Strawberry DNA Extraction

Put the isopropyl alcohol in the freezer for at least 30 minutes before you start. Warm alcohol gives a poor yield — this single step matters more than anything else in the method.

Stir together 100 ml water, 1 teaspoon washing-up liquid and half a teaspoon of table salt. Stir gently — froth traps your DNA and makes it impossible to see.

Fresh or frozen both work. Discard the hulls — leaf tissue adds fibre and pigment with no extra DNA.

Squash it to a smooth pulp with your fingers for a full minute. You are breaking cell walls mechanically — the soap cannot reach a cell that is still inside intact tissue.

Pour in about 50 ml of buffer, seal out the air and squash gently for another minute. Gently — vigorous shaking shears the DNA into fragments too short to see.

The soap needs time to dissolve the lipid membranes around each cell and around each nucleus. This is the slowest chemistry in the process.

Pour it through filter paper into a clean glass. You want the liquid, not the pulp — cell wall debris is what makes the final result look like porridge instead of thread.

It drips slowly. Do not squeeze the filter to hurry it — squeezing forces pulp through and ruins the clarity you just spent ten minutes gaining.

A narrow vessel concentrates the DNA into a visible band. About 2 cm depth of pink filtrate is plenty.

Add roughly the same volume of ice-cold alcohol, running it slowly down the inside wall. Do not stir. You want two distinct layers with a sharp boundary.

Within seconds a white, stringy, cloud-like mass appears exactly at the interface between the two layers. That is DNA coming out of solution.

Twirl a wooden skewer or a bent paperclip slowly in the alcohol layer. The DNA wraps around it in threads you can lift clear of the liquid.

It is not pure. The white mass is DNA tangled with RNA and protein. Calling it “DNA” is honest at this scale; calling it pure DNA is not.

Try a kiwi, a banana and an onion side by side with the same volumes. Record which gives the biggest clot and think about why.

Why the strawberry. The cultivated garden strawberry, Fragaria × ananassa, is octoploid: eight sets of chromosomes, 2n = 8x = 56, arising from a hybridisation between two already-polyploid American species in eighteenth-century Europe. Eight copies of the genome per cell means far more DNA per gram of tissue than a diploid fruit, and ripe strawberry flesh is soft enough to disrupt by hand. Polyploidy is common in cultivated plants — bread wheat is hexaploid — and it is one reason plant genetics developed differently from animal genetics.

What each reagent does. The detergent is an amphiphile: one end mixes with water, the other with fat. It inserts itself into the phospholipid bilayers of the cell membrane and the nuclear envelope and pulls them apart, releasing the nuclear contents. Salt supplies sodium ions that crowd around the DNA's negatively charged phosphate backbone, screening the repulsion between strands so they can aggregate instead of staying dispersed. Alcohol is the precipitant: DNA is readily soluble in water and effectively insoluble in ethanol or isopropanol, so adding alcohol drops it out of solution. Cold matters because solubility falls with temperature and because cold slows the nucleases that would otherwise chop the strands.

Why it looks like thread. A single DNA molecule is about 2 nanometres wide and invisible. What you spool is millions of molecules aggregated together with trapped air bubbles from the alcohol layer, which is why the mass looks white and fibrous rather than clear. The stringiness is real, though — DNA is an extraordinarily long thin polymer, and the human genome in one cell would stretch about two metres if laid end to end.

The historical thread. Friedrich Miescher isolated what he called nuclein from white blood cells in pus-soaked bandages in 1869 — the first DNA extraction, using alkali and acid rather than washing-up liquid. Its role as the hereditary material was not accepted until Avery, MacLeod and McCarty in 1944 and Hershey and Chase in 1952, and the double helix followed in 1953. For seventy-five years DNA was a known molecule with no known job. The extraction was the easy part; working out what it was for took three generations.