DNA: structure, extraction and Punnett squares
The double helix and base pairing, the size of the human genome, extracting DNA from strawberries, and predicting genotypes and phenotypes with Punnett squares.
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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.[1][2]
The double-helix structure was revealed in 1953 through the work of James Watson, Francis Crick, Maurice Wilkins and Rosalind Franklin.[1]
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.[1][3]
In replication the strands separate and each acts as a template for a new complementary strand, so each daughter cell receives a copy. More than 99% of the bases are the same in all people.[1][2]
Alleles are written with a capital letter (dominant) or a lower-case letter (recessive). Homozygous = two identical alleles (YY or yy); heterozygous = two different (Yy). The genotype is the allele combination; the phenotype is the observable trait.[5]
A Punnett square combines one allele from each parent. For Yy × Yy the expected genotypes are 1 YY : 2 Yy : 1 yy, so 3/4 show the dominant phenotype (3:1), as Mendel found in peas. These are probabilities: real families and small samples vary.[5]
Try it: Extracting DNA from strawberries
You need
- 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.
What to do
- Remove the green tops. Seal the strawberries in the bag and squash them gently for 2 minutes.
- Add the extraction liquid, reseal and squash for 1 more minute without making lots of foam.
- Filter the mixture into a clean cup or tube; gently squeeze the filter to collect the liquid.
- 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.
- Watch the boundary for 1–2 minutes. Wind the white strands that form onto the stirrer and describe them.
What happens, and why?
Squashing breaks the cells open. The detergent breaks up the lipids of the cell and nuclear membranes so DNA is released. Salt neutralises the charge on DNA's sugar-phosphate backbone and helps the molecules clump. DNA does not stay dissolved in cold ethanol, so it precipitates as white strands in the top layer. Cultivated strawberries have eight sets of chromosomes per cell, which gives a lot of DNA.[6][7]
Play: Structure, extraction or inheritance?
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Other ages: Ages 6–7 · Ages 10–11