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Restriction Endonucleases: When the DNA and its first cutting mechanisms were known, cleaving the DNA has never posed a problem. The Scientists working on it are very much aware of the fragile nature of the Nucleic acid. The popular methods used at that time are Pipetting and Sonication process. But we in genetic engineering need more than a random breakage. It is very much essential to cut the DNA reproducibly and predictably at the same point every time. So, Restriction Endonucleases, which have enormous practical application in fact when invented are of the most esoteric kind. The entire discovery began with a small interesting fact discovered by scientists. Some bacteria resisted the infection caused by phages of course the phage is grown in another bacterial strain. Phages grown in the same strain are recognized as 'self' and avoided degradation - First discovery of DNA cutting enzymes. Some examples of Restriction Endonucleases are given below: EcoRI (Escherichia coli) which has a recognition site between Guanine and Adenine. It produces small fragments of 4-8 kbp of length. It leaves staggered ends - Cohesive or Sticky ends. HaeIII (Haemophilus influenzae) and Sau3A (Staphylococcus aureus) cuts the DNA into smaller fragments with an average length of hundred base pairs. NotI (Nocardia otitidis-caviarum) cuts the human chromosome with an average length of million base pairs. Another interesting fact, I remembered while discussing about these enzymes. You may get a doubt, why do such enzymes do not attack the DNA of the cells in which they are found.??? A parallel set of enzymes exist that modify the DNA by adding extra methyl groups to bases within the recognition sequences. Such are marked as 'self' and avoid attack by the REs. Foreign DNA of course lack this modification. THIS BLOG IS FREE OF PLAGIARISM Ref: Studies in Microbiology- Principles of Gene Manipulation - An Introduction to Genetic Engineering RW Old & S B Primrose 5th edition Blackwell Science. Page no: 26-28

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