In the laboratory, restriction enzymes (or restriction endonucleases) are used to cut DNA into smaller fragments. The cuts are always made at specific nucleotide sequences. Different restriction enzymes recognise and cut different DNA sequences.
Where do restriction enzymes come from?
Restriction enzymes are found in bacteria. Bacteria use restriction enzymes to kill viruses – the enzymes attack the viral DNA and break it into useless fragments.
How do restriction enzymes work?
Like all enzymes, a restriction enzyme works by shape-to-shape matching. When it comes into contact with a DNA sequence with a shape that matches a part of the enzyme, called the recognition site, it wraps around the DNA and causes a break in both strands of the DNA molecule.
Each restriction enzyme recognises a different and specific recognition site, or DNA sequence. Recognition sites are usually only short - 4-8 nucleotides.
When are restriction enzymes used?
Restriction enzymes are a basic tool for biotechnology research. They are used for DNA cloning and DNA fingerprinting.
Different types of restriction enzyme
Scientists have identified and purified hundreds of different types of restriction enzymes. They are named after the genus and species of the organism they were isolated from and are given a number to indicate the order in which they were found. For example, EcoRI was the first restriction enzyme isolated from Escherichia coli strain RY13, whereas HindIII was the third enzyme isolated from Haemophilus influenzae strain R d.
DNA fragments: Blunt or sticky ends?
DNA consists of two complementary strands of nucleotides that spiral around each other in a double helix. Restriction enzymes cut through both nucleotide strands, breaking the DNA into fragments, but they don’t always do this in the same way.
SmaI is an example of a restriction enzyme that cuts straight through the DNA strands, creating DNA fragments with a flat or blunt end.
Other restriction enzymes, like EcoRI, cut through the DNA strands at nucleotides that are not exactly opposite each other. This creates DNA fragments with one nucleotide strand that overhangs at the end. This overhanging nucleotide strand is called a sticky end because it can easily bond with complementary DNA fragments.
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The article Catalysing chemical reactions with enzymes includes an animated video outlining in detail how enzymes work.
