Copying DNA

How do scientists copy the DNA of living organisms?

Until recently plant and animal breeders could only work with variations that already exist in nature. Even when breeders tried to add variation by introducing mutations, the changes they produced were unpredictable. Today genetic engineers can transfer certain genes at will from one organism to another, designing new living things to meet specific needs.

Recall from Chapter 14 that it is relatively easy to extract DNA from cells and tissues. The extracted DNA can be cut into fragments of manageable size using restriction enzymes. These restriction fragments can then be separated according to size using gel electrophoresis or another similar technique. That's the easy part. The tough part comes next: How do you find a specific gene?

The problem is huge. If we were to cut DNA from a bacterium like E. coli into restriction fragments averaging 1000 base pairs in length, we would have 4000 restriction fragments. In the human genome, we would have 3 million restriction fragments. How do we find the DNA of a single gene among millions of fragments? In some respects, it's the classic problem of finding a needle in a haystack—we have an enormous pile of hay and just one needle.

Actually, there is a way to find a needle in a haystack. We can toss the hay in front of a powerful magnet until something sticks. The hay won't stick, but a needle made of iron or steel will. Believe it or not, similar techniques can help scientists identify specific genes.