Extracting DNA

DNA (deoxyribonucleic acid) is the molecule that carries the genetic information (the instructions) for the cell, and DNA is found in all cells that have a nucleus. DNA is made of two chains of molecules known as nucleotides. The nucleotides in one chain link to the nucleotides in the other in pairs, and the two chains twist around each other in a spiral known as a double helix.

The nucleotides are called adenine (A), thymine (T), cytosine (C) and guanine (G). Adenine always pairs with thymine and cytosine always pairs with guanine. The genetic information in DNA is coded by these nucleotides – they form three letter ‘words’, and each word is a code for an amino acid. Amino acids are the building blocks for proteins, which make up all living things. A gene is a sequence of nucleotides that codes for a particular characteristic in an organism.

Extracting DNA

Cool down some methylated spirits by putting the bottle in a bowl and surround it with ice cubes. Measure out 900 ml of water, and gently stir in 25 g of salt and 80 g of washing up liquid (cheap, non-concentrated washing up liquid seems to work the best), without making too many bubbles. Peel the kiwi fruit, chop it and then put it in a small bowl and mash it with a fork. Pour in 100 ml of the salt and detergent mixture, stir, and put the bowl in a pan of hot (not boiling water). Leave this for 15 minutes, and then strain it through a sieve into a glass.

Carefully pour in an equal amount of cold methylated spirits, slowly dripping it down the side of the glass so that it forms a layer on top of the kiwi fruit paste. A white layer should form between the methylated spirit and the kiwi fruit paste – this is DNA.

The mashing, and then the salt and detergent break open the cells. Kiwi fruit contain enzymes called proteases. These break down the proteins in the nucleus that keep the DNA packed tightly together. DNA dissolves in water but does not dissolve in alcohol, so forms a layer between the kiwi fruit paste, which contains water, and the methylated spirits, which contains alcohol. Try it with an onion or pineapple – these also contain proteases.

Do All Cells Contain DNA?

The only cells in the human body that do not contain DNA are the red blood cells. The cells have a nucleus in the early stages, but lose these later so that they can carry more haemoglobin, which carries oxygen around the body.

DNA Fingerprints

A DNA fingerprint is a profile of a person’s DNA, and almost everyone’s DNA fingerprint is different (the only people will identical DNA fingerprints are identical twins). The police can use DNA fingerprints to identify people who have committed crimes, if they have left any samples of their DNA behind, such as hair or blood. They are also used to research and diagnose disease, and to identify the biological parents of children.