Extracting DNA from Strawberries

DNA is usually something you see only as a diagram — a neat double helix in a textbook. In this practical you’ll pull real DNA out of strawberries and hold it on a glass rod as a white, stringy mass. The method uses kitchen-grade ingredients, takes about 30 minutes, and every step is a small lesson in chemistry: detergents breaking membranes, salt neutralising charges, and alcohol changing solubility.

Purpose

To extract DNA from strawberry cells and explain, in terms of molecular structure and intermolecular forces, why each step is needed.

Why strawberries?

Strawberries are a favourite for this experiment for three reasons:

1. Lots of DNA: cultivated strawberries are octoploid — each cell has eight copies of each chromosome, rather than the two found in human body cells. More DNA per cell means a bigger, more visible result.
2. Soft tissue: ripe strawberries mash easily, so cells are simple to break open.
3. Enzymes on your side: ripe fruit contains pectinase and cellulase enzymes that have already started breaking down cell walls.

Bananas, kiwi fruit and onions also work, though usually with less DNA.

Principle: the chemistry of each step

To see DNA, you need to get it out of the cell and nucleus, separate it from proteins and other molecules, and then make it come out of solution. Here’s what each ingredient does.

Mashing: breaking cell walls

Plant cells are surrounded by a tough cell wall made mainly of cellulose. Mashing the fruit physically breaks many of these walls and increases the surface area for the extraction solution to work on.

Detergent: dissolving membranes

Inside the wall, each cell is wrapped in a cell membrane, and the DNA is inside a second membrane around the nucleus. Membranes are made of a double layer of phospholipids: molecules with a polar, water-loving head and non-polar, water-hating tails.

Washing-up liquid contains detergent molecules with a similar structure — a polar head and a long non-polar tail. Detergent molecules push into the membranes and surround groups of lipid molecules, forming tiny clusters called micelles. The membrane falls apart, releasing the contents of the cell and nucleus into the liquid. It’s the same reason detergent removes grease from plates. Detergent also helps unfold some proteins bound to DNA.

Salt: neutralising charge

DNA’s backbone is made of alternating sugar and phosphate groups (see DNA structure). At the pH of cells, each phosphate carries a negative charge, so a DNA molecule is a very long chain of negative charges. These charges repel each other and attract water molecules strongly, which keeps DNA dissolved.

Adding sodium chloride supplies positive sodium ions (Na⁺), which cluster around the negative phosphates and shield their charges. With the charges partly neutralised, DNA strands can come close to each other and clump together once the solvent is changed in the final step. Salt also helps separate DNA from the proteins that normally wrap it.

Filtering

Pouring the mixture through a coffee filter or muslin removes cell walls, seeds and other large debris. The liquid that comes through (the filtrate) contains the dissolved DNA along with proteins, sugars and other small molecules.

Ice-cold alcohol: precipitating DNA

DNA dissolves in water but not in alcohol. When a layer of cold ethanol or isopropanol (propan-2-ol) is added on top of the filtrate, DNA near the boundary comes out of solution — it precipitates. Alcohol has a lower relative permittivity than water, so it’s much worse at keeping the charged DNA dissolved. The shielded, now less-soluble DNA strands clump together into white threads that float up into the alcohol layer, carried partly by tiny bubbles.

Most other cell components — sugars, many proteins, salts — stay dissolved in the water layer below, which is why the DNA separates neatly.

Why ice-cold? Low temperature lowers DNA’s solubility further, making precipitation more complete. It also slows down enzymes called DNases released from the broken cells, which would otherwise start cutting the DNA into short pieces.

Equipment and materials

• 2–3 fresh or frozen (thawed) strawberries, green tops removed
• Sealable plastic sandwich bag
• Extraction solution: 90 cm³ water, 10 cm³ washing-up liquid, 1 level teaspoon (about 3 g) of table salt, stirred gently to avoid froth
• Coffee filter or muslin cloth and a funnel
• Beaker or plastic cup
• Tall, narrow test tube or clear glass
• Ethanol or isopropanol (propan-2-ol), chilled in a freezer for at least an hour
• Glass rod, wooden skewer or plastic stirrer
• Dropping pipette
• Eye protection

Safety

• Wear eye protection.
• Ethanol and isopropanol are highly flammable; keep them away from flames. Isopropanol is harmful if swallowed.
• Don’t eat the strawberries or taste any solutions.
• Wash hands after the experiment. The DNA isn’t dangerous, but the solutions contain detergent and alcohol.

Method

1. Put the strawberries into the sandwich bag, press out the air and seal it.
2. Squash the strawberries with your fingers for about two minutes, until there are no big lumps.
3. Open the bag and add about 10 cm³ of extraction solution. Reseal and mash gently for another minute. Try not to make too many bubbles.
4. Place the coffee filter in the funnel over a beaker. Pour the mixture in and let it drip through for a few minutes. Squeeze gently if needed.
5. Pour about 2–3 cm³ of filtrate into a test tube.
6. Tilt the test tube and, using a pipette, slowly run an equal volume of ice-cold alcohol down the side so it forms a separate layer on top. Don’t mix the layers.
7. Hold the tube still and watch the boundary between the layers for a minute or two.
8. Gently twist a glass rod or skewer at the boundary to wind up the white strands. Lift them out.

Results

Within seconds, a cloudy white layer appears at the boundary, and white, stringy or gel-like threads rise into the alcohol. With care, you can wind up a clump the size of a small fingernail.

What you are seeing isn’t a single DNA double helix — one is only about 2 nm wide, far too thin to see. It’s millions of long DNA strands tangled together, along with some RNA and other material that precipitated with them. Your extract is crude DNA, not purified DNA.

Questions to answer in your write-up

1. Why is the mashing step needed if detergent dissolves membranes?
2. Which part of the DNA structure does salt interact with, and why does that help?
3. Why does DNA appear in the alcohol layer rather than staying in the water?
4. Suggest why the experiment works better with ice-cold alcohol.
5. Would you expect this method to work on a banana? What might differ?

Sources of error and limitations

• Warm alcohol: less DNA precipitates and the strands are shorter and harder to spool.
• Mixing the layers: shaking or pouring too fast mixes alcohol into the filtrate so no clear boundary forms.
• Too much foam: vigorous mashing with detergent traps DNA in bubbles and makes the layers hard to see.
• Contamination: proteins, RNA and pectin (a carbohydrate from the fruit) often precipitate too, so the extract isn’t pure DNA.
• Unripe or old fruit: less DNA or more enzyme damage.
• No quantitative measure: the amount can’t be measured accurately without further steps. Research labs purify DNA using enzymes that digest proteins (proteases) and RNA, then measure it by UV absorbance at 260 nm.

Improvements and extensions

• Compare different fruits and record the mass of wet DNA spooled per gram of fruit.
• Test the effect of alcohol temperature (freezer, fridge, room temperature).
• Add a small amount of meat tenderiser or pineapple juice, which contain protease enzymes, to digest proteins and give cleaner strands.
• Compare salt concentrations to see how important charge-shielding is.

Key takeaways

• Mashing breaks cell walls; detergent dissolves the phospholipid membranes around the cell and nucleus.
• Salt shields DNA’s negatively charged phosphate backbone.
• Filtering removes debris, leaving dissolved DNA.
• Ice-cold alcohol makes DNA insoluble, so it precipitates as white strands at the boundary.
• The product is crude DNA — millions of tangled strands with some RNA and protein.

For the molecule you’ve just extracted, read nucleic acids and how DNA replicates. For another classic practical, try the food tests.