1. Different restriction enzymes commonly used in r-DNA technology are Alu I, Bam HI, Eco RI, Hind II, Hind III, Pst I, Sal I, Taq I, Mbo II, Hpa I, Bgl I, Not I, Kpn I, etc.

2. They are the molecular scissors which recognize and cut the phosphodiester back bone of DNA on both strands, at highly specific sequences. 

3. The sites recognized by them are called recognition sequences or recognition sites. 

4. Different restriction enzymes found in different organisms recognize different nucleotide sequences and therefore cut DNA at different sites. 

5. Restriction cutting may result in DNA fragments with blunt ends or cohesive or sticky ends or staggered ends (having short, single stranded projections). 

6. Restriction endonucleases like Bam HI and EcoRI produce fragments with sticky ends. 

7. Restriction endonucleases like Alu I, Hind III produce fragments with blunt ends

8. Type I restriction endonucleases fuction simultaneously as endonuclease and methylase e.g. EcoK. 

9. Type II restriction endonucleases have separate cleaving and methylation activities. They are more stable and are used in r-DNA technology e.g. EcoRI, Bgll. They cut DNA at specific sites within the palindrome. 

10. Type III restriction endonucleases cut DNA at specific non-palindromic sequences e.g. Hpal, MboII. 

11. In bacterial cells, REs destroy various viral DNAs that might enter the cell, thus restricting the potential growth of the virus.