Question

‘Restriction enzymes’ are called so because they restrict

• A. cleavage of phosphodiester bond within DNA
• B. cutting of DNA at specific position only
• C. the multiplication of bacteriophage in bacteria
• D. all of the above

Answer 1

The correct option is C the multiplication of bacteriophage in bacteria

Restriction enzymes that are commonly used in rDNA technology are a type of endonuclease that recognises specific sites and cleaves the phosphodiester bonds present between consecutive nucleotides at or near to specific positions within the DNA. They are also called “molecular scissors or biological scissors” because they cut the DNA and produce fragments.

These restriction enzymes are present in many bacteria as part of their defence mechanism which is named “restriction modification system”. It provides defense against foreign DNA such as those from bacteriophages. It consists of two components - restriction enzymes and modification enzymes. The restriction enzymes selectively ‘restrict’ the multiplication of the viral/foreign DNA in the cell by cleaving it into several fragments at or near to specific sequences. The modification enzymes add methyl groups to the bacterial DNA at particular sequences so as to make it unrecognisable by the restriction enzyme. This protects the bacterial DNA from getting cleaved by the restriction enzymes.

Cleaving phosphodiester bonds in DNA at or near to specific positions are only the mechanism of action of restriction enzymes. However, the term ‘restriction’ in restriction enzymes refers to the restriction of multiplication of foreign DNA within bacteria.