How Scientists Cut DNA with Restriction Enzymes
Scientists use restriction enzymes to cut DNA into smaller pieces so they can analyze and manipulate DNA more easily. Each restriction enzyme recognizes and can attach to a certain sequence on DNA called a restriction site.
You can think of restriction enzymes as little molecular scissors that slide along the DNA and cut the sugar-phosphate backbone wherever they find their restriction site.
The figure shows how a restriction enzyme can make a cut in a circular piece of DNA and turn it into a linear piece.
Some restriction enzymes make a straight cut through the DNA backbone, while others, like the one shown in the preceding figure, make staggered cuts. The enzymes that make staggered cuts leave small pieces of single-stranded DNA at the ends of the fragments they cut. Scientists call these single-stranded pieces sticky ends because they have complementary sequences to each other and tend to stick together by hydrogen bonds.
You can get two different pieces of DNA to stick together if you cut them both with a restriction enzyme that makes sticky ends. The two pieces tend to attach to each other, making it possible to combine them into a recombinant DNA molecule that has DNA from two sources.
A small, linear piece of viral DNA is shown in the next figure. The viral DNA contains restriction sites for two different restriction enzymes, called EcoR1 and HindIII.
The locations of the restriction sites are marked with arrows on the picture of the viral DNA. The length of DNA is given in kb, which stands for kilobase pairs. (Kilo means thousand, so one kilobase is 1000 base pairs of DNA.)
If you used just EcoR1 to cut a sample that contained many copies of this piece of DNA, how many differently sized pieces of DNA would result? What size would they be? If you cut the DNA with just HindIII, what would result? And what would happen if you cut the DNA with both enzymes?Example of cutting DNA with restriction enzymes.
For questions 2–4, test your understanding of restriction enzymes by using the information in the following figure to answer the questions.