Restriction endonucleases are used as molecular__________.

molecular knife cut DNA at specific site

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

Restriction endonucleases are commonly known as molecular scissors.

Restriction Endonucleases (Molecular Scissors): These are enzymes that are commonly referred to as "molecular scissors" because they cut DNA molecules at specific recognition sequences. These recognition sequences are often palindromic, meaning the sequence reads the same forwards and backward. When a restriction endonuclease encounters its specific recognition sequence on a DNA molecule, it cleaves the DNA at that specific point by breaking the sugar-phosphate backbone of both DNA strands. This results in the production of DNA fragments with "sticky ends," which can be used in various molecular biology techniques like DNA cloning.

DNA Ligase (Molecular Glue): DNA ligase is another enzyme used in molecular biology. After DNA has been cut by restriction endonucleases, DNA ligase plays the role of "molecular glue." It catalyzes the joining (ligation) of the DNA fragments back together by forming covalent bonds between the sugar-phosphate backbones. This process is crucial in DNA replication, repair, and recombinant DNA technology.

In summary, restriction endonucleases cut DNA at specific recognition sequences, and DNA ligase is used to rejoin these fragments. Together, they are important tools for manipulating and studying DNA in various applications, including genetic engineering and molecular biology research.