Assertion: Restriction endonucleases are also called "molecular scissors".
Reason: Restriction endonucleases are not used in genetic engineering to form ‘recombinant’ molecules of DNA.

Assertion (A) is true but Reason (R) is not correct.

The correct answer is Option (3) - Assertion (A) is true but Reason (R) is not correct.

Restriction endonucleases are also called molecular scissors. Restriction endonucleases are enzymes that cut DNA molecules at specific recognition sequences. These enzymes play a crucial role in genetic engineering and molecular biology by allowing scientists to precisely manipulate DNA sequences.

Restriction enzymes belong to a larger class of enzymes called nucleases. These are of two kinds; exonucleases and endonucleases. Exonucleases remove nucleotides from the ends of the DNA whereas, endonucleases make cuts at specific positions within the DNA. Each restriction endonuclease functions by ‘inspecting’ the length of a DNA sequence. Once it finds its specific recognition sequence, it will bind to the DNA and cut each of the two strands of the double helix at specific points in their sugar-phosphate backbones. Each restriction endonuclease recognises a specific palindromic nucleotide sequences in the DNA. Restriction endonucleases are used in genetic engineering to form ‘recombinant’ molecules of DNA, which are composed of DNA from different sources/genomes. When cut by the same restriction enzyme, the resultant DNA fragments have the same kind of ‘sticky-ends’ and, these can be joined together (end-to-end) using DNA ligases.