Restriction enzymes
Restriction enzymes, endonuclease enzymes crossing the DNA strand at a site determined by a specific DNA sequence. The recognized sequence is generally symmetrical with a length of 4 to 8 pairs, although there are frequent exceptions. Restrictions along with DNA methylation are a system of DNA restriction and modification, which in prokaryotic organisms is a defense mechanism that prevents the insertion of bacteriophage DNA into the bacterial genome. The non-specificity of restriction enzymes under some conditions is called star activity. Restriction enzymes naturally occur in bacteria and cyanobacteria. "modification-restriction" system. This system protects the cell against the penetration of foreign DNA. This system assumes the presence of two types of enzymes in the microbial cell: ▪ Restriction endonuclease - recognizes a specific site of cleavage ▪ Methyltransferase - protects against cutting This modification protects the DNA from attack by its own restriction enzymes.