Molecular Genetics

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Restriction endonucleases are protein enzymes that recognize specific nucleotide sequences and cleave both strands of the DNA containing those sequences. Recognition sequences for many enzymes are the same on both strands. Such recognition sequences are said to be palindromic. Commonly used restriction enzymes always cleave the DNA strands at a fixed position relative to the recognition sequence. Positions of cleavage relative to the recognition sequence depend on the enzyme. Products with flush ends or 5' or 3' overhanging single-stranded ends may be produced.

• Because the sites of cleavage of long DNA molecules are determined by their nucleotide sequence, the long DNAs are broken into discrete sized fragments.
• The size of restriction fragments is determined by the distance between restriction enzyme cleavage sites.
• On average, the frequency of cleavage of DNA depends on the probability of occurrence of a recognition sequence. Thus, enzymes with longer recognition sequences cut less frequently and consequently produce larger fragments than do enzymes with shorter recognition sequences.
• The overhangs produced by many restriction enzymes are used as sticky ends to "glue" DNA fragments from different sources together.
• Restriction enzymes and the fragments produced by them have been powerful tools of molecular genetics. They are used
  • to map DNA molecules physically,
  • to analyze population polymorphisms,
  • to rearrange DNA molecules,
  • to prepare molecular probes,
  • to create mutants,
  • to analyze the modification status of the DNA,
  • and other applications.