Restriction endonucleases are used as molecular__________.

The correct answer is Option (3) → molecular knife cut DNA at specific site

Option (1) molecular build up at nucleotides — Incorrect. Building DNA from nucleotides is the work of DNA polymerase, not of a restriction enzyme.

Option (2) molecular cement to join DNA sites — Incorrect. Joining pieces of DNA is the work of DNA ligase, the ‘molecular glue’.

Option (3) molecular knife cut DNA at specific site — Correct. A restriction endonuclease recognises a particular base sequence and cuts the DNA there. That is why restriction enzymes are called molecular scissors.

Option (4) molecular degradation to break up the DNA to nucleotides — Incorrect. Restriction enzymes cut DNA into fragments at fixed sites. They do not break it all the way down into single nucleotides.

NCERT Extract:
Principles of Biotechnology – “… Stanley Cohen and Herbert Boyer accomplished this in 1972 by isolating the antibiotic resistance gene by cutting out a piece of DNA from a plasmid which was responsible for conferring antibiotic resistance. The cutting of DNA at specific locations became possible with the discovery of the so-called ‘molecular scissors’- restriction enzymes.”