What are Restriction Enzymes?
A restriction enzyme (or restriction endonucleases) recognizes a specific base pair sequence in DNA called a restriction site and cleaves the DNA (hydrolyses the phosphodiester backbones) within the sequence. Restriction enzymes are widely found in prokaryotes and provide protection to the host cell by destroying foreign DNA that makes entry to it .It acts as a part of defense mechanism.
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Types of Restriction Enzymes
Restriction enzymes belong to a larger class of enzymes called nucleases. They are of two types:
They remove nucleotides from the ends of DNA.
Endonucleases are enzymes that produce internal cuts called cleavage in DNA molecules. Endonucleases cleave DNA molecules at random sites. A class of endonucleases cleaves DNA only within or near those sites with specific base sequences. These are called restriction endonucleases. Sites recognized by them called recognition sites. (These sites differ for different restriction enzymes).
Restriction Enzyme: A Cutting Tool for DNA
Restriction endonucleases serve as the tool for cutting DNA molecules at predetermined sites, which is the basic requirement for gene cloning or Genetic engineering. Many restriction enzymes cleave both strands of DNA at exactly same nucleotide position almost in centre of recognition site resulting in blunt or flush end. While some other restriction enzymes cleave recognition sequence asymmetrically. They produce short, single strands DNA fragments (hanging). Such ends are called sticky or cohesive ends (because base pairing between them can stick the DNA molecule again).
In figure below, restriction enzyme (EcoRI) cuts DNA between bases G and A only when sequence GAATTC is present in DNA. Here same restriction enzyme is used for cutting sequence of both vector and foreign DNA to form recombinant DNA.
Action of Restriction Enzyme EcoRI
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