Which type of nuclease digests nucleic acids from the middle of the molecule?

The correct choice is endonucleases, which are enzymes that cleave the phosphodiester bond within a nucleic acid chain. This allows them to digest nucleic acids from the middle of the molecule rather than from the ends, which is the characteristic action of other nucleases.

Endonucleases play a critical role in various biological processes, including DNA repair, RNA processing, and the restriction of foreign DNA in bacterial immune responses. The ability to cut nucleic acids internally facilitates the manipulation of DNA fragments for cloning or sequencing purposes, which is essential in molecular biology techniques.

In contrast, exonucleases act by removing nucleotides one at a time from the ends of nucleic acid molecules, whether from the 5' or 3' end, and are not capable of initiating cleavage at internal locations. Restriction endonucleases, while also capable of cutting DNA, do so at specific recognition sites rather than digesting at arbitrary internal sites. DNA methyltransferases are enzymes that add methyl groups to nucleic acids and do not perform any digesting function. Therefore, endonucleases are the correct answer for the question regarding nucleic acid digestion from the middle of the molecule.