Restriction enzymes cut the DNA at a particular point, determined by a nucleotide sequence (e.g. the EcoR1 restriction enzyme cuts the DNA wherever it finds the sequence GAATTC)

Some enzymes cut the DNA molecule unevenly i.e. they cut one strand of the double helix at one point, but then cut the other strand “downstream” of this point. EcoR1 is an example of one of these restriction enzymes. When it cuts the DNA molecule, it leaves two single stranded ends of TTAA. These are called “sticky ends” because they are complementary to each other and will bind again if brought close to each other

If the two sticky ends are bound to each other, another enzyme called DNA Ligase can reform the DNA molecule by rejoining the phosphate/sugar backbones.

These restriction enzymes are used to insert fragments of DNA into other DNA molecules – if the DNA is cut with sticky ends and a fragment with complementary sticky ends is introduced, the fragment may be inserted into the gap in the DNA. This is one of a ways that we introduce new DNA in genetic recombination.

Other restriction enzymes simply cut the DNA up into smaller fragments – every time the restriction enzyme finds its binding site on the DNA, it cuts the molecule. Because individuals with a different nucleotide sequence will have different frequencies of these binding sequences, each different sequence treated with a particular restriction enzyme will yield a different pattern of small and large fragments which can be demonstrated using gel electrophoresis. This is the basis of techniques such as DNA profiling (aka DNA “fingerprinting).

Visit The Dolan DNA Learning Centre and view the DNA Restriction animation (note : this link opens to an external website).