This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: DNA Replication , Subject name: Biochemistry
What do restriction endonucleases do - 
Select and explain. 
A. Cut ds DNA at specific sites, B. Cut RNA at specific sites, C. Cut ss DNA at specific sites, D. Break peptide chains
A: Cut ds DNA at specific sites
Answer: A: Cut ds DNA at specific sites 
Explanation: Ans. is 'a' i.e., Cut ds dna at specific sites o Endonucleases-enzymes that cut DNA at specific DNA sequences within the molecule (as opposed to exonucleases, which digest from the ends of DNA molecules)-are a key tool in recombinant DNA research,o These enzymes were called restriction enzymes because their presence in a given bacterium restricted the growth of certain bacterial viruses called bacteriophages.o Restriction enzymes cut DNA of any source into short pieces in a sequence-specific manner-in contrast to most other enzymatic, chemical, or physical methods, which break DNA randomly.o Restriction endonucleases cleave (cut) both strands of DNAQ.o Some restriction enzymes cut both strand at same level, so as to leave no unpaired bases on either end.o These ends are called blunt endsQ.o On the other hand, some restriction enzyme cut two strands in staggered manner, leaving two to four nucleotides of one strand unpaired at each end.o Thses ends are referred to as cohesive ends (or sticky ends42 or staggered ends).