Sometimes, DNA must be isolated from samples dried to patches of cloth or carpet, and getting the sample safely out of these fabrics adds additional steps to the isolation and purification processes. The huge genome is cut up with restriction enzymes to produce short, manageable DNA fragments. These bacterial enzymes recognize specific four to six base sequences and reliably cleave DNA at a specific base pair within this span. Cleaving human DNA with one of these enzymes breaks the chromosomes down into millions of differently sized DNA fragments ranging from 100 to more than 10,000 base pairs long. These DNA fragments are then sorted by size using gel electrophoresis .
Larger fragments move more slowly than smaller ones through the porous agarose.
Once you have separated the DNA, you can determine the relative size of each fragment based on how far it has moved through the agarose. DNA fragments that have been separated on an agarose gel will begin to disintegrate after a day or two. To permanently save the DNA fragments in this segregated state, you need to transfer and permanently affix DNA to a nylon membrane . The positively charged nylon membrane is then placed on top of the agarose gel and used to sop up the negatively charged DNA fragment, like you might blot ink off a newspaper with Silly Putty. Unlike Silly Putty, however, you can't actually see any of the DNA on your membrane.