Q.Each restriction enzyme cleaves a molecule only at :

A restriction enzyme (restriction endonuclease) cleaves DNA at or near a specific, short recognition sequence (a restriction site), first isolated from E. coli in 1963 as part of the bacterium's own restriction-modification defence system against bacteriophage infection. They split by mechanism into exonucleases (removing nucleotides one at a time from a DNA molecule's end, e.g. Bal 31) and endonucleases (cutting internal phosphodiester bonds, e.g. EcoRI, HindIII, BamHI); of the three known classes (Type I, II, III), only Type II - which cuts cleanly within a short, usually palindromic, 4-8 bp sequence - is used in recombinant DNA technology. Naming follows a fixed convention: genus initial + first two letters of species + strain + a Roman numeral for order of discovery (so EcoRI is the first enzyme discovered in Escherichia coli strain RY13). Depending on where within the recognition site the cut fall …