複選題7.Which of the following statements about restriction enzymes is/are CORRECT? (A) Type I restriction enzyme usually cleave DNA within their recognition sequences (B) They are part of a bacterial host defense system to degrade invading viral DNA. (C) Calcium (Ca2+) is typically required for their catalytic activity (D)Cleavage by restriction enzymes can be blocked when a specific base in the recognition site is methylated. (E) Restriction enzymes hydrolyze covalent phosphodiester bonds of the DNA, leaving either "sticky/cohesive" ends or "blunt" ends.