Why fruit is used

1. DNA can be extracted from any tissue whose cells contain nuclei, and soft fruit is the usual starting material because it is easy to work with at the bench.
2. Fruit tissue is soft, so it is broken up by mashing alone, without the grinding or heating that tougher tissue would need.
3. The cells of the fruit are packed closely together and each one holds a nucleus, so even a few grams of tissue contains a very large number of nuclei and therefore a large amount of DNA.
4. Strawberries and kiwi fruit work particularly well, because their cells contain several sets of chromosomes rather than the two sets found in a human body cell.
   1. Each extra set is a further copy of the same DNA, so a given mass of strawberry tissue yields more DNA than the same mass of most other tissue.
5. Fruit is also cheap, safe to handle and needs no preservation, so the whole extraction can be done in a single lesson with everyday materials.

Barriers around DNA

1. In a fruit cell the DNA is held inside the nucleus, which is enclosed by its own membrane.
2. The nucleus sits in the cytoplasm, and the cytoplasm is enclosed by the cell surface membrane.
3. Outside the cell surface membrane there is a rigid cell wall made of cellulose, which gives the plant cell its strength.
4. Four layers therefore lie between the DNA and the outside of the cell: the cell wall, the cell surface membrane, the cytoplasm and the membrane around the nucleus.
5. Every step of the extraction method exists either to remove one of these layers or to separate the released DNA from everything else that comes out of the cell with it.
6. The cell wall is broken by physical force, the two membranes are broken down chemically, and the DNA is then separated from the liquid it has dissolved in.

Releasing DNA from cells

1. Mashing or grinding the fruit breaks the cell walls by physical force, so the cell contents are released into the mixture.
   1. No reagent in the method breaks down cellulose, so the cell walls can only be broken mechanically and this step cannot be left out.
2. Washing-up liquid is added because it is a detergent, and detergents break down lipids.
3. The cell surface membrane and the membrane around the nucleus are both made largely of lipids, so the detergent breaks these membranes apart and the DNA passes out of the nucleus and out of the cell into the mixture.
4. Salt is added because a DNA molecule carries a negative charge along its whole length, since every nucleotide in it contains a phosphate group.
5. Two negatively charged DNA molecules repel each other, so on their own they stay spread out and dissolved in the liquid.
6. The positively charged sodium ions from the salt are attracted to the negative charges on the DNA and screen them, so the DNA molecules stop repelling one another.
   1. The DNA molecules can then clump together, which makes them come out of solution far more readily once the ethanol is added.

Separating the DNA

1. After mashing, the mixture contains the released DNA together with broken cell walls, lumps of tissue that were not broken up and the rest of the cell contents.
2. Filtering the mixture holds back the insoluble cell walls and lumps of tissue on the filter paper, so only a liquid containing the dissolved DNA passes through into the tube.
3. Ice-cold ethanol is then layered on top of this filtrate.
4. DNA is insoluble in cold ethanol, so as the DNA meets the ethanol it comes out of solution and appears as solid white threads.
5. Ethanol is less dense than the fruit extract, so pouring it slowly down the side of a tilted tube keeps it as a separate layer, and the threads form at the boundary where the two liquids meet.
6. The colder the ethanol, the less DNA stays dissolved, so more DNA comes out of solution and a larger, clearer mass of threads is seen.

Identifying the DNA

1. The extracted DNA is seen as white stringy threads at the boundary between the ethanol layer and the fruit extract, often with small bubbles trapped in them.
2. The threads are firm enough to be wound onto a glass rod and lifted out of the tube.
3. A single DNA molecule is far too thin to be seen, and cannot be resolved even with a light microscope.
4. What is visible is very many DNA molecules, released from the nuclei of thousands of cells and clumped together, so the threads are a mass of DNA rather than one molecule.
5. The threads contain the whole genome of the fruit cells, because every chromosome is released and precipitated together.
6. Nothing about the appearance of the threads reveals which genes the DNA carries, so the extraction shows only that DNA is present and can be handled.