extraction? DNA precipitates out of solution in it. The DNA comes out of solution and forms strands.[7]
About how many bases are in the human genome? 3 billion. NHGRI and MedlinePlus give about 3 billion.[1][2]

Going further

Draw Punnett squares for YY × yy and Yy × yy. Predict the genotype and phenotype ratios, then test one of them with 40 coin tosses and compare.

Sources used in this lesson

[1] Deoxyribonucleic Acid (DNA) Fact Sheet (National Human Genome Research Institute (NIH)): reliability High
[2] What is DNA? (MedlinePlus Genetics, U.S. National Library of Medicine): reliability High
[3] What is a gene? (MedlinePlus Genetics, U.S. National Library of Medicine): reliability High
[5] Concepts of Biology, 8.2 Laws of Inheritance (OpenStax, Rice University): reliability High
[6] How to extract DNA from a strawberry (National Human Genome Research Institute (NIH)): reliability High
[7] Lab Manual for Biology Part I: DNA Extraction from Strawberries (LOUIS, the Louisiana Library Network (2022)): reliability Medium-high

Full list with links and a note on each source's reliability: echoesof.science/school/dna/sources/

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