The problem is as follows. If we wish to clone a particular gene, either for study or to make a protein product, then we have to use methods that enable us to isolate this gene from all the other genes in the organism of interest. To show the scale of this problem, imagine that you want to clone a human gene – perhaps one that is responsible for an inherited genetic disease. Remember from Chapter 2 that the human genome is 3,289,000,000 bp long, encoding about 25,000 genes. If you are going to make a detailed study of the gene responsible for the genetic defect, you will need to get your hands on a manageable-sized piece of DNA, encoding just the gene of interest and perhaps its associated regulatory sequences. If
you extract DNA from human cells you will then have to locate the part containing the gene that is of interest to you, a task equivalent to the proverbial search for a needle in a haystack.