Notice that this sequence reads the same in the 5'-to-3' direction on both strands. It is palindromic, like the word "mom," in the sense that it is the same in both directions from the 5' end. The EcoRI enzyme has two identical active sites on its two subunits, which cleave the two strands simultaneously between the G and the A of each strand (see Figure 16.4).

The EcoRI recognition sequence occurs, on average, about once in every 4,000 base pairs in a typical prokaryotic genome, or about once per four prokaryotic genes. So EcoRI can chop a large piece of DNA into smaller pieces containing, on average, just a few genes. Using EcoRI in the laboratory to cut small genomes, such as those of viruses that have tens of thousands of base pairs, may result in a few fragments. For a huge eukaryotic chromosome with tens of millions of base pairs, the number of fragments will be very large.

Of course, "on average" does not mean that the enzyme cuts all stretches of DNA at regular intervals. The EcoRI recognition sequence does not occur even once in the 40,000 base pairs of the genome of a phage called T7—a fact that is crucial to the survival of this virus, since its host is E. coli. Fortunately for E. coli, the DNA of other phages does contain the EcoRI recognition sequence.

Hundreds of restriction enzymes have been purified from various microorganisms. In the test tube, different restriction enzymes that recognize different restriction sites can be used to cut the same sample of DNA. Thus, restriction enzymes can be used as "knives" for genetic "surgery" to cut a sample of DNA in many different, specific places.

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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