Biology 3.6 - Extracting DNA from fruit
Make DNA visible by releasing it from fruit cells, filtering away large debris and precipitating it with cold ethanol. Explain the purpose of each step, the expected observation and the precautions needed.
Extracting DNA from fruit
DNA is normally inside cells, enclosed by cell and nuclear membranes and mixed with many other substances. A successful extraction must expose the cells, release the DNA, separate large debris and make the dissolved DNA come out of solution.
| Step | What to do | Why it works |
|---|---|---|
| 1. Expose the cells | Mash a soft fruit such as a strawberry in a sealed bag. | Mashing breaks up the tissue and many cell walls, exposing more cells to the extraction mixture. |
| 2. Release the DNA | Add a mixture of water, detergent and salt; mix gently. | Detergent disrupts the lipid cell and nuclear membranes, releasing DNA. Salt helps the DNA strands clump together and separate from some proteins. Gentle mixing avoids breaking the long DNA strands into very small pieces. |
| 3. Remove large debris | Filter the mixture into a clean tube. | Seeds, fibres and large cell fragments remain in the filter while the liquid containing DNA passes through. |
| 4. Precipitate the DNA | Tilt the tube and slowly add ice-cold ethanol down its side so it forms a separate layer above the filtrate. | DNA is not soluble in the cold alcohol, so it precipitates where the two layers meet. The cold temperature improves precipitation. |
| 5. Collect the DNA | Use a glass rod or wooden stick to spool the material at the interface. | Many precipitated DNA molecules tangle together and can be lifted from the tube. |
The expected observation is a white, cloudy or stringy material forming at the boundary between the fruit filtrate and the alcohol. This is a mass of many precipitated DNA molecules and may still contain other cell substances; it is not one DNA molecule enlarged enough to see.
Ethanol is highly flammable. Keep it away from naked flames and other heat sources, keep its container closed when it is not being used, and wear eye protection to reduce the risk from splashes.