Q18 - Recombinant DNA and PCR

The correct answer is A) Endonucleases.

Break the word down: a nuclease cuts nucleic acids, and endo- means it cuts inside the strand rather than from the ends (exonucleases). Restriction enzymes recognise specific DNA sequences (EcoRI cuts at GAATTC, for example) and cut the sugar–phosphate backbone there.

A) Correct: They cut phosphodiester bonds inside the DNA molecule, so they are endonucleases.

B) Incorrect: Isomerases rearrange a molecule into one of its isomers.

C) Incorrect: Transferases move a chemical group from one molecule to another (kinases transfer phosphate groups, for example).

D) Incorrect: Oxidoreductases catalyse redox reactions.

E) Incorrect: Ligases do the opposite: DNA ligase joins DNA fragments together.