Which of the following statements are not true regarding restriction endonucleases ? A. They are called molecular scissors. B. These are the enzymes responsible for restricting the growth of bacteriophages in E. coli . C. They cut the DNA only at the centre of the palindromic sites. D. They remove nucleotides only from the ends of DNA fragments. E. They recognise specific palindromic base-sequences. Choose the answer from the options given below :
D. C and D only
Restriction endonucleases are commonly known as molecular scissors as they cut DNA at specific locations. Thus, statement A is true. They were discovered for their ability to restrict the growth of bacteriophages in bacteria such as E. coli . Thus, statement B is true. Restriction endonucleases cut the DNA helix a little away from the centre of the palindromic sites, but between the same two bases on the opposite strands. Thus, statement C is incorrect. Exonucleases remove nucleotides from the ends of the DNA, whereas endonucleases make cuts at specific positions within the DNA. Thus, statement D is incorrect. They function by inspecting the length of a DNA sequence and bind to the DNA to cut each of the two strands of the double helix at specific points in their sugar-phosphate backbones, recognising specific palindromic nucleotide sequences. Thus, statement E is true. Therefore, statements C and D are not true. Answer: C and D only
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