Q.Restriction enzymes are
a. Not always required in genetic engineering
b. Essential tools in genetic engineering
c. Nucleases that cleave DNA at specific sites
d. both b and c

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 falls, an enzyme produces either blunt ends (a symmetric cut straight through the centre, e.g. SmaI on CCCGGG) or sticky/cohesive ends (a staggered, asymmetric cut leaving single-stranded overhangs, e.g. EcoRI on GAATTC) - sticky ends being far more useful for cloning because complementary overhangs base-pair spontaneously, holding an insert and vector together before DNA ligase seals the join.