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the eye, the genes for eye color are “on” and make their product while other genes are “off” and make nothing. The fact that all the cells contain all the types of genes is very important in genetic engineering. GENETIC ENGINEERING Every plant or animal, such as an apple tree or a sheep, has a set of thousands of genes within its DNA which resembles tiny threads of pale jelly. This DNA can be taken out by chemical means. In genetic engineering, the DNA is split into shorter pieces, each of which is studied to see which particular genes it contains. Then a gene can be altered, moved, removed, or put into the DNA of another living thing. To obtain genes, you need just a tiny piece of a living thing such as a strand of hair, a flake of skin, or a flower petal. Even a tiny piece contains millions of cells, and every cell contains the full set of the living thing’s genes. The first step to identifying genes is to heat the cells with chemicals to make them release their contents. These are then spun around very quickly in a centrifuge machine (like a spin dryer), which separates them into different layers. The DNA layer is thin and pale and looks like damp coon. Its threads can be wound onto a glass rod. The long lengths of DNA are split into shorter ones by warming them with various proteins called restriction enzymes. These short fragments of DNA are identified by adding them to yet more proteins, puing them in a clear gel, and passing electricity through the gel. This makes various fragments move different distances along the gel. Called gel electrophoresis , this process creates a row of lines that look like a supermarket barcode. This identifies the DNA piece and so identifies the gene.

DNA “fingerprinting” may soon replace traditional fingerprints. Even a tiny sample of blood, skin or hair is enough for DNA testing to help us identify a criminal or victim.

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