Restriction Enzymes

Recombinant DNA refers to the construction of DNA containing DNA from more than one source. To form a recombinant molecule, a scientist uses restriction enzymes. Restriction enzymes are found in bacteria and evolved as a mechanism to breakdown the DNA of an invading virus. Restriction enzymes cut DNA at precise locations in the DNA molecule resulting in fragments of different lengths.

Restriction enzymes are more precisely known as restriction endonucleases. Over 100 of these enzymes are known. The restriction enzyme known as EcoR1, cleaves a DNA molecule at a particular sequence.

Notice that EcoR1 cuts very exactly between the A and G nucleotides starting at the 5' end of the molecule and that the sequence it cuts has a mirror image on either side of DNA molecule. These types of sequences are randomly found throughout the DNA molecule. If you were working in a lab and had an supply of different restiction enzymes, you could cut out sections of DNA that carried a specific gene.

• If there are only four different kinds of nucleotides, how can there be more than 100 restriction enzymes?
• The EcoR1 requires a sequence that is six nucleotides long to cleave a DNA molecule. Do you think that a bacterium would have greater survival advantage if its restriction enzyme worked with more or less nucleotides in a sequence?
• If you were interested in removing a DNA fragment with a gene, what do you think is the minimum number of nucleotides in a sequence that would be needed for a restriction enzyme to provide you with a fragment with the gene, rather than fragmenting the gene?